An exclamation mark as more and more people are jumping to this as a cause of running and hot-starting problems, without any diagnosis. Americans talk of 'vapor lock' and vaporisation and I think this is where people have picked it up from.

The first thing to remember is these cars have run in desert states - America, Australia and the Middle East for decades without these problems.

The second thing to remember when people start blaming ethanol is that America has had oxygenated fuels for far longer than we have, and in greater concentrations, and again don't report problems.

The third thing to remember is how our fuel systems work. We have a pump at the back of the car applying 3psi or so to the fuel. At the front we have the carbs, with float chamber and float valve, and the top of the float chamber is open to atmosphere via the vent port/over flow. No one has yet explained to me how, with 3psi of pressure from the pump, and the carbs open to atmosphere at the front, the pump can't push any air in the pipes out of the vent, and keep pushing until the float chambers fill up with fuel and the float valve closes.

Yes, modern fuels do have higher volatility, but as the vast majority of us don't get the problem it can't be the reason a few do.

Yes fuel can boil or vaporise, in hot weather I can hear it boiling at switch-off in the V8 carbs. But if that happens in the pipes why isn't the fuel pump pushing it out? The only way the carbs wouldn't fill is if the fuel was vaporising at the rate of a pint every 30 seconds, which is impossible in the relatively short length of pipe that is exposed to heat, and even if it were the pump would be chattering away like billy-oh.

Yes Americans do get vaporisation, but in cars with engine driven pumps which are pulling fuel from the back. With the pump pulling the pressure is lowered, so it is more likely to boil (see comment here when a pump was mounted in the engine compartment of an MGB). With our pusher pumps the pressure increase reduces the chance of boiling. Also an engine-driven pump is only running at engine speed, so with vaporisation at idle they can get fuel starvation from the pump not being able to keep the float chambers full of fuel. Our electric pumps should be capable of delivering a pint in 30 seconds whatever the engine speed. There are any number of American YouTubes around talking about vaporisation/vapour lock and percolation (?). And the recommended solutions? Fit an electric pump at the back!

There is also the question of just how hot things have to get to cause a problem. With the best will in the world the UK doesn't get heat waves very often. But when they do, on organised runs that can get 100 or more MGs, one would expect to find several cars stuck by the roadside or failing to start if it were a generic problem, and I've never come across anyone with that problem. Very occasionally something else (I always stop and check unless they have other cars with them), but never that. Incidentally problems hot-starting are unlikely to be vaporisation anyway, more like fuel expansion with heat-soak and overflowing into the inlet manifold i.e. flooding.

Another possibility is when at switch-off heat-soak causes the fuel in the line between pump and carb to heat up and expand, which can increase the pressure to higher than normal pump pressure, and if the float chambers are full with the float valves closed that could force them open raising the fuel level there and in the jet. But there is more chance of the fuel in the float chamber expanding and raising the level in the jet, possibly to the point where it overflows into the inlet manifold causing a grossly rich mixture at restart. And it was only offered as a theory anyway, from tests sponsored by the FBHVC carried out by Manchester University on an XPAG engine. Some notable observations:

• "At a temperature of 75°C 20% to 30% of the classic petrol would have evaporated. While at 75°C nearly twice that volume of modern petrol has evaporated." But what does that actually mean in an engine?
• "With over 40% of modern petrol evaporating at typical under bonnet temperatures, it is surprising classic carburetted engines manage to run at all." But as we know they do, and most of them without problems in the hottest weather - in the UK at least.
• "Two thermocouples, one in each carburettor, at the bottom of the transfer pipe connecting the float chambers to the carburettor body (shown in photo 2 and photo 3). Typically, this was 42°C, which was surprisingly low considering that this part of the carburettor is positioned under 1" away from the 400°C exhaust manifold."
• "When the engine was running, the highest petrol temperature of 42°C was in the transfer tubes. At this temperature, less than 10% of modern petrol will evaporate, insufficient to cause any problems."
• It puts running and hot-starting problems down to two factors - "As the petrol in the carburettors gets hotter, more of it boils. The pressure of this vapour forces petrol out of the carburettor jet, which collects in the inlet manifold making the mixture temporarily richer. The vapour bubbles in the jet then cause the carburettor to deliver a much weaker mixture when the engine is running or cranking." I.e. one enriches the mixture and the other weakens it, therefore compensating for each other at least?
• It suggests: "However, if the problem is not too bad, it is possible to nurse the engine back into life using the choke to enrichen (sic) the mixture." which is all a bit vague as you wouldn't know whether the problem was excessively rich and choke would make it worse, or excessively weak. But this doesn't seem to have been quantified, only theorised from the temperature readings.
• It has been suggested that adding a return pipe to MGB fuel systems will prevent problems: "Modern cars do not suffer from these problems for two reasons. Firstly, the petrol in the pipes and injectors is held under high pressure, which increases the boiling point. Secondly, as soon as you switch the ignition on, the hot petrol in the engine bay is recirculated back to the fuel tank, allowing the engine to start on a new charge of cold petrol." But the return pipe for a carb-equipped MGB would have to come directly off the feed pipe, not through the carburettors, so it cannot have any effect on what is inside the float chambers, jets or inlet manifold. Injectors are completely different as there is no reservoir and the circulating fuel is within a few mm of the nozzle injecting it into the throttle body or intake.
• Insulating fuel pipes has also been mentioned, about which they say: "Unfortunately, insulation does not stop the transfer of heat, it only slows it down. Once the engine has stopped and the petrol is no longer flowing, the petrol will heat up, no matter how well insulated the parts of the fuel system are. Benefits will only arise if the heating is delayed for a sufficient time to allow the under-bonnet temperature to fall below 45°C."
• As far as preventing vaporisation problems goes their tests on various fuels show that higher octane vaporises less quickly than standard 95 octane. I've always used higher octanes in the roadster, but only ever supermarket 95 in the V8, and only once in the V8 have I experienced anything that could possibly have been put down to heat effects on fuel, and that was flooding not vaporisation.
• An Australian writes: "Temperatures measured are no where near what we experience during an Australian summer. This makes me question how does my MG TF run when the ambient air temperature is 45 degrees Celsius. No modification to the original design specifications have been incorporated, I do use 98 octane rated fuel." To which the author replies: "Australian summers - The distillation curves shown in the article are for UK winter fuel (intended to work at around 0 degrees Celsius). I would be very surprised if fuel in Australia were not supplied in different volatility grades chosen to match ambient temperatures, just as we get winter/transition and summer fuel in the UK. Hence the reason your TF continues to run in your summer." To which I can only say "Can it really be as simple as that? And why don't we have summer grades that allow UK cars to run in hot weather?"

Over the years I've done some simple tests in hot weather:

• In the roadster at 26C ambient the engine compartment in stop-start traffic got up to 50C.
• On another occasion of 30+C ambients the V8 engine compartment on two days got up to 64C.
• Testing the V8 cooling system shut in the garage on a 30C day (exhaust piped outside), at a fast idle (from wedged throttle), I measured the air going into the grille at 41C.

The bottom line? I would love to get my hands on a car where the owner is claiming vaporisation as the cause of his problems, but until then - with the possible exception of vaporisation in the exposed jet-pipe of HS carbs, I simply don't believe that it's even possible in our cars. Even with the jet pipe it is at atmospheric pressure both ends, so apart from when actually boiling the levels in the float chamber and jet should still be the same and hence at a normal level. If it is the jet pipe that is causing the problem, then holding a wet cloth round both of them should pretty-well instantly get the car going again. I did wonder about my new heat shield as it only has a thin layer of shiny cloth as the insulator instead of the asbestos slabs, but just as there were no problems with the old one despite having a large chunk of one of the slabs missing for 30 years, there have been no problems with the new one.

Not got my hands on one but a pal in America has been having problems including hot-starting for some time which could be attributed to vaporisation, however he has a second MGB with the same carbs and that has no problems at all which bears out my contention that it's not generic to the MGB but to particular cars. He's having a number of other problems with stalling, carbs oveflowing and pumps clicking way more than they should which we have been looking into for a couple of years.