1981 CB750C - RPM Too high after start

[scott_pockrant]

Hi Everyone,

I recently purchased a 1981 CB750C from a guy who had intended to restore it. It was still in good overall condition and was running when I bought it, but it was obvious that there were some carb issues. It had some fuel leaking from at least one carb. Since the bike was still running and only about 10 blocks from my house, I rode it home. By the time I got home, I noticed that the engine wouldn't drop below about 3000-3500 rpm when in neutral. I assumed that was related to the amount of dirt and grime on the carb and that the throttle was just sticky.

When I went to rebuild the carb, I pulled it out of the bike and discovered two things. The first was that someone had previously opened up the carb. Some of the screw heads had some wear/damage. Not much, but you could tell it had been opened. Second, I realized I had no idea how to rebuild a carb. I decided to take it to a shop to get rebuilt, then I installed it back on the bike myself. When I went to fire it up, she went right up to about 3000 rpm again. No matter what I try with the choke or throttle, it won't drop down. I haven't let it run for too long to see what happens once the engine is warm. I don't want it running at that high of rpm when its cold.

Where I'm running into problems is what to do next. Is it a carburetor setting issue, or a fuel/air mixture issue, or a choke issue, or a bigger issue with the engine? When I was installing the carb, I noticed the tubes on the air box were getting fairly worn, but I'm not sure if that would cause my problems or not. Also, since the carb had previously been opened, I'm wondering if the last guy messed up the settings. When the shop rebuilt it, they would probably have returned everything to those same, potentially incorrect settings. Can I adjust the carb while the bike is off, or do I need to have it running to adjust the carb? And could running it at high rpm like that - before the engine is warm - cause damage (other than potentially making my wife really upset)?

Any suggestions on what I could do to try and diagnose the problem would be appreciated. I'd really like to avoid hauling the whole bike into a shop until I know what's wrong.

Thanks in advance

Scott

[TOOLS1]

It sounds like you have a vacuum leak. The carbs are not sealing onto the rubber boots that connect them to the engine. New ones are pretty cheap on ebay. I would recommend going on and replacing them and the clamps since they are old and have had the carbs removed and replaced several times now.
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