RPM Sticking around 3000

[Vgm]

Hi Guys,

I've been fixing up a 75 cb750 thats been sitting for almost 2 years, garaged but still sitting. I dunno how much of a difference it makes but I also believe it has a 78 cb750K engine. Heres the problem I finally got the bike to idle on its own however if i hit the gas the rpms jump and when the fall they stop at 3000 sometimes they will fall back to idle but typically they will stay there. Also while idling for a periods of time the idle will increase on its own until about 3000 and again typically will stay there and not come back down. I've taken the carbs apart more then i'd like to count and they're clean. Doe anyone have any ideas? Thank you either way.

[Re-run]

Usually 2 things cause this. Either there is a air leak around the boots between the carbs and engine, or the slides may be sticking. Can you manually close the carbs and drop the idle?

To check the boots, spray wd-40 around the boots and if the idle changes, you found it.

[Vgm]

I opened the tops of the carbs again and the slides arent sticking and I sprayed wd-40 on the boots but nothing happened. IF I cover one carb it will sometimes drop but not always. Anything else that it could be? Thank you for your help.

[Re-run]

Well the 2 main causes of high idle are air leaks or the carbs hanging open in some way.
So lets look at the hanging part first.
First have the airbox off. Now check to see the choke is fully opening. If you have a 78 engine, you might also have 78 carbs. Do they have an accel pump? If so, that model of carb has a fast idle adjustment. if the choke is not opening all the way, then it might be triggering that fast idle which is around 2800 rpm or so.
If you have no accel pump, then you do not have this BUT you may have a throttle stop knob under the throttle. If you have this knob, turn it out and see if that helps.

next, are you running 1 throttle cable or 2? How easy do they move? If they are old or dry, then the cable could be binding really bad and not allowing the slides to close all the way. The cables may also be mis-adjusted. That could also cause the slides to not close.

Another thing to be aware of. your bike will not idle as high when cold as it does when warm. So you may have the idle knob set higher for cold starting and then you idle higher when warm. Try turning down the idle knob when the bike is warm. Set idle for when warm. If you have it, that throttle screw under the grip can be used to hold idle when warming up from a cold start.

Finally, if none of that seems to help, pull the carbs off the bike and check to see how far open the slides are in the front(not the rear or airbox side). If they are like half an inch, then my guess is something is holding them open somehow. As you will have probably disconnected the cables to pull the carbs, it is something internal then. If the cables are still connected, remove them. If the carbs close more, then it is the cables.

After this, check the boots to the engine. Look for cracks and splits. They should flex some. If not, then they probably need replacing BUT you could try boiling them in water and wintergreen oil. Yes you heard me! Put the rubbers in a pot of water deep enough to cover them and add 1 - 2 tbsp of the wintergreen oil. Get the water boiling first. Apparently this is supposed to work.
I never tried it but never heard of anyone melting the rubbers.

If you got cracks or splits, then new ones are needed.