1961 B40 cutting out

[Cargers]

My 1961 B40 runs great until around 20-25 miles then starts to hesitate at large throttle openings then it cuts out completely and will not restart. I have sparks and plenty of fuel but it will not fire back up. Leave it for a couple of hours and it starts straight away. It suggests an electrical component overheating and when it cools it is ok again, but before it cuts out completely it appears to be a fuel issue missing and backfiring at bigger throttle openings. I have set the valve clearances larger in case they were holding a valve slightly open but there is still good compression when hot.  Any ideas???
Ian

[Stubaker58]

Hi
Hope you get it sorted soon, sometimes a coil will break down when hot so that’s worth checking.
Could it be the carb overheating? Do you have an insulating (Tufnell?) gasket between head and carb?
Regards

[Mike40M]

Had a similar problem with a Landcruiser. Though it managed 150 miles. Turned out to be an after market completely air tight tank cap. I suggest checking if you have vacuum in the tank when it cuts out. If so check that cap can breathe.

[Cargers]

I do have an insulator fitted and the fuel cap breather is clear. I have just taken the monoblock carb off and fitted a concentric  this morning to see if that makes any difference. The only problem is that it is harder to start from cold with the concentric as there is no choke fitted just a tickler. The monoblock usually fires up 1st or 2nd kick with the choke from cold.
Ian.

[Beesy]

About 75% of Carburettor problems are electrical! I spent ages tinkering with mine. Turned out it the coil was kaput.

It is frustrating, just a case of trial and error

[Steve.S]

I wonder how you're checking for sparks. Laying the plug on the head won't tell you much. I would invest in a spark plug gap tester. I don't think they cost much. This will tell you if it's firing under compression.
From what you have said, I would go for a new coil and condenser.

[rhyatt]

dodgy condenser?? (unless you have electronic ignition)

[cdsdorset]

Do you have an inline fuel filter fitted.

[chaz]

an outside couple of thoughts.
first , but more often with 2 strokes, if the cylinder is getting too hot and/or the piston clearance is too tight, it could nip up, freeing itself when colder.
secondly, has the fuel tank been sealed inside? maybe the sealer is peeling off and covering the fuel filter/tap starving the carb of fuel. When the vacuum from feeding the carb goes, the sealer floats again.

otherwise as other suggestions.

[Cargers]

Tried the concentric carb today, ran well but still started to misfire and cut out. not as bad as the monobloc but still not right. i think i may have a dodgy coil, (new one one order) and also a worn out monobloc carb.
I have owned lots of 2 stroke race bikes and it is definitely not seizing. No tank sealer or inline filter.
Still running 6v electrics with points.
thank you for all your suggestions.
Ian

[John Dynostar]

I noticed the last sentence included points :-)
this is my 1st post .
condensor if fitted would be my 1st port of call.
I have had problems with new chinese copys never mind old origionals
if possible sit a 12v battery on the back of the bike or in your pocket and
borrow a timing light and gaffa tape it to the handlebars whilst riding the bike and see if the spark/light  dissapears .
this should split the fuel/ electronic issue for you, a bit of a faff I know.

re: condensor ;
doing a quick search I found this which may be of help  but may muddy the waters also if its not the condensor.
hope this helps
https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=3601.0

[neil1964]

I noticed the last sentence included points :-)
this is my 1st post .
condensor if fitted would be my 1st port of call.
I have had problems with new chinese copys never mind old origionals
if possible sit a 12v battery on the back of the bike or in your pocket and
borrow a timing light and gaffa tape it to the handlebars whilst riding the bike and see if the spark/light  dissapears .
this should split the fuel/ electronic issue for you, a bit of a faff I know.

re: condensor ;
doing a quick search I found this which may be of help  but may muddy the waters also if its not the condensor.
hope this helps
https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=3601.0

I echo the comments re: aftermarket points and condensors. 

Although not a BSA (mine are both magnetos) I had all sorts of problems with premature  points and condensor failures on my MGB - hugely frustrating as superficially all ok but the pivots wear and short, the points themselves fail as the condensors fail.  Particularly as I replaced them more out of duty with each service (same time as filters, plugs etc).

I found electronic conversion transformative (same for my Series Landrover and Triumph Daytona 500s) and am contemplating one for my Goldie (10:1 comp is quite testing of even a good MOV1).

[Cargers]

Cutting out problem sorted. Turns out the coil was breaking down when hot. New coil fitted and it runs like a dream now. Thanks for all your suggestions.
Ian

[John Dynostar]

2nd post    stubaker58    well done 

[neil1964]

Thanks for the update.  Well done.