Idle speed won't come down

[rcuttler]

I've got a '77 CB750K with 25,000 or so miles on it. I always had problems with the bike starting on only two cylinders but the other two would come in after some warming up. Recently I decide to do something about and pulled the carbs, cleaned them up, replaced some gaskets\o-rings. I also replaced all the air tubes connecting carbs to the motor and air box to carbs, as well as changed out a clapped out air box for a solid one, with good recent use filter. Bike now starts up like a champ on all cylinders. But now I've got a new problem. Once it warms up the idle speed won't come down below some 2,800 rpm. The cables had been lubed in the last year, but I don't see how it can be a cable problem anyhow. The throttle turns freely closed with no drag. The idle speed adjuster is now out of play as I turned the screw out sufficiently that it no longer comes near to touching its seat. I've sprayed around the whole area with ether, getting no reaction at all. The pull cable pulls (throttle up) and the push cable pushes (throttle down). The only thing left I can think of it the problem is in the bell crank the cables connect to, but I can't imagine how that would work, as the first couple of minutes of riding everything is fine. The problem starts as the bike warms up. Any ideas?

Bob

 

[dirtdigger]

Make sure you have some free play in the throttle cables. Can you see the carb slides verify they are closing? have you done a running carb sync? Check timing, verify advance is free and returning to base timing. Make sure the choke high idle isnt hanging up or out of adjustment that is keeping the idle from coming down. Have you tried adjusting idle mixture? Lean will cause high idle as well.

 

[rcuttler]

Some good ideas there I haven't thought of. I'll check the timing and the choke functioning. I haven't done a running bike carb synch but I did do a bench synch, and though not as good it I got it pretty good to the naked eye. I'm pretty sure I set the air\fuel at 1 1\2 turns out. Even if one carb is a little lean I don't see how it could have this much of an effect, and I can't see how multiple carbs are set very wrong. Thanks for the info.

Bob

 

[UK K6]

I don't know the later model's carbs, but I recall some had two idle speed screws. One adjusted the idle speed when starting on the choke. I think called 'fast idle'. The second was the more traditional idle adjustment when warmed up. If yours are like this, are you adjusting the correct screw?

Otherwise, incorrect slide heights would be my guess. This can happen if you've removed the carbs from the gantry, especially when you go into the top of the carb for needle height adjustment (if yours are built that way). If you have, also check they're seated properly in the gantry. Also check they appear to be in synch whilst on the workbench, that helps make vacuum synching simpler.

Also, check for any air leaks, especially the rubber inlet manifolds and the vacuum synch screws (the washers can get a bit worn).

 

[dirtdigger]

you have to look at it as the engine is 4 individual engines since they have their own carb. Bench sync in reality is pointless, each cylinder needs its own amount of air due to how well it seals mechanically. If one cylinder has less compression or rings are worn more then it will use less air then one that is not worn as well and seals better, this is why you do a running sync and adjust each cylinder to what it needs for air via the slide position. Same thing with the mixture screw, you cant set them the same and expect it to be perfect for each cylinder. Each cylinder will need to be adjusted for what it needs to run correctly. Next to air leaks and bad carb sync the ignition advancer not working correct is the biggest thing that causes a hanging idle.