Stops When Hot

[Billington]

My W32-6 1932 BSA 500 cc side valve starts and runs fine now I’ve adjusted the Throttle Stop and Pilot Air Screw so that it will tick over slowly when the throttle is closed. However when on a long run the other day I came to a long downhill section, I closed the throttle because I did not need to increase the speed, when I opened the throttle again at the bottom of the hill the engine simple didn’t pick up. The motor died and I pulled over to the side of the road. I had been riding for about 20 minutes in 20 C temperature.

By looking at the Primary Air Holes I could see that no fuel was present. I allowed the bike to cool down for a few minutes and pressed the Tickler on the Carburettor, fuel could be seen bubbling an evaporating through the Primary Air Holes. I waited another few minutes then pressed the Tickler on the Carburettor, fuel could now be seen following out of the Primary Air Holes as normal; hence I started the bike and road the 6 miles home without a problem; however I made sure that I didn’t allow the throttle to completely close to avoid a re-occurrence of this problem.
I think what may be happening is that the Carburettor is getting so hot that the fuel is evaporating before it enters the cylinder. Does this make sense and if so how can I prevent this from happening.

[Rex]

Vapour lock in the fuel line? Tufnol spacer between carb and head?

[Billington]

Thank you Rex this was helpful.

[R]

Something else to look out for, although maybe not related here, is that the fuel cap
has a tiny little hole in it to prevent a vacuum forming in the tank as the fuel is used up.
If this hole is blocked, the fuel supply can starve after a while.

Although it sounds like your problem is heat, like you say.
We see you investigating the spark timing on another thread here,
good idea, running retarded timing can make things extra hot....

[john.k]

Heat does sound right....on  along downhill with throttle closed,no air or fuel to cool carb..................a lot of early SV s had the carby on a long intake pipe......maybe a good reason why,although it seems the pipe is needed to clear a magdyno.

[iansoady]

Many years ago I had a Norton 16H that always refused to start when hot & left for half an hour or so. That was fuel vaporising in the carb. The Norton had a stub type carb fitting so no opportunity to fit a tufnol insulator. The only way to start it was to flood it liberally. I was actually told this by an ex-Don R who happened on me kicking vainly one day - which just shows how long ago it actually was!

[Billington]

Thank you everyone for your kind replies, I’ve a lot to consider. I will try running the bike half way between advanced a retarded to see if it will run both smooth and cooler. If it is still hot I will go down the tufnol spacer route.

This is my first vintage motorcycle; there has been a lot to fix and a lot to learn, but I’m really enjoying riding it.