Motorcycle Repair: CB350 carbs etc., throttle levers, adjustment screws

Question
Hello Rich,
I’m new to motorcycling and have just started doing maintenance on my 1968 CB350. On this bike, the dual exhaust was removed and replaced with a two into one type. I’ve been trying to sync the carbs and was wondering if I need to remove the exhaust pipes. Otherwise, is there another way to do this (since it says to feel the exhaust pressure levels from behind both pipes)? Also, the gas cap seal is not very good and when the tank is full, it leaks. I just don’t fill up the tank all the way but are there any problems that this could cause performance wise? Oh and last time the bike was serviced, the mechanic said my valves had been adjusted completely wrong before and that it would cause the ignition timing to go out more quickly. Could you possibly give a brief explanation of the valves? Finally, the bike smokes a lot when starting (smoke goes away after maybe 30 seconds) but the idle is perfect around 1200. However, after 20-25 minutes of riding, the idle stays pretty much around 2500-3000. I set the ignition timing and am wondering, what else could be causing this problem? Thank you very much sir.
Sincerely, Marta


Answer
Marta, the only way to sync those carbs is by watching the throttle levers and adjust the cables so they start to open at exactly the same time.  If you back off the idle adjustment screws until both throttles are closed and then adjust the screws until they just contact the stops before adjusting the cable.  Then adjust the cables so they start to move the throttle levers at the same time. Then you have to adjust the screws a little at a time until you get the idle speed you want.

The cap is just worn out.  There should be a rubber or cork gasket in the cap and if that's broken or cracked, it can cause the leak. There's also a spring that puts pressure on the seal so if that's broken it could also cause the leak.  

The valve adjustment won't change the ignition timing.  I'm not sure what he meant by that but there's no connection.  The engine has an overhead cam to actuate the valve.  There are four sliding rocker levers that run on the cam lobes to open and close the valves.  On the end of the cam there's a shaft that has the timing advance unit. The advance unit has two springs that control the advance curve.  Many times those springs relax allowing the timing to run more advance than it should.  I usually tell folks like yourself, to pinch the loops on the ends of the springs just a little to increase the tension.  Just not too much or the timing won't advance properly.  

The idle speed should always be adjusted on a warm engine. It's not unusual for a cold engine to idle too low or not idle until the engine worms up a little. The engines don't have a fast idle cam.

Regards
Rich