QuestionHonda CB250N 1984. Recently had to strip top end of motor to repair stripped thread in crankcase. rebuilt with new gaskets etc. Decoked engine, reground valves, reset valve clearances, stripped and cleaned carbs, set float heights Starts well, ticks over fine, runs ok, but hangs on to revs when throttle is closed (ie when changing gear). Both throttle cables lubed, and throttle butterflies return to shut position, but still holds revs for 4 or 5 seconds, so there is no engine braking. Any ideas?
AnswerLynton, we didn't get the 250N version in the US, only the 400-450cc one. Hopefully, the carb design is still the same.
US bikes have an anti-backfire diaphragm on the side of the carb body, which introduces a little air into the intake system to prevent a rich mixture on the overrun. If the diaphragm is worn or the passageways are blocked or restricted, then you may have a problem from there, however, there will be a short period of higher revs as a part of its normal function.
Do set the idle speed down to the suggested range.. probably 1000-1200 rpms, to ensure that the throttles are not holding the idle higher than specifications.
The only other things that affect hanging idles are intake manifold leaks, any loose or damaged vacuum lines connected to emission control units and the spark advance curve, which is preset.
Bill Silver
