vapour lock

[BILLINGTON]

Today I took my 1932 BSA 500cc sv, out for the first time this year. After checking it over and adding fresh fuel it started on the second kick. I then when for a drive, after just under 2 mile the engine stuttered and can to a halt. The carburettor was very hot, hence the petrol had become so hot it vapour locked the carburettor. I let it cool down for 10 minutes and it started up again so I rode home.
I had is problem in 2019 when the air temperature was over 20 °C. HenceI fitted a Tufnol flange spacer, which I did. After fitting the 5mm spacer the bike ran okay in temperatures up to 28 °C. However the temperature today was only 16 °C.
I know what the problem is, its vapour lock. My question is why is it now happening at a lower temperature and is the only solution a thicker carburettor flange spacer?

[spaceman]

Out of curiosity, where in the carb do you think the vapour lock is happening or is it in the fuel line? I think one of the problems with modern fuel is that it boils at a lower temperature and so this type of problem could occur more often than it used to. I would have thought E10 fuel is worst but I assume you are not using this?

[BILLINGTON]

Hi Spaceman, I use regular petrol in my bike. If I’m honest I did not look at the E value when I purchased the petrol but I will have a look next time I’m filling up. I did however put about 25ml of Redex Lead replacement fuel additive. I’m not shore if this could have an effect.
I don’t know where the fuel is turning from liquid state to gas in the fuel delivery system. How would I find out if it happens in the cooper pipe, carburettor bowle or carburettor body, and how would this help me to resolve the issue.
Thank you for any helpful contributions.

[idie]

I can make a guess on your problem. If your fuel is old in that you have bought it during the cold spell it will be a different formula to that made in hot weather. Fuel made in cold weather will have a higher evaporation rate to aid better combustion. This maybe is why you had the vapour lock.

[DAVE BRADY]

Hi,

I know that the formula for diesel changes for winter as it can freeze but never thought that petrol was different or is it a new thing because of the ethanol?

Dave.

[spaceman]

Regular petrol is now high (10%) ethanol E10 petrol with the more expensive premium petrol usually rated at E5 although it is normally closer to E0; that is, a low ethanol content and probably the highest boiling point.

As far as I understand the problem, vapour locks normally occurs in pressurised fuel systems (ie in cars with fuel pumps) due to fuel turning to vapour in fuel lines due to a high temperature affecting the pressure of the delivered fuel.

In this case, it's difficult to see how the formation of vapour in the fuel lines could affect fuel delivery to the carb even if they were getting hot. There's no pump and the system simply relies on gravity.

However,  it's possible that the elevated temperature of the carb is causing problems with the atomisation of the fuel that gets sucked into the inlet manifold. It could well weaken the fuel mixture and cause running issues.

I'm pretty sure the carbs don't get particularly hot on the bikes I ride and I'm surprised the Tufnol spacer is not doing a better job. I'd have to check how good an insulator Tufnol is but there may be better choices available.

I would definitely check the petrol tank breather is working properly and also check that the carb float chamber breather hole is operating. If you are using regular petrol, I would recommend changing to slightly more expensive premium or E5 which may ease any problems.

[idie]

I was an AA patrolman for 26 years and often found this problem back in the 70s and 80s when most cars still had carburetors especially when the car had been laid up and was running on winter fuel during a hot spell. Try putting in some fresh fuel and see if this solves the problem.

[BILLINGTON]

Thank you for the advice idle, you make an important point. I drained all the petrol from the fuel tank and carburettor last autumn then filled it with fresh petrol yesterday when I had the issue.

[BILLINGTON]

When this problem happens the  carburettor is very hot, you could not hold it without burning your hand. Also is you press the tickler no fuel come out of the wholes that you can see on the lower part of the carburettor until the carburettor has had enough time to cool down.

Could the issue be due to me running the bike nearly fully retarded rather than fully advanced?

[idie]

If you are running the bike retarded it will definitely overheat.

[ducati2242]

Not going to say this would work or not but would a thick fiber washer under the nuts holding the carb on help to reduce the heat transmitted to the carb .

[Mike Farmer]

 

May I suggest that if your carb is untouchably hot then your engine is also getting too hot. So maybe you have a general overheating problem. It would suggest to me a weak mixture, that combined with running it retarded will easily produce your noticed affect.

If it wont run properly in a more advanced state your basic problem could include a timing issue.

It could also be as simple as a sticky float especially as you say petrol doesnt come out when you tickle it. This because the fuel flow thro the float bowl etc assists the cooling.

Mike

[BILLINGTON]

I went for a ride today to try the ideas suggested in this link (air temperature 10 C). My first ride I tried the air lever full open and the ignition half advance, however the results were poor, the bike did not get so hot that it vapour locked but is ran poorly an cut out on the one mile run.
In the afternoon I changed the oil and played with the throttle stop; I could set it a tick over but if I stopped the bike and started it again it would then stall on closed throttle which seems inconsistent. However I went for a 3 mile ride (air temperature now 17 C), with the air lever closed (because I had forgot to open it!) and trying different ignition positions as the road dictated (I live in a very hilly area of England). The bike did not stall because I was managing this with the throttle. At the end I could touch the carburettor but it was still warm.
Thank you for all your advice and questions.

[Mike Farmer]

 

Any chance your exhaust valve has a little chip in the lip. Doesnt take much to cause overheat and consequential problems.

Mike

[BILLINGTON]

Thanks for the thought Mike, however if the exhaust value was chipped I think it would go over compression without using the exhaust valve lifter, which it will not.