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

 

[Dave]

Have you tried adjusting the idle? (Technically the "Throttle Stop")

Knob made for turning by hand, underneath the carbs, right in the middle. It's actually easiest to use while sitting on the bike. (You'll learn to keep your hand flat while turning it by burning the back of it on the starter cover LOL)

A good shop would set everything to base specs from a manual.

Idling at 3000 rpm won't hurt it as long as everything else is ok, like oil pressure, these engines don't redline until 9000 rpm.

If you had no issues with letting a shop rebuild the carbs, you should have no issues with letting that same shop synchronize and adjust them. This is done while it is running.

You really should do a compression check before spending any more cash on it since you just got it. If the numbers are too low, anything else you do is a waste of time and money.

 

[amc49]

Wow, you ARE a rookie or you would know there are FOUR carbs there and not just one. The four black rubber manifolds you pop the 'carb' into and out of commonly crack to leak air and then bike idles super high.

 

[Dave]

You can spray some ether** (a little at a time, allowing it to evaporate before spraying more to reduce fire hazard*) around where the carbs slide (with extreme force usually) into the rubber manifolds while it is running. If there's a leak there, the idle will go up when it sucks the ether in.

*Unless your like me, fuck it, I'd hose the the rubber manifolds at first, then narrow down the exact location with smaller amounts if the rpm went up. My eyebrows would grow back, they always have and I have a fire extinguisher handy.

** Ether because it evaporates very quickly.