Motorcycle Repair: left cylinder not working., rubber diaphragms, plug caps

Question
Hello,
I have a '70 cl350 and '71 cb350, both have developed the same problem. both ran fine when i first got them, now for some reason the left cylinder is not firing. the spark plug is wet with gas and when i place the plug next to the head i see a spark. i can see spark in the points gap, but the pipe on the left never gets warm.

Any help would be great.
Thank you,
MIke

Answer
Mike, Always start at the beginning when troubleshooting a cylinder miss issue.

Compression... How much? If low, always check valve clearances, then recheck. Standard readings will be around 150-175 psi.

After that, I always suggest that the spark advancer be examined for proper advance/retard function. Clean and lubricate. Be sure to mark the point cam, if you slide it off the advancer base, as you have a 50-50 chance of intalling it backwards and fouling up the ignition timing.

Once that is checked, clean the point faces thoroughly, then set the gaps to .012-.014" at the highest spot on the point cam. Afterwards, check the ignition timing to assure that the points just open at the LF and F mark alignments.

Put in FRESH spark plugs (once they are fouled, it is hard to get a reliable spark again). Actually, before you put them in, check for a good clean spark with plugs lying against the head. While you are cranking the engine over, check the points for excessive arcing. If one side has persistent arcing across the point faces, then check the condenser connections, up under the fuel tank. If they are clean and tight, then you probably have a failed condenser set. They come as a unit of two, so both have to be replaced. They are pricey from Honda, so check Ebay or even try one from a small Japanese Car from that era, spliced into the circuit.

Check the spark plug caps for resistance readings. Should be around 5k ohms and be tightly installed on the plug wire end.  So much for the electrics and basic tuning.

CB350 carbs have rubber diaphragms for the slide that often tear or get pinholes in them. Check this first! If they are damaged, they will never work properly. Check cable syncronization and idle mixture screw settings. Set both side the same to start. Try about 1 turn out on the mixture screws first.

The carbs have some numbers/letters stamped on the bodies, indicating a certain calibration set and they changed them quite a bit during the production run. Find out what the stampings are, so I can provide the correct settings.

Other carb issues are float levels incorrect, the floats have pinholes in them, causing them to sink, numerous O-rings inside can fail and jets/emulsion tubes and air passages can get blocked with varnish and dirt. Everything has to be clean and to original specs.

Petcocks can get blocked, sometimes, allowing more fuel to one carb than the other. Be sure that the gas cap vent holes are clear, too.

If you let these bikes sit with old gas in the carbs, it will evaporate quickly, leaving deposits that can upset the calibration of the units.

Always start with a fully charged battery and be sure that the charging system is fully functional.

Bill Silver