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Newb with mint 1981 and hanging RPM issue

DogBunny

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My first CB750. It only has 2,715 miles!
When cold it takes forever until it runs without the choke, and it won't go below 4000 rpm when warm and riding. It will eventually come down to normal idle when I put it in neutral, but it takes a while. Barely ridable like this.
I've done a little research on other threads. The carb boots look very good -- all of the rubber parts on this bike looks pristine. New filter in air box.
The previous owner resurrected the bike, and re-built and synced the carbs, but I don't know how good a job he did, although he seemed competent.
Did 1981 still have a mechanical advance?
Thanks...
 
It sure looks sweet on that pic.. And what you're describing sounds like an air leak to me. Try and spray start gas on each intake boot while running, one at the time, and listen for the motor sound to change. Sometimes there's such a small hole/crack that you wont catch it with the naked eye. And if you got that on more than one carb you could get it to act as you describe yours is acting. This is assuming the PO knew his shit when putting those carbs back together.

It could also maybe be the vacuum operated throttle slides that doesn't slide as they should. Or a very lean condition. How does your plugs look?
 
Thank you for the response.
I'm beginning to think that I am going to have the problem fixed by a local expert. I am very good with carbs, but I think my time is better spent working on '70s era Yamaha twins, which is what I do and which I know well. There is a local retired Honda mechanic who I think I am going to take the bike to. I got a very good deal on this CB750, so I think I can afford to have him work on it and it will still be a good deal.
 
Stabler, you were right, air leak. The previous owner never put the carbs all the way into their rubber manifolds after he re-built them. They were about a quarter inch out, which meant that the little lip in the rubber wasn't slipping into the groove in the carb body. He hadn't tightened the clamps enough either.

It still hangs a bit, but it's rideable. I've got the air mix screws way open. I've been told that it probably still has an air leak or a fuel blockage problem, possibly involving the air cut. Or, it might just need a sync.
 
When I got the bike each air mix screw was set at exactly two turns out -- which indicates to me that the PO never adjusted them -- he would have adjusted them while syncing, so I'm sure he never did that either. Not seating the boots also indicates general sloppiness which also leads me to believe that he never synced the carbs.
 
Not having the carbs all the way in manifolds often signals hard rubber from time and sitting. Why worker can't get them in to begin with. If so you still could easily have cracks in the rubber, as forcing them the rest of the way is often what cracks them (more). 95% of the cracks you will never see other than the effect.

I set all mine with mixture screws at exactly whatever turn I choose, that's no evidence at all that someone never touched them, in fact evidence they HAVE. You attempt to avoid setting the idle mixture screws while syncing, you move the screws on the links between carbs to sync. Sometimes you may have to possibly move the mixture screws but not always. If already set to best idle at the idle speed you are looking for then no need to touch them at all. Taking too long to do the sync can affect that as the engine will run richer the hotter it gets while idling with no air running over engine, I always throw a box fan on the motor when doing it.

At the low mileage this one has I would expect the idle circuits to be clogged in some fashion as they do it at the drop of a hat on these. The idle fuel feed restrictions are almost always clogged. '81 may or may not be screw-in pilots, the earlier ones are pressed in and much harder to guarantee clean as you cannot get to the air crossdrilled holes in the top of pilot without the jet pulled and in front of you. The actual fuel restriction can be cleaned with say the smallest string on a guitar, or E string, around .013" diameter, the hole size of the jet there (a #35 pilot).

Old aircut valves can leak to affect idle too.

Valve clearances affect all around running too and the vast majority of these never get them set right again, why you can buy one and knowing that get it running right to double your money in a few days. Screw the Honda OEM spec, look for .005" minimum or risk holding the valves open. For a fact these can burn valves set at .003" OEM recommended, the cams move around in the heads more than the clearance at valve when running and what you get for clearance is never a real world number. The cam hole can be up to .006" clearance, and the cams move around up/down due to that and the neighboring valve springs pushing them around in the big hole. Another reason to open the spec up is that unlike most engines these usually CLOSE UP gap with wear rather than get looser as most engines do. The valves are only nitrided and the seats commonly wear more than valve tips and opposite most other engines.
 
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