The #1 problem like this I find on these CB/CL/SL350 engines is: the spark advancer itself. (It shows up immediately when riders try to fit the Transistorized Ignition to a bad one.)
It goes like this: there were (once in its life) 2 or 3 thin plastic washer shims behind the spark advancer's points cam that shimmed it outward to be square against the face of the back of that washer that holds it onto the end of the cam. (whew.)
When these shims melt and disappear the first symptom is: the gap on the 2 sets of points must be different in order to make timing meet the marks. (Sound familiar?) Typically the L points end up around 0.012" and the R points around 0.016", which then causes the L coil to have a weak spark and sparkplug that fouls. (Am I getting warm?)
When this happens, you must make new shims, at least 2, to push the little points cam back out toward the back of the washer. The clearance between the cam and the washer should be no less than 0.002" and no more than 0.006", and if you set it up on a typical summer day it should be about 0.004". You can measure how much gap is there now, then get some brass sheet at the hardware store and cut some washers out with a razor blade, as you will be cutting brass sheet of about 0.002" thickness to make these. Brass ones will outlive you.
This MUST be done in order to use the Transistorized Ignition on these bikes (and the CB/CB450, too) or else the L coil's spark doubles below 3000 RPM and the bike idles not at all, or is so hard to start it's silly. The simple shims fix this right up.
