The SU carb is brilliantly simple in its design, with very little to go wrong. However the later HIF (which stands for 'Horizontal Integral Float', by the way) is a bit more complex than the earlier HS, which I think stands for 'Horizontal Side float'. Why not HSF then? Who knows? The 'Horizontal' in both cases refers to the direction of air flow into the engine, as opposed to the 'down-draft', or 'semi down-draught' you might see applied to some other designs of carb. Visual identification is simple - on the HIF the float chamber is contained within the main body of the carb and actually surrounds the bottom of the jet whereas on the HS the float chamber is to one side of the main body of the carb and has an external pipe connecting it to the jet. Whilst technically the HIF is an improvement over the HS, for a number of reasons there is no good reason to convert to HIFs if you already have HSs, and if you are converting to SUs from Zenith/Stromberg or an aftermarket conversion and have the choice of HSs or HIFs then HSs would be marginally preferable for their simplicity. The diagrams below are of an HS unless otherwise indicated, click on a thumbnail for a full-size image in a different window.

That is generally the case, but in practise there is a physically large but quite weak coil spring between the top of the piston and the outer cover pressing down on the piston and so restricting its rise somewhat. This is another feature to ensure the correct balance of air to fuel across the range of throttle opening, and means that a progressively larger vacuum is required the higher the piston rises. However the difference in vacuum between idle and full throttle piston heights is relatively small compared to the up to 20 in. Hg or so available in the inlet manifold. The spring strength, carb throat diameter, needle shape and jet size are all chosen to give the correct mixture across the operating range of the carb in any particular application. For a larger or smaller engine, single or multiple carbs, with or without supercharger, etc. the carb throat size, needle shape, jet size, spring strength etc. will all be chosen to give the correct results for that application. You can't just bolt on a bigger carb without doing anything else and expect your car to go faster, indeed it will probably perform worse. Likewise if you make changes to engine capacity, breathing, valve timing etc. you probably won't get the best out of them unless you change the carb parameters as well. What changes to make under what circumstances is a huge subject.

It is vital not to get pistons and covers mixed up. The original part numbers are for a pair matched in the factory to meet a specified drop-test characteristic, and mixing them up can significantly change the operating performance of the carbs.

There are some misconceptions about covers though, one being that you have to mark the cover relative to the carb body to get it back at the right orientation. Incorrect as the fixing holes are not equidistant round the periphery so each cover can only go back in one orientation. Whilst two of the holes are either side of the intake, the third one has to be one side or the other of the port on the engine side otherwise it would project into the port.

Another one is that you must mark each cover and piston so it goes back on the right body. Ideally yes, but not strictly correct as within each era of carb there is only one part number for each cover piston/assembly so they can be fitted to either the front or the rear carb.

The interesting thing is that HS carbs and early HIF carbs have a breather rib in the cover neck, and that rib faces in different directions - towards the air-cleaner on the front carb and forwards on the rear carb. This is because the fixing holes in the carb body are mirror images with the hole nearest the engine being one side of the port on one carb and the other side of the port on the other carb. So the cover is at one orientation on the front carb and a different orientation on the rear.

But later HIFs have the both ribs facing the air cleaners, and that is because instead of the fixing holes being mirror images, the hole nearest the engine is to the rear of the port on both carbs, so the covers are at the same orientation. At first I joked that it must have been someone with OCD not being able to stand seeing the offset any more. But that is highly unlikely, and I think it must have been for emissions reasons. Because they are a matched pair to meet a drop-test, unless the piston and the cover were perfectly circular you will get small difference in the drop-test depending on the relative orientation of the piston in the cover, and so you would get different operating characteristics depending on whether the cover/piston assembly were fitted to the front or the rear carb. With how the emissions limits tightened in later years for North America, maybe that difference was enough to cause problems meeting them. And by moving the position of the engine-side hole to be the same on both carbs, each cover and piston could be tested at a known orientation, and that would be the same when fitted to either carb. Maybe.

Dampers

Drop-test: April 2019

North American emissions-controlled vehicles may have a different arrangement with a three-port float chamber lid on the front carb although this is not listed in the Parts Catalogue. One of the ports is the fuel inlet from a filter on the inner wing, another is a fuel outlet that goes to the rear carb, and the third is the overflow going to the charcoal canister. The rear carb should have a conventional 2-port with fuel inlet and overflow.

HIF carbs:

October 2020:
The 4-cylinder HIF vent/overflow arrangement is much the same as for HS carbs but the V8 is quite different. Hoses from the carbs go under the air-box to a plastic washer tee GWW401 behind the off-side carb. Another short piece of hose goes down to metal pipe BHH1570 which has a support bracket under the top off-side bell-housing bolt, then a longer section of hose goes down past the off-side of the bell-housing. Vee had none of that when she came to me and I had no idea what was supposed to be there, didn't fancy fuel running down onto the exhausts, so I did basically the same with hoses and tee and in the same position, but without the metal pipe to support it all. Apparently these pipes are very rarely seen in the wild, and I can imagine that unless someone remembers to attach the metal pipe to the bell-housing bolt when the engine and gearbox are reassembled out of the car, they won't go back on at all as you can't access that bolt once the two are back in the car!

The air-filter gaskets (HS and HIF are the same) are handed and must be installed the right way up. The flange has breather holes that maintain the underside of the piston at atmospheric pressure, so when inlet manifold vacuum is applied above the piston it can rise correctly to deliver the correct mixture. V8 HIFs are different in that there are no less than nine holes in the carb flanges. Again the gaskets must be installed the right way up to leave the breather and fixing holes clear, and that cover the other holes.

While doing the clutch change on a friend's 78 I found these gaskets, which have holes in both upper and lower positions, and so cannot be fitted the wrong way round. However! You still have to fit the base-plate for the air filter the right way round or the holes will be blocked whichever gasket you use or how you install it! It wasn't obvious from the running of the car that they were blocked, but Keith's car failed it's emissions test this year having passed just a year earlier, done very little mileage since, no changes other than a new choke cable (which was fully releasing the choke) even during the clutch change apart from having the air-filters removed. When I went to setup the carbs for air-balance and mixture I found the filter bases upside down, hence the auxiliary ports were blocked. In the end the balance and mixture were just about spot-on, only the balance under choke was out, which wouldn't have affected the emissions test anyway, so I can only assume I put the filter cans on the wrong carbs hence the bases upside down, and that was enough to affect the reading. Correcting that and a precautionary weakening of the mixture by just 1/8th turn to show 3.8% on my Gastester passed the retest at almost the same figure.

Originally the air-filter cans were clearly handed and as long as you kept them, the bases, bolts and gaskets together as two assemblies you are unlikely to get them on the wrong carbs. But at some point they seem to have been modified so the cans are identical, so if you keep them assembled but mixed up and fit them to the wrong carbs you will get the bases upside down. The Parts Catalogue shows different part numbers for the front and rear cans for all years, but you can definitely fit the later ones on the wrong carbs and it isn't immediately obvious. Originally common to all markets they changed for the 72 model year (from straight intakes to curved?) to three different sets for UK, North America, and the rest of the world (export cars changed to HIF the previous year so it wasn't to do with that). UK changed to use the 'rest of the world' set in November 73 with the 18V 779/780 engines and all twin carb engines had those to the end.

Inlet Manifold:

Together with the distance pieces 12H712 there are three gaskets AEC2083 per carb to secure the carbs and heat shield to the inlet manifold. These gaskets are also handed but with these it's not a case of blocking holes but simply aligning the outline with the carb flanges and distance pieces. I think these were originally fitted dry (no fluids normally) but I use a smear of copper-grease on all the gasket surfaces in preference to conventional sealants, both hardening and non-hardening.

One of the biggest benefits of the HS carb over the HIF is that the floats and valves are so easy to access on the HS compared to the HIF, which really need to be removed from the car. The bottom cover on the HIF is also submerged in fuel, and any weakness in the seal can cause significant seepage, enough to drain the float chamber while parked. The HS float chamber lid does have a gasket, but it is above the level of the fuel, so really only protects against seepage if the fuel is sloshing about inside, or if the float chamber should flood due to a faulty valve or float as the overflow port is above the join between chamber and lid.

See here for a bench test of a float chamber.

Floats
Valves

  Floats:
Float Height

All HS carbs used the same float for both front and rear carbs - originally AUD9904, currently it seems to be WZX1300.

North American 4-cylinder HIF carbs used AUD3571 in the front carb and AUD3570 in the rear, currently WZX1510 and WZX1509 respectively.

UK HIF and V8 carbs used CUD2774 in the front/left carb (as viewed from the driving seat) and CUD2773 in the rear/right, however these also seem to use WZX1510 and WZX1509 respectively now.

There are also 'StayUp' float kits for all versions which have a closed cell construction and Burlen claim they are unsinkable. However on one forum someone had one sink, returned it to Burlen, who said it must have been damaged during handling ... which surely makes it no different to any other float ... apart from being nearly twice as expensive!

If the float cracks it can take in fuel, which makes it heavier, so the level of fuel in the float chamber and hence the jet has to rise higher than it should before it shuts off the fuel flow. This can give rise to mixture imbalance between the carbs and eventually overflowing. If you have repeated overflowing always check the float for fuel by shaking before automatically changing the float valve.

With the lid (HS) or bottom cover (HIF) removed, on HS carbs grip the thicker end of the float hinge pin with a pair of pliers and withdraw to release the float. On HIFs unscrew the hinge pin. On HIFs it is wise to replace the cover seal, they harden in use.

  Float height: This is given quite a close tolerance in the workshop manuals if not a single value, but according to SU Burlen typically for the HS carbs on MGB it is 3 to 5mm with the adjustable float and steel needle, or 1.5 to 5mm for the 'fixed' nylon float and Delrin needle, and for HIFs 0.5 to 1.5mm. If having problems with a car or carbs new to you or that have had work performed on them and it could be fuel related, check this height. Easy on HS's - just remove the lids, invert, and slide a bar or drill between the lowest part of the float and the machined edge of the lid. HIFs will have to be removed and the whole carb inverted, a straight-edge laid across the centre of the float chamber at right-angles to the pivot i.e. crossing the centre of the 'U' of the float, and estimate (measuring being tricky without wire gauges) the vertical gap between the nearest part of the float and the straight-edge. Note that an excessive gap could be due to an incorrect or faulty valve holding the float too high or too low, and if the float is way too high it could contribute to fuel starvation at high speeds by restricting how far the valve can open hence low fuel level in the jet.

Valves: With the float removed the inner of the float valve should fall out. This has a conical point at one end, and a spring-loaded pin at the other. The outer can be unscrewed with a 11/32" socket.

Original float valve inners were steel tipped against a brass seat. The tip of the inner eventually develops a wear ridge, and this can cause seepage when the valve should be closed, and eventual overflow. Someone in America developed an alternative - Grose Jets (jets?) which used a ball valve, and were said to be superior. They may well have been, but then a Viton tipped valve was developed by SU that was equally as good. Subsequently the Grose products were produced by a different company, and people started finding those became worse than the original SU items, let alone the Viton-tipped versions. Outers for Viton-tipped seem to have a conical seat, i.e. different to the earlier flat seat. This is probably Viton is a resilient material, and the sharp edge of the original seats would almost certainly cause a ridge to be developed in the Viton in short order. It makes one wonder if a conical seat for the original steel-tipped inners would have delayed the development of a wear ridge, if not prevented it altogether.

Three different part numbers were used at various times:

My V8 uses the larger bore i.e. 0.096, spring-loaded. And whilst originally they were steel tipped it's probably best to get the Viton-tipped now, i.e. WZX 1096. However neither the old or the replacements had filters. As that is the only spring-loaded Viton-tipped, it's probably best to get those for 4-cylinder HIFs as well.

  Seepage and Overflow: Small particles can cause slight flooding, getting trapped as the valves would never normally be fully open. Sometimes these can be washed out by disconnecting the fuel pump, running the engine to empty the carbs, then reconnecting the fuel pump. The resulting rush of fuel through the now wide-open valves can dislodge particles. These may not be a problem in HS carbs as the needle to jet clearance is large, and there is room for them to lie in the bottom of the float chamber. They can be more of an issue in HIFs as there are more orifices to get blocked, and they will lie directly under the jet.

Many years ago I started getting overflow from the right carb in the V8. The above trick did not help, so I changed the float valve, but still had the problem. It was only then I discovered the float had fuel in it, impossible to see in the brown float. I tried putting it in hot water to discover the source of the leak, but that, pressure and shaking did not give any indication of where it might be, so replacement was the only option.

May 2016: Just before Vee's MOT I discovered a leak from where the supply hose replaced just a few months ago attached to the near-side carb, fixed by tightening the clip although it didn't move very much. The day after the MOT while looking at something else I discovered the overflow hose was leaking on the same carb. Obviously the float valve wasn't seating properly (a hang-over from the filter change last year?) and timing the ticks from the fuel pump they were about every 15 seconds. They shouldn't occur more frequently than once every 30 secs, otherwise a problem with a float valve or pump inlet non-return valve is indicated. The first trick is to run the engine with the fuel pump fuse disconnected to drain the carbs, then when the engine has stopped reconnect the fuse in the hope that the now wide-open float valve and high-rate of fuel flow flushes out any debris that might be there. After that clicks were more than 30 secs apart, although less than the 60 secs plus after I had changed the pump hoses. So not leaking now, but that still left the overflow hose to deal with. A few days later after a run - and when cool - I gave up waiting after 2 minutes without a click, but I'll lay a couple in for stock anyway.

See here for information on needle selection.

Originally HS carbs had fixed needles and the jet has to be centred on the needle to prevent the one rubbing on the other, inhibiting piston free movement, causing wear and upsetting the mixture. North America changed to swinging/bias needles on AUD326 carbs/18GH engines in 1969. Home market cars didn't get them until 1971 with AUD492 carbs on 18V engines. The jet bearing kits differ between the two - WZX1341 for fixed needles and WZX1442 for bias/swinging needles. The difference is that the jet bearing for fixed needles has a larger clearance in the jet nut to enable centring, that for bias/swinging needles is basically a snug fit in the nut to hold it in one position, the needle moving to fit the jet.

Fixed needles have a screw that locks it into the piston, swinging have the needle going through a guide AUD4288 and the fixing screw secures that into the piston. The needle is loose in the guide and there is a spring AUD3306 above to keep it pressed down onto the top of the guide.

4-cylinder cars have 0.090" jets, V8s 0.100". HIF carbs have different jets to HS and in both cases there are different jets for front and rear (left and right from the driver's perspective for V8) carbs.

There are two different types of jet bearing for the HS4 with different threads which affects how far the jet moves with each turn of the mixture adjusting nut - coarse and fine. However everything encountered on an MGB should be the coarse thread, the fine thread only applies to other marques such as the Mini where it has Waxstat temperature-controlled mixture.

Jet Height: Some recommend getting the starting point for setting-up from a depth gauge. Personally I've never seen the need when you are going to use the lifting pins and engine note for the final setting, it is only a starting point, and it means removing the cover and piston and putting them somewhere safe, as well as poking things down inside onto the jet. But Miles Bannister recently commented that when checking his he found them at different heights, and when setting them to the same height the car ran noticeably better. My first thought was that using the lifting pin to determine final settings is 'dynamic' based on what the engine is actually doing and not a theoretical measurement, much like dynamic timing is preferable to static, which again is only a starting point. Carbs like anything else have a tolerance in component manufacture, which may well result in very small differences in settings between carbs for an identical mixture, and that would be preferable to a theoretical jet height. But ever interested I decided to check Bee and Vee.

I found Bees at .0445" for the front and .0440" for the rear. Now on the one hand this is pleasingly close, just 1/12th of a flat different based on .0064" of movement per flat, but on the other hand it is significantly less than the .060" mentioned by various people as being 'correct' for the 4-cylinder, which should mean they are weak. Nevertheless Bee has always run really well, no flat spots, choke fully home within a mile or so in winter, so no need to change.

There has been a lot of discussion as to how many return springs were fitted to the SU carbs, in particular whether the choke has one, and it's all quite confusing. The Parts Catalogue for up to September 1976 has one page entitled Accelerator pedal - choke control - all versions except V8' listing item 12 'spring-cable return' AEC2075 qty 2 for UK car numbers 101 to 332032. The next page is entitled 'Accelerator cable and fittings - manual not V8' lists the same item three times - 'spring-return' qty 2 for the same UK car numbers 101 to 332032 i.e. HS carbs which seems to be correct i.e. two cable return springs and two accelerator cam return springs. Then it has 'spring-choke return' qty 1 UK car number 332033-on i.e. HIF carbs; and 'spring-throttle return' qty 2 'except Zenith carb' with no indication of car number which implies both HS and HIF which wouldn't be correct for HIF. For September 76 on 'Carburettor fittings not USA Canada' it lists item 10 AEC2075 'spring-throttle return' qty 3 which is definitely incorrect and item 21 AEC2075 'spring-choke return' qty 1.

HS carbs have torsion springs on the choke quadrants and not on the accelerator quadrants, but HIF carbs have them on both accelerator quadrants. Logically where there are torsion springs additional springs on each carb are not required (as per V8 practice), but they are where there are no torsion springs, plus a cable return spring regardless. So again logically HS carbs need three on the accelerator (two quadrants plus one cable) and one on the choke (cable, which is what mine have), and HIF need one on the accelerator (cable) and one on the choke (cable), and the Workshop Manual does only show two springs. However several cars seem to have them on HIF choke quadrants making three for the choke (including a pal's 78), with others reporting that with only one on the choke it does not return properly. Moss for the HIF only shows one spring for the throttle and none for the choke, and Brown & Gammons do not differentiate between HS and HIF showing 3 throttle and 1 choke which is what Moss shows for HS.

For completeness both Catalogues show one 'Spring-throttle return' with the Zenith carb, the 76-on catalogue also lists one 'Spring-throttle return' with the pedal parts but the earlier Catalogue doesn't.

V8:
Only one throttle return spring - BHH1781 which is shorter than the SU items at a stated 1.75" 'free' length, plus two torsion springs AUD4272 and 4273 on the butterfly spindles and two more AUD4417 and 4418 on the choke spindles. BHH1781 is not listed in the Parts Catalogue for the V8 and no one seems to have one. Similar to the 4-cylinder springs AEC2075 at a stated free length of 2.75", the springs themselves on my two cars are the same length it is only the wire ends that are longer on the 4-cylinder, so it would be easy to shorten a 4-cylinder spring to suit the V8. Whilst my V8 spring is shorter overall than one from the 4-cylinder it is only by about 1/4", not the 1" that is indicated by the stated lengths. Both are barely stretched when fitted to a released throttle which logically seems to be correct, if the V8 spring was 1.75" closed it would have to be stretched at least 1/2" to be fitted. I've never had any indication of throttle sticking open, and there are two torsion springs on the spindles after all making three in all, the same as the 4-cylinder HS (three coil springs) and HIF (one coil spring plus two torsion springs) installations. However BHH1781 is also used on the Stromberg carb which was fitted for several years on North American spec and a couple of suppliers there list it - if they will post it and you can stand the postage cost. At least one of them cross-references BHH1781 with AEC2075 which is obviously incorrect so you need to be careful.

MGOC show it for the Stromberg here (but I can't find it on their web site) in April 2022. At least they did, after finding this for someone and them getting one I got an email saying there was only one left, so I'm afraid I bought it. Well, I say 'bought', but after a week it hadn't turned up. Complained, and they said they still had one on the shelf so it obviously hadn't been sent, and to order it again. But of course because it had been the last one it had been processed as 'sold' so that page showed 0 available and no 'buy' button! So back onto them and they corrected that, so I was able to purchase it - but it was 50p dearer!! That's neither here nor there but I moaned about it anyway, and they said I should have contacted them direct!!! Maybe they should have got it right - three times. At least this one came in two days.

Just intending to put Bee back in the remote garage and bring Vee back to the house, I was suddenly reminded that I had been intending to look at the throttle cable for a while - like 15 years - as it was a bit sticky and jerky especially in the lower gears, so much so I've always slipped the clutch for small pedal movements in low gears/low speeds. Got the inner out to find a broken outer strand - ah ha! Thinking one wouldn't make much difference I started peeling it off only to discover the cable kinked a couple of inches from the pedal end to reveal another broken outer strand plus one or two inner strands. Oh well, have to replace it now and in fact it is a wonder it hasn't broken already, but had to put it back to get the car back in the garage, up the slope of the drive. By now I had found that the pedal end of the outer had become dislodged, because although it was wedged in the guide, the flanges had been broken off the guide so it, and hence the cable, was floating about in the cavity in the inner wing. As the inner was damaged where it passed through the body it could well have been caused by the broken guide, so now I need one of those as well. But how to get the bits of the broken flange out which were still screwed to the body inside the cavity? I removed the pedal cover which gave me just enough room to get a Pozidrive bit and a selection of 1/2" and 1/4" socket extensions and a UJ to undo the screws, which fortunately were under a layer of old grease or Waxoyl and not corroded. The cable should be easy to source but what about the guide? And if and when I do find one it's going to be fun getting the screws started without cross-threading them.

Stripped and repainted the cover with two coats of Hammerite Smooth, then stuck on the self-adhesive upper seal. Replaced the bottom seal, which was fun. In the end I chose to remove the pedals, then slacken the four bolts holding the pedal frame to the horizontal part of the firewall. I left the two upper bolts to the vertical part, but had enough play to lever the frame upwards gently while I removed the old seal with a flat blade, and slid the new seal into place. Note that it is handed left and right as well as back and fore. Retightened the four bolts, ensuring that the holes in the seal for the cover lined up with those in the firewall, and refitted the pedals, greasing the pivots and clevis pins with copper grease.

Three of the cover screws are easy to fit, but the fourth is in the very narrow gap between the cover and the edge of the wing. I chose to put the screw in the cover, then carefully move the cover into position but raised up a bit so it didn't push the screw out, then got it started using a screwdriver bit handle, 1/4" extension, and appropriate bit. Fitted the other three screws loosely, then tightened all four down. Job done.

Update August 2005: Having done a few hundred miles now the cable seems nearly as sticky and jerky as before. Oh well, at least I know it is sound. Subsequently I removed the 3rd spring from the throttle cable which has improved things. 2022: At some point (no idea when) I put the 3rd spring back and I've not been aware of any problems for a long time.

Cable adjustment:

Four aspects:

• Leaving a little free-play in the cable with the pedal released so the butterflies are held open at idle by the idle screws and not the cable. If the butterflies are left hanging on the cable it can result in an inconsistent idle speed.
• Having enough pedal travel to fully open the butterflies. If the pedal can't fully open the butterflies you are obviously missing out on some performance.
• Making sure that the pedal hits its stop when fully open fractionally before the carbs are fully open so you don't stretch the cable and stress the linkage at the carb.
• Finally adjust the pedal back-stop so that there is just a fraction of free play with the pedal released and the back-stop isn't pushing the pedal down and pulling on the cable.

The final aspect is pedal back-stop adjustment that takes up free play in pedal movement, i.e. if you can lift the pedal up when the carbs are on their idle screws. It's important that this adjustment does not start applying a force to the cable by pushing the pedal down. The RHD back-stop screw is adjusted from the footwell and LHD from the heater shelf.

HS set-up
HS components
HIF set-up
HIF enrichment valve
V8 set-up

I had assumed that early versions of the 'C' knob at least were 'push-pull' rather than 'twist to lock' based on my (from memory!) 64 Mini, but Dave O'Neil has sent me a scan of a Mk1 handbook which describes it as the 'twist to lock' type. There were two types - BHH526 for North America, Sweden and Germany, and BHH653 for elsewhere. BHH526 has a safety symbol in the Parts catalogue so I suspect that was a soft knob, with the other being hard.

The shutter-valve symbol is not very appropriate and neither is the word 'choke', as they really apply to fixed-jet carbs where mixture enrichment is performed by restricting the air supply, whereas SU carbs perform enrichment by adding fuel to the same air volume.

Which type of knob (hard or soft) was provided when - and even what part number they were is pretty confusing. According to the Parts Catalogue the original UK choke cable changed from BHH653 to BHH651 with HIF carbs (and the new 18V 779/780 engines) for chassis number 332033 (some) to 336650 (all). This cable has the CB round knob, but a longer inner needed for the HIF fittings. It changed again to BHH650 at chassis number 3602301 and the start of rubber bumpers, and the 77 and later catalogue lists it as BHH2064 which suppliers show as the T-handle. However Clausager says the T-handle type was provided on all cars from the start of rubber bumpers, whereas BHH650 as indicated by the catalogue is shown by suppliers as the soft 'shutter' type. Various suppliers list the two 'shutter' types interchangeably, and even the 'hard' shutter type is shown by some for the earliest UK cars - a case of "what we have is the one you want", I'm sure.

September 2019:
The V8 inner is different in that it has a nipple on the end that fits into a slot in a trunnion between the carb choke cams, and the outer is clamped to the air-box to set the correct adjustment. It may also be shorter than the 4-cylinder cable as the carbs are right at the back of the engine compartment. But after that it gets confusing. My 75 has a T-handle and an original brochure shows the same, although the brochure is for the RB models. Clausager shows a 74 with a T-handle but says it probably should have a round knob. The Parts Catalogue only shows one - BHH1121, and Googling that comes up with a round knob from most suppliers, you have to search on BHH1121a for the T-handle. For the 4-cylinder the Parts catalogue lists both - round knob changing to T-handle with the RB, so if the V8 was the same I'd have expected that to have two entries as well. The drivers handbook dated 1976 shows a CB on the front and only the CB front lights, but it's drawings initially show the T-handle ... until it comes to using the choke, then it shows a round knob, and for the remainder of the handbook, even though it shows the V8 binnacle. The only CB V8 brochure I have seen doesn't show the V8 dash, and modern pictures online are no help as they could well have been replaced.

On the 'twist to lock' types wear occurs on both the knob shaft and a 'wedge' clipped into the body that attaches to the dash. So by purchasing a new cable it is possible to fit the inner and the 'wedge' from the new cable to the old outer, leaving the run of the outer in position and just undoing the nut on the back of the dash to pull the outer forward to access the wedge, rather than removing the whole thing then having to thread the new cable (inner and outer) through the most advantageous path to get smooth operation. Of course if the old outer is damaged, or is stiff in operation because it is poorly routed, then you will have to remove and replace the whole thing anyway. May be advantageous to pull out the knob, put a spot of grease on the shaft, push it back in till the grease lines up with the wedge and twist it back and fore a few times to distribute it. Then pull the knob in and out a few times wiping the shaft each time to wipe off any excess or you will get grease on the handles of your handbag.

May 2011: John Tait in Australia found his new choke cable failed to lock after just 10 starts. Opening it up he found the locking wedge is made of plastic and had deformed!

June 2008:

September 2018:
When export models from 1971 (18v engines) and UK cars part way through the 1974 model year gained HIF carbs, they retained the 'upside down' HS choke arrangement even though the throttle cable bracket had provision for a choke cable, which was used to secure the inner.

February 2009:

HS choke components:
Quite complicated with no less than 11 components. At least they are (nearly) all visible, unlike the HIF carb.

HIF choke set-up:
Although the external components of the HIF choke are much simpler than the HS things still need to be assembled and adjusted correctly for the choke to work properly.

In both cases the clamps on the interconnecting shaft need to be positioned quite carefully - these operate the choke cams as the cable is pulled and the shafts rotate, from a finger on the clamps sitting in a hole in its cam. As well as ensuring both carbs operate in unison - rotational positioning on the shafts, they also need to be in the correct position along the shaft. Pressed too tightly against the shaft sockets on the carb they can cause the choke to bind, but positioned too close together the shaft can slide back and fore and one or other clamp may become disconnected from its choke cam.

V8 setup:
Simpler again, with a nipple on the end of the inner lifting a trunnion that engages directly with the enrichment and fast-idle cams. The only adjustment consists of a simple clamp on the cable outer so you just have to slacken that while pulling the outer up to the required position, then tightening the clamp.

HIF choke valve:
John Maguire in Oz rigged up a bench-test after having problems with a float valve towards the end of a full restoration. As part of that he took the choke valve out and describes its operation. Also shown is an exploded view of the valve components.

North American 18GK engines in 1971, despite still having HS carbs according to Clausager, then seem to have the distributor connected to a port on the inlet manifold. They had AUD 465 HS carbs, but whether these carbs still had the port but it was capped, or whether there was no port, isn't known.

With the first 18V engines for the 1972 model year all export models gained HIF carbs. North-American models had different carbs to other export markets and did not have the vacuum port, other export markets may have had HIFs with the port, but it's not known whether it was used for vacuum advance.

UK didn't get HIFs until November 1973, and these have the vacuum port on the rear carb as before, but now on the bottom as the butterfly opens the other way. UK carbs were changed to FZX1001 in September 74 with rubber bumpers and this is when the vacuum port was deleted and the source became the inlet manifold. Clausager shows a 1975 with a vacuum connection to the top of the front carb, which would be wrong for an HIF on an MGB, the same picture shows a manifold port capped off so the carb vacuum is almost certainly a user modification. Some people have spoken of drilling the body to add a port to change from manifold vacuum to carb vacuum, even though it makes no difference under normal driving conditions, and if this is what has been done here it has been done incorrectly as the port should be underneath.

From the start of the 1977 model year all export models were to North American spec, including cars for Japan which meant they got LHD cars despite being an RHD country.

V8s always had HIFs with a vacuum port below the throat of the near-side carb.

Whether the port is above or below the throat depends on which way the butterfly pivots as it opens. If the top of the butterfly moves towards the carb, then the port will be at the top, to be presented to manifold vacuum as soon as possible, and this is how it is on HS carbs. If the bottom moves towards the carb then the port will be on the bottom, and this is how it is on HIFs. How the accelerator cable acts on the throttle spindle indicates which way the butterfly moves. On HSs the cable lifts a lever on the manifold side of the spindle, showing that the top of the butterflies will move away from the inlet manifold, and the vacuum port is at the top. But on HIFs the cable lifts the lever on the air-cleaner side of the spindle, showing the bottom of the butterflies moves away from the inlet manifold, so a vacuum port would need to be on the bottom. The V8 pulls a lever that is above the spindle away from the carbs, so again the bottom of the butterflies moves away from the inlet manifold, and hence the vacuum port is on the bottom. The parts catalogue indicates the change from the cable pulling up the inlet manifold side, to pulling up the carb side, occurred at chassis number 332033 in the UK and 25800 elsewhere, which coincides with the change from HS to HIF in each case, indicating that all HIFs - if they had a vacuum port - would have had it underneath. Clausager also shows a 'post-76' car with the vacuum pipe going to the inlet manifold on p67, and on p80 he says the final carbs on UK cars 'possibly for the 77 model year' have much shorter necks to the bellhousings of the suction chambers, but the carbs on p66 and p67 seem to be identical.

Certainly the V8 suffers from fuel trickling down from the port into the vacuum capsule and rotting the vacuum advance diaphragm. 4-cylinder cars may not as the pipe always has to rise to go over the rocker cover. After two V8 failures of the very expensive capsule I fabricated a small separation chamber which I mounted above the carb port, so fuel trickles back to the carb.

Note that if carbs have been removed and refitted check there is some free-play in the throttle and choke spindles that run between the two carbs. The flange nuts may need to be slackened and the carbs repositioned slightly to achieve this. Also the spindle clamps may need to be slackened and the amount of engagement with the cams reduced to give a little fore and aft play.

The engine must be fully warmed up to begin with, and the engine at run 2500rpm for one minute if not driven at normal road speeds immediately before. For the duration of the setting-up process each three minutes the same must be done, to 'clear its throat'.

The basic requirement for good twin SU set-up is that the carbs should be matched - and that means matching springs, needles, jets, air flow and mixture. Springs, needles and jets should always be replaced in pairs. If you have modified the 'breathing' in any way - air cleaners, carbs, combustion chambers, exhaust - you may benefit from a different needle to standard. For example K&N filters may need a richer needle, see SU Needles.  While the earlier metal floats can be adjusted to give the same fuel height in the float bowl, it looks like the later plastic ones cannot easily be, except by placing washers between the needle valve and the housing if the fuel level is too high. The float height on HSs is supposed to be such that, with the float chamber lid held upside down, the float should just rest on a 1/8" to 3/16" round bar placed across the middle of the lid parallel to the hinge pin.

Fixed needles on HS carbs must be installed with the shoulder near the top flush with the bottom face of the piston.

Swinging needles (HS or HIF) must be installed with the bottom of the needle carrier, flush with the recess in the piston.

With the earlier fixed needles the jet must be centred so that the piston drops smartly and freely onto the bridge with a metallic click. This test should be done with the jet screwed up to be flush with the bridge, if it drops cleanly here then it will definitely do it when the jet is lowered to its normal running position. If it binds with the jet raised the jet needs to be recentered by slackening the jet bearing nut, pressing the piston down then retightening the jet bearing nut. Recheck, and repeat until the piston drops cleanly. Note that whilst it is easier to do this check with the damper unscrewed, with the damper fitted and oil in the reservoir the piston should still drop smartly, the damper should only retard the upward movement.

Tip: A float valve can sometimes stick in the closed position, particularly if the car is not used for some time. Running the engine will empty that float chamber which will cause poor idle and running. Rapping the top of the (HS) float chamber with the handle of a screwdriver can often free the valve.

Tip: The opposite effect is dirt in the float valve that stops it closing when the float chamber is full and it overflows. Disconnect the fuel pump and run the engine until the float chambers empty and the engine stops. Reconnect the fuel pump and the resulting rush of fuel into the float chambers will usually clear the dirt away. If it happens again immediately either the float could be punctured and full of fuel so it doesn't float, or the valve could be worn. If it happens frequently change the in-line filter (if fitted) or investigate the causes of dirty fuel e.g. internally corroded fuel tank. Check the float height after replacing the float valve, or float.

Tip: Many HIFs, and possibly some HSs, have a 'poppet valve' in the butterfly which opens under conditions of high manifold vacuum i.e. the overrun. This was an emissions measure which simulates opening the throttle slightly until the manifold vacuum drops closer to its normal value at idle. This valve can stick open and cause a high idle, sometimes only during certain circumstances e.g. warming up and be OK the rest of the time. One of my V8 carbs was doing this so I soldered them shut, which needs minimal dismantling to perform. Some recommend replacing the butterfly with the solid item, which has the same effect plus removes a small obstruction from the throat of the carb, but needs much more dismantling and it can be fiddly to get the new butterfly to seat properly in the throat, which leads me onto my next tip.

Tip: A high idle that cannot be brought down to normal by use of the idle screws is not being caused by a vacuum leak. A vacuum leak only lets in air, whereas the engine needs fuel to run. Therefore, if the engine is still running when the idle screws are backed right off, there is some other problem causing one or both butterflies to be partially open. This could be one or more of the following:

• Maladjusted fast idle screws holding the butterfly open, see below for correct adjustment.
• No slack in the throttle cable i.e. the throttle pedal stop is causing the cable to hold the butterflies open. There should be 12 thou (for manuals, 20 thou for automatics) free play between the finger on the throttle interconnecting spindle and the choke spindle. My WSM incorrectly states 0.12" i.e. 120 thou, but it also states 0.3mm which is correct.
• Maladjusted throttle interconnecting clamps and spindle - one carb fully closed is holding the other one slightly open. Go through the full set-up sequence below.
• Butterfly poppet not seating - solder it closed or fit a plain butterfly.
• Butterfly not seating properly - check the carb throat seat is clean and reseat the butterfly.
• Worn throttle spindle and/or bushes - this can cause a vacuum leak as well as allowing mixture to bypass the butterfly.

The main adjustments: Updated March 2023

  Note: There are two different types of jet bearing for the HS4 with different threads which affects how far the jet moves with each turn of the mixture adjusting nut - coarse and fine. However everything encountered on an MGB should be the coarse thread, the fine thread only applies to other marques such as the Mini where it has Waxstat temperature-controlled mixture.

• Remove the air filters, slacken the throttle and choke bar clamps, two on each bar, and back off the fast idle screws that bear on the choke cams.
• Screw each jet up (nut under the carb on HS, screw on the side of the body for HIF) until it is flush with the bridge (NOT as high as it will go), then screw it down two full turns to give the basic start-point for the mixture.
• Start and run the engine up to temperature, adjust the idle screws to give a reasonable idle speed.
• Using a tube to listen to the hiss in each intake, or by using a balance meter such as Gunson's, independently adjust each idle screw so you get the same hiss or meter indication in both carbs while still retaining a reasonable idle speed.
• On each carb in turn adjust the jet height to give the correct mixture for your spec:
  On UK cars first registered after 1st August the mixture is set by adjusting the each jet in turn to get the highest idle speed, rechecking back and fore as the balance tube makes them interdependant. You should find that there is a small range of adjustment where nothing happens, beyond that the idle speed starts to fall as the mixture goes rich or weak. From the highest idle point weaken first one then the other until the idle speed just starts to fall, no further. The MOT requires these cars to have a CO reading below 4.5%, but adjusting to achieve that should only be done after the carbs have been fully balanced for air flow and mixture, and then only by adjusting both carbs by the same amount in the same direction.
  Otherwise mixture is set by adjusting for highest idle as above, then fine-tuned by using each piston lifting pin in turn. When a piston is lifted 1/32" it should cause the engine speed to momentarily increase then settle back down - note that the pin itself can be pushed up quite a way before it starts lifting the piston. This isn't easy both from the point of lifting the piston the correct amount as well as hearing changes in the engine note, you may need to 'train your ear' as here, or if still struggling you could try the emissions-controlled HSs method as here. If the speed stays up the mixture is too rich, if the engine speed immediately falls the mixture is too weak. Each carb should be adjusted independently so that it gives the correct, and more importantly the same, results. If you still struggle to detect the momentary rise with the lifting pins then use the method for UK emissions tested cars above i.e. go for the highest idle, about mid-way between where it just starts to drop when weakened and just starts to drop when enriched. After adjusting each carb the other should be rechecked as the balance tube between them makes them interdependent. Adjusting HS jet nuts may well need a special spanner even with the air-cleaners off, even more so with them in-situ. If you have had to adjust one carb noticeably more than the other from the starting point, that indicates there is a problem on one of the carbs, probably the one you had to adjust more. See here for more info on jet height.
• Recheck the air balance again, adjusting idle screws independently as before if required.
• The throttle spindle clamps should be set such that there is a small amount of free play in the throttle cable and interconnecting spindle before the butterflies start to open, and that both butterfly cams start moving at the same time. There is a lever on the throttle spindle that rests on the underside (HS, above on HIF) of the choke spindle when the throttle is closed. Insert a 12 thou (for manuals, 20 for automatics) feeler gauge between the lever and the choke spindle, lightly press down on the part of the clamp that engages with the throttle cam and tighten each nut. Note that my WSM incorrectly states 0.12" i.e. 120 thou, but it also states 0.3mm which is correct. Check afterwards to ensure that the slight play described above exists, that both butterflies start opening at the same instant, and also that there is about 1/32" end-float on the interconnecting spindle. The 12 thou isn't critical but if too small and you need to reduce the idle speed you may not be able to as that reduces the clearance. Likewise if the clearance is too large the throttle pedal will have to be moved more before it starts opening the throttle, and increasing the idle speed increases the clearance.
• Run the engine at 1500 rpm and check that the air balance is still correct. If it is not the throttle spindle clamp(s) will have to be readjusted. Persevere with this, it is important to get both air balance and clearances right - and more important to get them balanced off-idle than at idle for obvious reasons. If you find this difficult to set your throttle spindles/bushes may be worn i.e. can be waggled up and down or from side to side.
• From now on only ever adjust the idle screws by the same amount and in the same direction to obtain the required idle speed. If you start altering them independently you will upset the balance and have to go back several steps.
• The throttle pedal should reach the stop on the floor (or the floor itself for later pedals) just as the butterflies reach fully open but without stretching the cable - the butterflies and cable should not act as the throttle stop. Adjust this with the cable clamp on the throttle spindle (HS) or adjuster on the outer at the carb end (HIF). Take up any free play in the throttle pedal with the screw and lock-nut bolt located near the pedal hinge, make sure there is still the required clearance between the finger on the throttle interconnecting shaft and the choke interconnecting shaft.

Choke:

• The choke operates in two phases - opening the throttle slightly first (fast-idle), then enriching the mixture. Make sure the choke cable is routed such that it has a clear run when the choke knob is out, otherwise stiff operation can result.
• Adjust the choke interconnecting spindle clamp screws at the carbs such that enrichment commences at the same time on both carbs as the choke is operated i.e. after the fast idle movement has taken place. For HSs the movement of the jets can be observed, for HIFs there are index marks on the enrichment cams and enrichment commences when they are under the middle of the fast-idle screws.
• See here for how the chrome bumper 4-cylinder choke cable attaches at the carb end, it is unusual in that it has a fixed inner and moving outer. There can be quite a lot of movement of the choke spindle lever and fast idle cams before the jets start to move, so set the choke knob at about 1/2" to 5/8" out, then with the cable inner clamp screw slackened pull the inner through the clamp to move the choke lever (for the HIF top-down choke cable lift the choke lever up while keeping the cable under tension) until the jets are just about to move, and tighten the clamp screw. NB: John Twist covers this briefly on the MGA but just says to 'pre-load the cable somewhat'. Unless you pre-set the control first you will be guessing how much cable needs to be pulled through the lever clamp to give half an inch of fast-idle before enrichment starts.
• Adjust the choke cable lever on the interconnecting shaft so that when the choke knob is about 2/3rds out the lever makes an angle of about 90 degrees with the cable, again to avoid stiff operation. You will probably have to go through a couple of iterations of adjustment of the choke lever and the trunnion to get both the angle of the lever and the fast-idle distance of the choke knob satisfactory. Pictures
• Independently adjust the fast idle screws such that as the choke is operated both throttle butterflies start to open at the same time, and gives the correct fast idle speed (e.g. 1000rpm when the engine is hot) when the choke knob is in its maximum 'fast idle' position i.e. just before enrichment starts, and check the air-flow is balanced.

Damper:

• Check the oil-level in the piston damper. The most sensible way to do this is to unscrew the damper cap lift it up, and press it down again. If you feel the resistance of the oil before the damper cap reaches the dashpot cover you have enough oil. If you try and maintain the oil level at the recommended top-up position of 1/2" above the top of the hollow rod you will be forever topping up which will wear out the damper cap threads and the damper cap will shoot up out of the dashpot cover under hard acceleration. The correct top-up oil is engine oil of whatever grade is correct for your local climatic conditions, e.g. 20W/50 for temperate climates. When you do have to top it up there is no point in filling it to above the top of the hollow rod as any oil above this point is rapidly drawn into the engine, hence the frequent topping-up if you try and maintain it at this level. However see Damper Oil Level

Herb Adler describes using a Colortune. I never got on with them on single-carb engines, so for years never contemplated it for the MGB where you either have to buy two or keep swapping them between pairs of cylinders. Herb describes how it is easy to tell between rich and not rich, less so between just right and slightly either side. Eventually he bought a second one, and even more eventually I first bought one then a second on eBay. I still don't see the colour change between slightly weak and slightly rich any clearer, I can hear the revs change just as easily for coarse adjustment. The only thing I did find with two is that with one carb a bit weaker than the other there is a visible difference in the increased richness when blipping the throttle, but lifting the piston reveals that anyway.

Important: Once the carbs are correctly set up only ever make further adjustments to both carbs by the same amount and in the same direction. Once you start adjusting the carbs independently you will have to go through the above set up to get them balanced again.

  July 2014

November 2018: