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Fuel evaporation - possibly

Last couple of days have been glorious - weather wise - here in Suffolk, two days of 18 degrees, however, as has happened in previous years when the temperatures are high my 1934 RP will not start after a good run, that is until I wait some ( several) minutes for things to cool down.

The pipe work, pump, and carb are standard, I blame the modern petrol. My question is, do others suffer from this problem and is there a remedy?


Location: Suffolk

Re: Fuel evaporation - possibly

Joe - it would be interesting to see if you get some scientific replies, but my experience is that some engines won't start after previously running hot, others (most Sevens) don't have an issue. Older Ford V8s were notorious for this fault, never heard of anyone curing the problem, either. Despite the hotter weather in this Country, (so maybe not evaporation?) I don't know of anyone having trouble since the newer petrol mixes were forced upon us. Good Luck, Cheers, Bill

Location: Euroa, Australia

Re: Fuel evaporation - possibly
Re: Fuel evaporation - possibly

For what it is worth....
Paul Ireland of the MG CC is running a whole series of tests on this subject using an XPAG engine at Manchester School of Mechanical Aerospace & Civil Engineering (MACE)& is very long & complex subject. However there are some interesting simple conclusions so far for the layman:
Apparently the fuel at the pumps in the summer(June to August) has less low temperature distillates (which they find are the root cause of "the hot restart problem"), so if you run your tank low at the end of the summer don't fill up too early!
Correct ignition setting is critical (interesting with manual control!)
Their advice is to use summer super grade fuel & don't get stuck with a tank full of winter fuel in the summer. Add 10% kerosene(yes!) and keep the ignition properly tuned. Note adding kerosene is illegal for vehicles post 1956 (not our problem!) BUT you need a licence from HMRC! AVGAS is good too!
Reducing underbonnet temperature is vital- ie get the hot air out of the top & make sure cool air can get in under the engine, exhaust pipe insulation will help, but only delay the transfer of heat when the fuel stops flowing. On the XPAG engine they found that the reason the carburettors got hot when the engine was stopped was not because they were near the hot inlet manifold, but because hot gases had escaped along the manifold and into the carburettors! A blip of the engine then switching off the ignition while the engine was still revving apparently caused the hot gases to be vented & colder gas to enter the cylinder as the engine runs down! This apparently helped a great deal, but I think would be difficult to do in a traffic jam (apart from being nail biting to keep switching on & off!)
They are also looking at corrosion etc caused by ethanol, but point out that "Cleveland Discol" (1928-1968) advertised its ethanol content (unspecified%) advantages in keeping engines cleaner & cooler!

Location: Stratford upon Avon

Re: Fuel evaporation - possibly

I understand the use of AVGAS is illegal on the public highway, but how the authorities are ever going to detect it, I don't know. It doesn't have the yellow tint of petrol. Importantly, it contains no ethanol, presumably because of the suspected damage it does to fuel components in older systems and it wouldn't do to have aeroplanes falling out of the sky!
There's two grades of AVGAS available. 100 octane low lead (contains more lead than there ever was in petrol), 91 octane unleaded. Old cars seem to run quite happily on the lower octane stuff (don't ask how I know) as when Austin 7s were made the octane rating of petrol would have been low and probably never had any lead in it.
AVGAS costs around 50P/litre more than petrol.

Re: Fuel evaporation - possibly

Back in the 80's when working for Volvo , we had occasional problems with some customers cars which would refuse to restart after a short stop.This was ,it was determined, caused by fuel vapourisation ,causing an over rich mixture. It was cured by introducing an insulating spacer twix the carb. and manifold. On my RP, I have a 2mm. aluminium plate shielding the heat from the manifold. It also ducts any fuel leakage away from the exhaust.

Location: Piddle Valley

Re: Fuel evaporation - possibly

Hi Jo
My RP is standard, never had such an interlude.
Its not the petrol.q
Are you sure it is a fuel related issue ?
I think from what you have said I might be tempted to think it could be an ignition problem.
How about trying a new condenser ?
If that makes no difference try swapping the coil.


Location: Centre of the Universe

Re: Fuel evaporation - possibly

You might be right Nick, could be any of the things you mentioned, trouble is the problem is never there in the cold months so I tend to forget about the problem, in fact probably suspect it's gone away.

Still, I will try - progressively - all the various suggestions during the coming months until the problem is sorted, not that its worrying me a great deal, when the car fails to start I just lift the bonnet, sit and watch the world go by for 10 minutes, prime the carb, close the bonnet, start up and ' chunter' away. Lovely !

BTW, I have checked the fuel pipes from the tank to the pump - all clear. Next will be the pump to see that it's delivering what it should, I know the carburettor is clean and correct, then it's start on the electrics, looking forward to it all, much better than gardening.

Regards all.


Location: Suffolk

Re: Fuel evaporation - possibly

Both the Chummy and the Saloon play up in hot weather (around 30) - often stalling when left idling at traffic lights, but restarting after a short time then running roughly until settling down.

Chummy is fitted with a four blade fan, Saloon the original two blade.

I have been blaming the fuel and vaporisation but I have just been told by a club member that in his experience some new 6 volt coils are prone to lose spark when hot- a cooling spray with water often causing a re-start- has anyone had this happen ?

I realise that my version of hot weather may be a bit more uncomfortable than some

Cheers, Tony.

Location: Melbourne. Victoria, Australia.

Re: Fuel evaporation - possibly

In the hot weather my Ruby will sometimes not want to restart. The engine seems to have poor compression as there is very little resistance when tuned over on the handle, but it always starts well from cold and runs fine. I know the timing is right - I can feel a slight kick back on the handle when turned over slowly with the ignition switched on. So I have always thought the hot starting problem is due to vapour lock as a result of the more volatile modern fuel - though I do use the premium BP petrol. As it only happens in the warmest weather, I have done nothing more about it other than contemplate fitting a four blade fan.

Location: North Cheshire