Topic: Idle problems?

[afkrejci90]

I have an 80 CB750 Custom, when it starts it idles fine, but whenever it warms up it winds up to 5000+ RPM and doesnt come back down until it goes into gear.  Any ideas on what could be the cause?

[grcamna2]

You may have a spark advance unit that is stuck..or dirt in you're CV pistons on the carbs.

[afkrejci90]

I dont really know what the spark advance unit is or what you meant by the last part of your post.  Does it have anything to do with the points on the end of the crank?

[grcamna2]

I dont really know what the spark advance unit is or what you meant by the last part of your post.  Does it have anything to do with the points on the end of the crank?

Yes, The spark advancer unit is behind the points breaker plate on the end of the crank;it's a good idea to remove that points plate and remove your advance mechanism and make sure it is moving correctly.The little weights fly out when engine speed increases and the springs return the weights when engine rpm slows down, it should move easily & work smoothly.They get stuck in an advanced position sometimes and keep the engine rpm too high at idle.
The last part of my post is the carburetor slide mechanisms..one in each carb.
Try posting some pictures of your advance mech. when you have it out and it'll be easier to explain.

[afkrejci90]

heres the advance

[afkrejci90]

The advance seems to work well, its not stuck.  After firing it up it remained at an idle for longer than it usually does, until I took it around the block for a test drive.  Back to the same problem.  The carbs are a total mess, gas slowly leaks out of them so I am going to get some rebuild kits and take a shot at it.  I also checked for vacuum leaks around the boots and they look like they are tight on the manifold nor cracked.

[afkrejci90]

I've rebuilt the carbs and cleaned them thoroughly.  It starts up with no hesitation, but now gas pours out of the drain holes ( I've left the hoses off for ease of removal), and now smoke pours out the exhaust.  Is the mixture off or too much fuel in the bowls?

[TwoTired]

The carbs have a float which rides on the fuel level in the carbs.
The float arm also actuates and float valve to close off the incoming gas when the float reaches a predetermined (set) level.
Part of carb refurbishment is cleaning those parts, and checking they shut off level.  If the float doesn't shut the fuel off properly, the carb overflows through the bowl stand pipe.  And if it comes in too much too fast, it overflows into the intake runners, past the intake valve, past the piston rings and dilutes the oil supply.  If the engine is running during the overflow, it goes out the exhaust, as well.

Not that any of this applies to your bike, because you told us:

I've rebuilt the carbs and cleaned them thoroughly.

[afkrejci90]

Not that any of this applies to your bike, because you told us:

I've rebuilt the carbs and cleaned them thoroughly.

Never said I rebuilt them correctly, but thanks for the advice.

[afkrejci90]

These carb floats are plastic and non adjustable.  I checked them anyways and they are almost spot on where the should be.  Since this isn't a float issue, where else could the problem be coming from?

[w1sa]

The float valves may not be seating correctly enough.......... to shut off the fuel at the correct level.

Did you clean/replace the float valve/seats when you did the carbs...?
 
Some other things that affect fuel flow in the carbs is sh1t from the tank/lines....clogs jets and sticks float valves open.    Did you check/clean the tank/tap....? Tank/tap filter in place...? ...Also,incorrect operation of floats due to leaks, binding at pivots etc, and /or incorrect installation.

A good way is to overhaul/clean/set and static test on the bench....When installed use the clear tube method to verify float operation and fuel level correctness.

[lucky]

First check the rubber diaphrams on the carb pistons. If they have any tears in them it will never idle properly.

Any kind of vacuum leak will cause the idle to be high, or lean or both.

[afkrejci90]

I've got the leak stopped, apparently a good whack to the bowls fixes that problem.  The intake boots are pretty solid and its hard to get the carbs into them, so what I might have is a vacuum leak which is causing the bike to maintain a 3000 RPM idle.  A new set of boots or refurbishing this set is in order.