NARROWING DOWN THE PROBLEM
Paul Stallard replaced most of the wearing parts in his narrow-case Ducati 250 motor, but the camshaft and rockers looked good enough to reuse. Bought as a tired non-runner, he also replaced the Dell’orto carb with a Mikuni VM28 – but despite trying various settings and jets he can’t get the bike to run right.
It starts and accelerates OK, but tends to misfire on a level throttle and won’t rev beyond 5000rpm. Checking the valve timing revealed that although the inlet valve opens at the correct point it closes at 80° before TDC instead of 76° after BDC – that’s 24° too late.
“Is this the problem?” Paul asks, “or should I carry on with the carburation?”
Valve timing is about gas inertia. The descending piston sucks in gas from the carb, but the inertia of the gas stream means it will continue coming in, even after the piston is on its way back up – until the point where the piston starts pushing it back out. The trick is to shut the valve just before that happens, bearing in mind that it will vary with engine revs. The overlap (inlet opening/exhaust closing) figures are supposed to be the most critical, but at 90° after bottom dead centre the piston is halfway up, so even 76° is leaving it pretty late and another 24° means it’s past halfway, so this could well be the problem.
However, I would expect starting and idling to be affected, too – and I’d probably expect signs of fuel blowback out of the carb. Personally, I haven’t had much luck setting up Mikunis and I would go back to the original carb to see if the problem changes. If not, then it clearly isn’t a carb issue and it’s time to look for another cam.
It’s generally wise to correct anything that’s obviously wrong first – but equally I’ve often found faults on bikes that should have caused problems but somehow didn’t. So if the obvious fault is also the most costly, it’s maybe worth trying the easy things, like swapping the carb, first.