BSA Owners' Club Forum

The BSA Workshop => Pre War => Topic started by: BILLINGTON on 21 May, 2022, 23:12:29

Title: vapour lock
Post by: BILLINGTON on 21 May, 2022, 23:12:29
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 įC. HenceI fitted a Tufnol flange spacer, which I did. After fitting the 5mm spacer the bike ran okay in temperatures up to 28 įC. However the temperature today was only 16 įC.
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?
Title: Re: vapour lock
Post by: Spaceman on 22 May, 2022, 13:12:13
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?
Title: Re: vapour lock
Post by: BILLINGTON on 22 May, 2022, 17:32:49
Hi Spaceman, I use regular petrol in my bike. If Iím honest I did not look at the E value when I purchased the petrol but I will have a look next time Iím filling up. I did however put about 25ml of Redex Lead replacement fuel additive. Iím not shore if this could have an effect.
I donít 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.
Title: Re: vapour lock
Post by: idie on 22 May, 2022, 17:55:21
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.
Title: Re: vapour lock
Post by: DAVE BRADY on 22 May, 2022, 18:04:56

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?

Title: Re: vapour lock
Post by: Spaceman on 22 May, 2022, 18:20:12
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.
Title: Re: vapour lock
Post by: idie on 22 May, 2022, 18:48:48
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.
Title: Re: vapour lock
Post by: BILLINGTON on 22 May, 2022, 18:57:21
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.
Title: Re: vapour lock
Post by: BILLINGTON on 22 May, 2022, 19:04:11
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?
Title: Re: vapour lock
Post by: idie on 22 May, 2022, 20:48:33
If you are running the bike retarded it will definitely overheat.
Title: Re: vapour lock
Post by: ducati2242 on 22 May, 2022, 22:05:09
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 .
Title: Re: vapour lock
Post by: Mike Farmer on 23 May, 2022, 11:40:24
 :) :) :) :)

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 8) 8) 8) 8)
Title: Re: vapour lock
Post by: BILLINGTON on 25 May, 2022, 21:19:14
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.
Title: Re: vapour lock
Post by: Mike Farmer on 26 May, 2022, 20:25:12
 :) :) :) :)

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

Mike 8) 8) 8)
Title: Re: vapour lock
Post by: BILLINGTON on 26 May, 2022, 21:50:59
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.
Title: Re: vapour lock
Post by: BILLINGTON on 26 May, 2022, 22:06:54
Part of the problem is that the ignition advance/retard lever does not stop in position I have turned it to. It slowly returns to closed which equates to full retarded. The friction available on the lever does not seem able to overcome the spring in the magneto.
Title: Re: vapour lock
Post by: LightningAndStarfire on 27 May, 2022, 16:57:17
An infrared thermometer would perhaps be a sensible investment. You could easily measure the carb body temp and much more.
Title: Re: vapour lock
Post by: idie on 27 May, 2022, 18:04:28
That is why the magneto was changed to slack wire advance.
Title: Re: vapour lock
Post by: BILLINGTON on 29 May, 2022, 08:42:35
Iím a little embarrassed to tell you this, however the reason why the ignition lever would always return to retarded was because the nut had worked lose, after tightening the nut I can run on constant full advance. I had previously identified that this nut would work lose so had put some lock wire around it to stop it turning, however over time the lock wire around the nut was insufficient. When I first realised this I thought that I had finally found the solution, but Iím afraid Iíve only found part of the puzzle, because after riding four miles on full advance at 17 C (air temperature) the bike was so hot that smoke was coming from the cylinder head. So itís not the ignition timing.
Should I take the cylinder head off an look for heavy carbon deposits, could it be that I need to decarbonise my bike, would that make it run so hot? 
Title: Re: vapour lock
Post by: Spaceman on 29 May, 2022, 09:02:50
I would still double check the ignition timing is accurately set - if you inadvertently run it retarded, the engine will get very hot.

I would then check the mixture to see whether the engine is running lean. Ideally, you need to run the engine on the road at constant throttle and, if possible, stop the engine and coast to a stop before checking the colour of the plug. A moderate amount of black deposit would be normal but a light brown and very little deposit would signify a lean mixture.

If not ignition timing or mixture, it might be necessary to check valve timing and the condition of the valves. I'm not sure a coked up engine would lead to over heating but it would certainly produce poor running and erratic idling.
Title: Re: vapour lock
Post by: BILLINGTON on 29 May, 2022, 09:14:45
Thank you spaceman for your post, I will do what you say and post an image of the spark plug.
Title: Re: vapour lock
Post by: BILLINGTON on 29 May, 2022, 16:46:30
My thinking has moved on to the point people have been making  about weak fuel mixture and leaks at the head gasket. I had checked and tighten the cylinder head bolt as part of my spring service notes after taking the bike off the road in the autumn, but I now realised that I had set my torque wrench to 30 newton meters, but my notes state 30 foot pounds, which would equate to 40 newton meters.

When I stopped the bike yesterday smoke was coming out of the head gasket, which would indicate that is not hermetically sealed, there was also a shiny oil mark on the cooling fins that come from the cylinder head. All this would point to the cylinder head not being completely airtight. If smoke can get out air can get in during the intake stroke.

I will set the cylinder head bolts to their correct torque setting and see if this makes a difference. If it does not I will remove the cylinder head anneal the copper gasket again and refit the cylinder head.
Title: Re: vapour lock
Post by: Spaceman on 29 May, 2022, 19:46:36
I doubt if a leaking head gasket would make much difference to the mixture - it leaks because the pressure inside the cylinder during combustion is >> 500 psi whereas it is very low during induction where the path of least resistance will be through the carb. Therefore, it is still worth doing the plug colour test.
Title: Re: vapour lock
Post by: BILLINGTON on 31 May, 2022, 12:35:23
Yesterday, I tightened the cylinder head bolts to 30 foot pounds, then set the engine at TDC using a gauge I made from a wire coat hanger. After this  I set the contact breaker points gap to 0.003 with the ignition fully retarded, these are the instruction in the BSA book for my model.

Today I went for a half mile ride; I stopped early because it had started to rain hard; however I could see smoke coming from the cylinder head again. Hence the next thing I intend is remove the cylinder head anneal the copper gasket and use Wellseal to improve the seal.

I will take photos of the head and post them.

Thanks again for your advice.
Title: Re: vapour lock
Post by: BILLINGTON on 31 May, 2022, 18:48:40
The pictures with this post are as a result of me taking the cylinder head off today, because it was leaking smoke which was not solved by tightening the head bolts. Also the bike is running hot possibly due to the air fuel mixture being lean due to the air being drawn in past the cylinder head gasket.


1, The spark plug with a light brown indicating hot or lean
2, Contact breaker when the piston is at TDC. The point gap is 0.003 in this picture. This has been set in accordance with be information given by BSA for this bike.
3, Copper head gasket before cleaning. You can see the black marks at the top and bottom of the gasket where smoke was coming out.
4, The Copper head gasket after being cleaned with a Brillo pad, scotch brite and annealed.
5, Cylinder head before cleaning. You can see the shiny marks where smoke was escaping.
6, Cylinder head after cleaning.
7, Piston and valves before cleaning. Again you can see the shiny marks where smoke was escaping.
8, Piston and valves after being cleaned with a Brillo pad, scotch brite.

Title: Re: vapour lock
Post by: BEVANC on 01 June, 2022, 20:23:59
An interesting difference in spark plug position compred to my 600 sv Ariel - my plug is directly above the piston.
You are lucky the gasket did not blow out - my Ariel has blown two gaskets over the years. I lapped the head  flat on a surface plate to try to prevent a repeat. Both the heads and the barrels do distort due to the heat.I may yet need to get the barrel face machined flat. A fellow Ariel VB owner has had his head & barrel machined and lapped together and is running without a gasket.

If you haven't already put the head back on, i'd suggest checking it for flatness on a surface plate or sheet of glass.

Initially, I had to tighten up the screw on my advance retard lever a lot to prevent it shifting as i ride - but it was my fault as i had sprayed CRC on the mechanism to inhibit corrosion, making it too free to turn. Took it apart, cleaned it and reassembled and its now ok.

As with your BSA, after a ride, the carb on my Ariel is far too hot to touch. Lack of clearance over the magdyno and from the carb mouth to the oil tank means that it would be impossible to add anything more than a very thin a tufnel spacer, but it never seems to cause any issue. Side valves do run really hot.

I hope your bike runs better once you getthe head back on. I must say, your initial descriptions made it sound like fuel starvation (e.g. blocked breather hole in the petrol cap; clogged fuel tap/filter etc..)

Title: Re: vapour lock
Post by: Mike Farmer on 01 June, 2022, 21:08:23
 :) :) :)

Wow I'm seriously surprised that you needed to use the valve lifter with that problem. Just dont know.

Mike 8) 8) 8)
Title: Re: vapour lock
Post by: Spaceman on 01 June, 2022, 23:12:12
I think you definitely need to check the flatness of the mating surfaces on the barrel and the cylinder head - the head in particular may be distorted giving rise to the leaking head gasket. One way of doing this would be to gently bolt on the head without the head gasket and then use a feeler gauge to check for the size of any gaps. Worst case, you may need the head skimming.

I have to confess I have no experience with side valve motors but the thing that struck me is the lop sided combustion pattern shown on the top of the piston, possibly due to the spark plug being positioned above the valves. I assume there was a reason the plug wasn't positioned above the centre of the piston? From the photos, it looks almost as there is a crack in one of the valves or is that just a surface flaw?
Title: Re: vapour lock
Post by: BEVANC on 02 June, 2022, 02:14:11
That will be machined slots in the valve head - my Ariel has those too. Here's a picture of my barrel.

And my previous comment about spark plug position was bollocks - don't know what I was thinking ! Heres a picture of the head with some bluing on it when I was checking for flatness.
Title: Re: vapour lock
Post by: Spaceman on 02 June, 2022, 06:31:34
It wasn't the machined slots I was referring to but the strange feature pointed to in the image below. For the Ariel, it would be interesting to know whether this shows the same combustion pattern on the top of the piston as the BSA? This could be normal for a side valve engine but does not look ideal!
Title: Re: vapour lock
Post by: BILLINGTON on 03 June, 2022, 21:49:49
Hi spaceman, I can see what you mean about the inlet valve looking as if it has a crack, however it is simply the angle of the photo. The other photos Iíve taken show the inlet valve does not have a crack.

After a top end overhaul, which included torqueing the head bolts several times, when cold and after running the bike to get it worm the bike now seems to be air tight at the cylinder head. It was pulling strongly up the hills where I live.

There are other issues Iíve been working on with the advice from people such as how to ensure the ignition lever has enough friction to stop where Iíve put it. Also getting it to idle just right, but these are all problems I will persevere with overtime.

Thank you for any ideas and advice you may have given.