Motorcycle Repair: CB350 -engine wont return to idle, 1971 honda cb350, cylinder head repair

Question
Hello Mr. Silver,

I am writing on behalf of a friend that does not have access to the internet or a competent motorcycle mechanic. He recently left the bike at a motorcycle shop for three weeks and still has this problem.

1971 Honda CB350

Once warmed up, the engine will not return to idle speed when the throttle is released, and will sometimes increase engine R.P.M. without any throttle being applied at all.

The motorcycle seems to run OK when it is cool.

I know that you may not be able to accurately diagnose the problem from this very brief description, but perhaps you may be able point out the most likely cause(s).

Thank you in advance,

Doug Offield


Answer
Doug, this not an uncommon problem with the older twins. If I assume that the shop correctly adjusted the valves and have cleaned and set the points and timing, along with a good carb system check, then usually the problem lies behind the point plate. The mechanical spark advancer often gets gummy and hangs in one position or the other.
Remove the point plate, inspect the mechanical advancer for free motion and return. Sometimes the springs get loose and allow too rapid of an advance, so be sure that they are bringing the weights back to rest by themselves.

Mark the point cam on the advancer base, if it is to be removed for cleaning and lubrication, otherwise there is a 50-50 change of installing it backwards which will result in a non-running condition.
Once the point plate is reinstalled, reset the initial timing to the F and LF marks respectively. That should take care of the problem.
If it doesn't, then there could be issues with gas cap venting, incorrect float level settings (there are at least 3 sets of carb calibrations for 350s through the years) and the possibility of an air leak at the intake manifolds or the gasket where they mount to the cylinder head. Repair or replace as necessary.

Bill Silver