Motorcycle Repair: 72 cb350, fuel tank cap, vent holes

Question
I bought a 72 cb350 twin k4 with very low mileage about year ago that did not run. Did the regualr stuff, carbs, points, tank and got it running.  It seems to be running realy rich on the right side and constantly backfiring when its running and really undependable. Sometimes it will start sputtering and die like its out of gas but its not. It seems to run better with full tank? All these questions seem to baffle any local(saint louis, MO)help and am getting kinda of desperate. I love the bike and physically its in great shape. This is my first mechanical experience with motorcycles so I dont claim to be that smart with what Ive been doing.
I really have not been able to find any explanation of what to do if it runs rich, even in clymer.
any help is much appreciated

Answer
Brian, Be sure that the compression is the same on both sides. If not check valve clearances and check again. They should be within 15 psi of each other.

Double check your spark timing and the spark advancer for proper function. Watch the points when the engine is running to see if there is excessive arcing on one side. If so, then replace the condenser set.

When you cleaned the carbs did you remove the emulsion tube up in the carb body where the primary main jet is located. It is small and must be unscrewed with a tiny slot screwdriver. The tube is cross drilled with holes so it can mix air and fuel. That is a common oversight in carb repairs for these bikes.

Be sure that the fuel tank is clean, petcock is clear and flowing properly and that the fuel tank cap vent holes are open.

Rich mixtures come from an imbalance of air and fuel. If carb air passages, emulsion tubes are restricted then there will be a rich condition. Likewise, excess fuel from a high float level (float with pinhole leak will cause that too), O-rings not sealing the jets and float valve can cause rich conditions. Be sure that the carb settings are correct for that series of carbs. There were several carb sets for the 350s through the years. Carbs are stamped with a code on the bodies.

Be sure that the spark plug caps have equal resistance values (around 5k ohms) and that the battery is fully charged and charging system is fully functioning.

Bill Silver