Well I'm stumped. I have just put a used bottom end on my mc21 as the last one had a worn crank, seals and bearings. Now for some strange reason the lower cylinder is sucking air into the exhaust while idling. It has a little putt every now and then with a bit of smoke coming out but that's it. When I give it a little throttle around 2500rpm it gets a bit better but the exhaust gas seems colder than the top cylinder. At around 5000rpm it heats right up and appears to be good.

The top cylinder is running great, no complaints at all.

I have cleaned the carbies, rebuilt the reed cages, swapped the coils over and had a good general look over to eventually find nothing out of the ordinary.

Any advice or idea would be great as I can't think of a single thing to try now. Don't hold back, I'm willing to try anything.

Cheers

Andy