Trial the other night my bike was running like sh!t, smoking like a good un. Cutting out a few times, particularly near the end but also smoking a fair bit all night. Now I had the usual mixture in the fuel, the carb had just been cleaned the night before, and the bike has always ran well, so what might be the issue ??

Crank seal, drop the gearbox oil & see how much comes out

The smoke will smell awful too probably at it;s worst when the motor is really hot.

The Bultaco blew a crank seal on Wed night nearly killed half the entry,couldn't see me dab for the smoke though.

White or blue smoke?

Are you sure it's the bike!

So it was running fine until you took the carb off the night before?

I'd go back there.

When I say I did the carb the night before, it was the week before, but it's the first time the bike has been run since doing the carb. I shall drop the oil over the weekend and see what comes out.

I checked the plug and it was black and soaking, changed it, and the bike fired up and ran ok then put bike in the van then headed home. I didn't want to say I'd replaced the plug to see what replies I got.

You've blocked a jet when doing the carb and it's choking.

This explains the smoke , plug and cutting out.

Was she also down on power, and slow to wind up?

Get the carb, sonically cleaned and try again.

Good luck.

Can't say I noticed it being down on power.

After some thought, it wasn't like mental smoking just more than normal, so I'm not sure how badly they smoke when a crank seal goes.

Back to my question then!

Am I sure it's the bike ?

I'm not getting your question I'm afraid ??

Am I sure it's the bike ?

I'm not getting your question I'm afraid ??

Point being it may be just the plug. Now that you have provided more info!

Now if they kept on fouling prematurely, unusual smokiness persisted, bike is more than 2-3 years on, I may be more concerned about crank seals, yet there may usually be other symptoms such as unexplained floaty idle even with a perfectly clean carb.

See how it goes! I dump a plug in a heartbeat at the first sign of hard starts or rough running or anything else!