Motorcycle Repair: 73 CB350 eats sparkplugs, 1973 honda cb350, resistance values

Question
Hi Bill,

This one has been stumping me for a while, here’s the full scoop:  I have a 1973 Honda CB350 that I brought back from the dead.  It had several issues, and about five years ago I tore it down completely and rebuilt it, replacing several parts, including the head.  After getting it back on its feet, it ran well for a quite a while.  It never wanted to idle correctly, but I figured it just needed a few miles on it to get used to itself.

Now about three years ago it’s stopped functioning correctly.  It eats sparkplugs.  While riding it, the left cylinder would kick on occasionally, or sometimes wouldn’t even fire at all.  I cleaned and rebuilt the carbs a couple of times, but that didn't help.  The compression tested strong.  Now this year I decided to replace the coils and condenser, and this seemed to work.  I put a new set on, and the bike started up and it took right off.  The left cylinder was a bit rough from lack of use, but it was definitely firing.

Went to start it up the next day, and the left cylinder quit on me.  Wondering if it was the plug, I switched the right and left plugs, and the left cylinder fired but not the right one – the original left sparkplug was the issue.  I went down and picked up 2 brand new plugs, put them in, and both cylinders started right up.  Took the bike for a short ride (10 mins), and the left cylinder once again stopped running.

I’ve been through at least a half dozen sets of plugs, and they all are quickly eaten by the left side.  They do not appear to be fouled, no burnt spots or buildup.  Any idea what could be causing this?

Thanks a ton for your time!

Answer
Adam, I had a similar problem with a CL350 and it turned out to be a bad coil. All these original OEM coils are falling apart internally. Check the primary winding resistance values on each one. Honda coils used on points/condenser ignitions usually have about 4.5 ohms resistance read through the + and - coil connections, disconnected from the wiring harness.

When my coil went bad, I had spark at the plug AT IDLE. When you pulled up the throttle arm on one side, it quit running, due to its inability to provide spark under load.

Other issues for 350s have been the condensers not properly grounded to the frame due to paint or corrosion build-up and a more subtle problem with insufficient point gap settings. If the points are not opening wide enough there is a brief moment where power is being routed to both coils at the same time, which weakens the voltage/current to both coils. Point gap should be about .016" to prevent this problem, assuming that there is no damage to the point cam itself on the end of the spark advancer.

Check spark plug resistor caps for 5k ohms and you should see about 25k ohms from the spark plug lead (w/o cap) to ground. Compare both coils for all of your readings.

Make sure that the carbs have good diaphragms, all the jets are clean, including emulsion tubes up inside the carb body.

I have seen the rubber carb intakes separate causing big air leaks, as well.

Make sure your valves are adjusted properly. The ends of the adjustment shafts should look like:
(-I)   (I-) with the little punch marks on the edges of the screwdriver slots facing away from each other.  

Hope that helps...

Bill Silver