Motorcycle Repair: 1970 CB 750 poor running motor, air fuel mixture, spark plug wire

Question
QUESTION: I have replaced the plugs, condensers, coils, spark plug wire ends, cleaned the carbs (several times), cleaned and gapped the points, set the timing and still cannnot get the thing to stop missing. It accelerates fine but misses at constant speeds. Any Advise??

ANSWER: Wow, you've checked about everything.  It sounds like you have a lean air fuel mixture to me.  When it misses out try opening the choke slowly and see if it doesn't quit missing.  If it get better, we're looking for something that is leaning it out such as float level adjustment. The intake manifolds leaking can cause this too.  It could also be a spark problem if the energy from the plug is insufficient to ignite the fuel mixture.  This could be too narrow or two wide of a plug gap.  Is there any sparking at the points when it's running?  A bad condenser could also cause something like this but it would run bad all the time and not just at lighter loads.  Let me know if this helps.  

Good luck
Rich



---------- FOLLOW-UP ----------

QUESTION: Rich,
I have been running without the air cleaner as I am waiting for new rubber connectors between the air box and the carbs. Could this do it?? Also, this bike has sat for a long time, could a valve be hanging up?? I have no sparking at the points and the condensers are new. When I have pulled the plugs they are black with carbon build up??
Dave

Answer
That bike needs the added restriction of the air box and filter to maintain the right air fuel mixture. Running without the filter will certainly cause it to lean out.  It's not the valves.  Those either work or don't work.  

Looks like you figured it out.

Rich