Monobloc

[Tun up]

1957 B31. Will not restart after being run to normal operating temperature and switched off and then allowed to cool a little. All other starting is fine. All jets are correct and the needle is on the second notch. However on a non start when turning the engine with the plug removed no petrol/air mixture can be felt through the plug hole; yet the carb has petrol.
There is a spark on ever plug used and the magneto is fine.
Any pointers much appreciated please.
Cheers

[wetdog]

with the plug out no fuel will be drawen into the engine , it sounds very much like the mag failing when warm (windings condenser) or plug

[Tun up]

Thanks for the info wetdog.
Regarding the fuel issue; even with the plug in there are no traces of petrol on the plug despite repeated attempts to start the bike. If fuel was getting through and the bike won't start oughton the plug to be wet; as in flooded?
The mag has recently been fully rebuilt. The bike restarts from hot straight away; the non starting only occurs if the bike is left to cool down a little from hot. Also there is strong spark at the plug.
If after all that you still think magneto, then I'll get it checked out. Does the extra detail ring any bells?
Cheers
Tun up

[wetdog]

so the bike wont start from warm , have you tryed flooding it when warm , have you an insulation block between head and carb , has the armiture been rewound ( not that this would account for no fuel) its an interesting one , i have had plugs sparking when out of a engine with a mag but would not spark under compresion , if all else fails put the mag on test ond run off the machine and watch for spark fail

[Goldy]

As wet dog says check that you have at thick insulation gasket between the carb and the head which prevents heat transfer to the carb body.
All the best

[Revband]

Another thing to look out for, if you are using a new NGK plug there are a lot of fakes around I had the same problem, I spent a long time going round in circles before I found it.

[Rex]

What's the compression like with the engine warm? If it's low the tappets could be set too tight and holding a valve off it's seat. Petrol pipe routing good? Air locks aren't uncommon either.

[wetdog]

Also watch suppressed  plug caps on mags they can give problems

[twolitre]

Also watch suppressed  plug caps on mags they can give problems

Many people seem to think that a spark at the plug in air is a good enough test.
In fact many times that voltage is required to cause a spark at the plug electrodes under compression. Suppressed plug caps and supression plug leads increase the required voltage even further. Air gap testing tools are available. A typical airgap test for a car is around 1cm. Your B31 is probably quite low compression ratio, but I have no idea of the minimum air gap required to test.
Jim