Hot Start Problem

[Billington]

My 1932 BSA W32-6 500cc side valve will not start when the engine is hot. It will start when cold with just one kick and run beautifully, however if I stop the engine and attempt to restart it when it is hot it will not start.

I had this problem in 2017, which I solve by having the magneto rebuilt. I’ve done approximately 800 miles since the magneto was rebuilt by Tony Cooper (I believe he is now retired).

Could the magneto have failed again with such little use or should I look for other issues. I would be grateful to hear your kind advice and details about how to diagnose this and correct issue.

[R]

Sounds like a failing condensor again, doesn't it. !
Do you know what the rebuild used - not a NOS one, we hope. ?

Another possibility is vapour lock.
Have you checked to see how warm the carb/manifold is when you pull up.
Those heat insulating tufnol type gaskets were added to many a model,
to help prevent this happening in hot weather.

So if you pour icy water over the carb and it still won't start, then try it over the maggie.
Preventing water ingressing to inside the works - a tricky situation.
Have fun, tell us how it goes ...

[33d6]

Of course the magneto could fail after little use. Age and how it has been stored since last done can have serious consequences regardless of mileage done.
I’m currently amusing myself with three radio controlled helicopters. All N.O.S, all received in their original unopened packaging, all looking perfect but all non running due to battery failure through sitting around for the last ten years or so. Electrical equipment can do that if left sitting. Look good externally but moulder away internally. Very frustrating but that’s how it is.
As R says it could also be vapour lock. Side valve engines do tend to run hotter than their OHV equivalent and I have seen some poorly thought out fuel line arrangements as well as R’s failure to fit a Tufnol heat barrier twixt carb and cylinder.
Nevertheless I tend towards electrical failure as my own experiences with vapour lock have been on stinking hot summer days with the engine smelling like a fifty year old fish and chip shop on a busy Friday night. Not just a mere hot start issue.
Tell us how you get on.

[john.k]

Fail to start can also be flooding of a hot motor ...........I diagnose start problems with a can of starting spray ......this stuff will fire with flooding or with fuel starvation .......but it wont fire with no spark.

[Billington]

Thank you everyone for your kind advice I will try your ideas.

[Billington]

Based on your diagnostic advice I went on a long ride. The air temperature was 26°C, 69% humidity (hot and sticky). The bike started easily on the second kick, then ran beautifully.
I then stopped the bike at the top of the hill an immediately attempted to restart the bike, however it simply would not start, not even any signs of firing. So I tickled the carburettor until fuel was seeping out of the bottom wholes, then kicked it over but it still refused to fire. So I put the bike in second gear rolled it downhill until it picked up speed then let the clutch out, it started and rode all the way home without any hesitation.

When I got home I stopped the engine and immediately attempted to restart the engine, but again it would not fire up. I then sprayed some Holts Easy Start in the carburettor at which point it started and ran without any problem.

About 3 years ago I started to have issues when running the bike at temperatures over 20°C. After just under 2 miles the engine would stutter and come to a halt. The carburettor would be very hot, hence the petrol had become so hot it vapour locked the carburettor. I was using E10 fuel (in the UK this means it can have up to 10% Ethanol). I resolved this by using E5 in a see through container then removing the Ethanol by employing the water method (Ethanol is hydrophilic), after 2 days I remove the water, leaving just fuel. I also added a 6mm Tufnol spacer. Finally, I changed the route of the copper fuel line from the fuel tank to the carburettor from behind the cylinder head to the right-hand side, where it could be in the air stream.

Because the bike started when rolled down hill I thought that this pointed to the condenser, however because the bike will start on Easy Start, I’m not sure. What are your thoughts?

[R]

Did you feel the carby and manifold - was it warm/hot. ?
Sure sounds like fuel, doesn't it.

Have you drained the carby floatbowl, and inspected what comes out.
I had a rash of rust particles and little water droplets.
Must have bought some dirty fuel someplace
(fuel by law is allowed to contain up to 2% water !!).
Sure didn't like burning that stuff ...

P.S. Tried a new spark plug ?
They don't last forever...

[Rex]

Thing is, if it was shite in the float bowl it would cause problems whether hot or cold..

[Billington]

Thank you R and Rex, I drain the carburettor and fuel tank once a year so I think there is no issue with rubbish in the fuel. I would welcome some more advice on weak sparks and vapour lock.
I have produced two videos of me kicking the bike over with the spark plug out of the cylinder. The first video is when the engine is cold and the second is when the engine is hot. Rather than being blue the spark looks white.
I’m using a KLG M30 spark plug; I mistakenly said “M20” in the video.
Click HERE https://www.youtube.com/watch?v=3qxDJLp57u8 for cold engine.
Click HERE https://www.youtube.com/watch?v=9qWQgNpN07Q for hot engine.

[R]

Thing is, if it was shite in the float bowl it would cause problems whether hot or cold..

While this is quite true, I have found that cold engines are very sensitive to this,
and once warmed up they will start near regardless.
Even if they then run like a dog - but this instantly tells you something is up.

Could the easy start spray to make it start be because of a weak spark ?
I have enough old iron that I keep a spare magdyno on the shelf.
This ensures they never play up !
The 1932 version may be a bit pricey for this, and it can be a lot of work to swap over.
But worth considering if this persists ?

Someone here had persistent starting problems, and borrowed such a spare maggie to try.
It proved equally difficult, but in the course of this the earthing path was improved.
Instant success !  Think laterally ?

[33d6]

I’ve been thinking of other possibilities. My vintage side valve experience mainly comes from operating a 1926 Matchless. This has a splash and hope total loss lubrication system with exposed valves and sloppy provision for valve guide lubrication. This created some entertaining stoppage issues that took some time to diagnose and even longer to effectively resolve. As your BSA is only a little younger  I was wondering whether you might have the same.
The one that most closely resembles your problem was a sticky exhaust valve. This was over lubricated as the piston did not have provision for an oil control ring (pointless with a total loss lubrication system) much oil disappeared down the exhaust pipe clagging up the exhaust valve guide on the way. This caused mysterious stoppages and refusal to start for no apparent reason. Any sort of fiddling with the engine, checking fuel, checking spark, etc, allowed the engine to cool down, the valve freed itself and it would happily restart with me thinking it was my successful fiddling that did the trick, not the cooling off.
I’m not saying that is your problem precisely but I am saying your problem may lie in the valve operation. Guides slightly too tight, valve lifter a fraction tight and so on. It doesn’t take much for valves to not quite seat when hot yet be fine when cooler.

[cardan]

Hopefully you have set the points gap to 12 thou and set the magneto timing correctly. https://classicmotorcycleforum.com/index.php?topic=6299.msg31348#msg31348

If you haven't, do it now.

Leon

[Billington]

I did my usual 41 point maintenance schedule on 10/05/2025, which I do every year when I get the bike out of it’s hibernation. I also have a winterisation check list which I do in October each year, which includes things like removing the fuel from the tank and carburettor.
The bike starts on the first or second kick and runs beautifully, however if I stop the engine after riding the bike at 25°C or above I can only immediately restart the bike by using Easy Start or running it down hill, which will eventually draw fuel through the carburettor.
I’m beginning to suspect that the spark and hence the Magneto may not be the cause of the problem, because once the bike is restarted it will run without any problems. I think the facts are pointing to the fuel boiling off as soon as I stop, requiring me to restart by either using Easy Start, running the bike down hill or simply giving it enough time to cool down.
Are my expectations of a 92 year old side valve motorcycle running on modern fuel unrealistic? Should I just live with the fact that restarting my hot bike in hot weather will require a little rethinking (hence the problem is with me) or should I continue to explore more ways of getting the bike to run cooler?
I’ve based my maintenance schedule on BSA guidelines for a W32-6 500cc side valve motorcycle, written in the 1930s, which means they are based on 1930s fuel.

1, Contact breaker point gap when the piston is at TDC, is 0.003 inch with the lever set at fully retard. This has been set in accordance with be information given by BSA for this bike.
2, Tappet clearance Inlet = 0.004 inch, Exhaust = 0.006 inch. Again this has been set in accordance with be information given by BSA for this bike.
I’m open to your thoughts about the points gap being set to 0.003 inch at TDC, given that the main variable change in this case is modern fuel.

[cardan]

Is there any time in the rotation of the magneto that the points open to 0.012"? I assume no.

Do your bike a favour and get someone who knows what they're doing to have a look at it. You will be amazed at how a 92-year-old side valve runs when it is properly set up.

Leon

[R]

Ah, we're back to this case are we 
Yes, if the ignition timing is so off that the engine is on the point of meltdown,
then all bets are off.

Get the ignition set correctly, as suggested ....