Question bill,
honda z50- put new int and exh valves in.lapped in good. tight seal. timing correct. have spark. put head back on. bike fires right up .ride bike a few
miles ,get home try to restart bike. wont start. ocassionaly backfires
through exhaust and at times ocassionaly through carb.
pull head back off and check .put back on without doing anything bike starts up ride a few miles runs good shut it off wont start tear apart put back together, runs, done this 5 times now. valves sealing good timing is checked before i tear back apart. im stumped! can a new spark plug fire out of engine but under compression not fire and fouling out due to saturation of fuel? coulc it be when i pull it apart
the combustion chamber is drying out and when i put it back together the plug is less dry of fuel? can the plug be misfiring causing the ocassional backfires noted and when it does spark there is a build up of fuel in combustion chamber causing backfire? thanks for your time!-gary
AnswerGary, you are getting to be a real expert at top end jobs on these things! Must be frustrating and I'll throw in my 2 cents worth here and hopefully you can solve the problem.
First, I am not sure what year your bike is, whether it has points or CDI ignition. Condensers and CDI components can malfunction when they get hot, so your problem could be electrical not mechanical.
Get a compression gauge, so you can check cold and hot compression readings. If they are both in the 150+ psi range, then the problem is probably not mechanical. Stock Honda readings are actually closer to 170 psi. I'll assume that you have checked the valve clearances, since the rebuild. At .002" cold they tend to sink a bit after a valve redo and can lose their clearance, as the stems heat up and expand.
A fuel-fouled plug will tend to fire down the side instead of across the gap. Try a new one when in doubt. Check the plug cap for high resistance. Normally they are in the 5k ohm range.
Backfires originate from incorrect cam or ignition timing, lean/rich mixtures, tight valves, air leaks at intake manifold and/or exhaust system connections and faulty ignition components for the most part.
Make sure that the valve guide clearances are sufficient and you aren't experiencing any piston seizure episodes. Any metal being thrown into the combustion chamber will short circuit the spark plug firing.
Check your carburetor settings for main jet size, needle position, float level and idle mixture screw adjustment. Be sure all jets are secure and all air passages are clear and clean.
Bill Silver
