Motorcycle Repair: RPM surges on 1974 Honda Engine, honda engine, air leakage

Question
I have a 1972 Honda CL175 powered by a 1974 CL 200E-1000734 Engine.  After it sat for several years in my garage, I decided to get it running again. First it would not start but after cleaning the carbs and replacing the points and condenser (the spark plugs were fine)it started and ran fairly well except for the idle.  The engine would die when the idle speed dropped to 2000 RPM (1200 is recommended.)  I decided to disassemble the two carbs again.  When I reassembled them and started the bike, the idle speed would surge to 6000 to 7000 (10500 is the red line).  When I rode the bike it would continue surging erratically from 3000 to 7000 or 8000 which made the bike non functional.  Since I did not soak clean the carbs when I first worked on them, I bought a can of recommended cleaning solvent, totally disassembled the carburetors and soaked all the parts over-night.  When I reassembled the carbs and installed them back on the bike, the problem was worst.  I should add that I replaced all the gaskets both on the carburetors and the intake manifolds, so I am fairly certain that air leakage at the manifolds is not the problem.  Frustrated, I decided to remove the air fliters and see if that would make any difference ( I had removed them before to clean them with compressed air.).  I did so, and started the engine.  It started right away and warmed up to the point that I could totally close the choke.  It was idling steadly at about 3500 RPM.  I placed my thumb on the right carburetor intake opening and felt what appeared to be some suction. I could feel air movement.  When I placed my thumb on the left carburetor intake opening, I could not feel any suction or air movement.  I then turned the idle speed screw on the left carburetor and the idle speed changed.  Turning it counter-clockwise I reduced the idle speed to about 2200. I then tried to reduce the idle speed further by turning the idle speed screw on the right carburetor. Turning the screw had no effect on the idle speed.  I then started turning the air mixture screw on the right carburetor and suddenly the RPM surged and I managed to shut down the engine as it reached the red line.  I waited a couple of minutes and started the bike.  The RPM immediately shot up to the red line.  I am at a loss as to how to solve this problem. I don't understand why there would be no air suction felt on the left carburetor yet turning the idle mixture screw changes the idle speed while on the right carb turning the idle mixture screw has no apparent effect although there is definite air movement at the carburetor air intake opening.  Thanks,  Mike Sullivan

Answer
Mike, that is an interesting swap, but not useful at the moment, obviously. Are you using the 175 or 200 carbs? Different settings for each version. CL200 carbs require #95 main jets and #35 idle jets (CB200s have #88 and #38 sets). CL175s have various jets depending on carb model, but the vary from #90 to #98 with #35 or #38 idle jets. Both carbs do use the same float level settings =21.0mm
The main difference between CB and CL applications is the exhaust systems. The length of the primary exhaust pipes affect the carburetion greatly.

Things to consider.... Check compression on both sides, check valve clearances (.002" cold for both intake and exhaust valves on TDC).

Look at the carb slides... the cutaway has to go to the air filter side.. out where you can see them. They are side specific and can easily be reversed.

Take the point plate off and inspect the spark advancer. Sounds like it is stuck/sticking to me, which is common because the are generally ignored. The spark advancer has to be clean and lubricated so it can advance/retard with engine rpms.

Check your throttle cables for incorrect routing, internal damage or sticking. Be sure you have just a bit of free play at the throttle and that both carb slides are synchronized together. Leave the air filters off until you get them set correctly, so you can watch what they are doing.

Carb bodies can get out of round and cause slide sticking when opened. Be careful not to over-tighten the flange bolts/screws, which can cause slide seizures.

If you have good compression on both sides, no intake air leaks, carbs set to specs and the ignition timing verified and advancing properly, you should be back on the road again.

Bill Silver