Very High Rpm At Idle..help!

[MJMiller86]

My motorized mountain bike idles VERY high like its going to blow up when I idle. It seems to do it mainly if I give it give it a little gas when idling. I tried replacing the carb with a "rebuilt" spare, replaced the fuel cable with a brand new one, and took off the twisty throttle and put a squeeze handle on instead. It still idles high as ****. The carb is on all the way and tight, haven't really checked for tiny cracks in the motor for an air leak, but I scanned it for big cracks and nothing showing. I even tweaked with the air/fuel knob, choke, and all that and nothing is working. I hope it's not a cracked motor.

 

[2door]

If you're talking about a 2 stroke engine (wish people would tell us what they have when asking questions) There is no air/fuel mixture 'knob'. There is a small screw, left side of carburetor, the one with a spring, that is the idle speed screw. It does nothing but block downward movement of the throttle slide or barrel. Screw it in and the idle speed is increased. Screw it out and the idle speed is lowered. If this screw has no effect on the idle speed its a good bet you have an air leak somewhere. Start with the intake manifold gasket then go to where the carburetor slides onto the manifold. Those are the two most common areas for air (vacuum) leaks. Replace the kit supplied intake gasket with good quality gasket material sealed with a good gasket sealer. Also the slots that you can see in the throat of the carb where it slides onto the manifold need to be filled/sealed with a good sealer. Give these a try and get back to us with the results.
Tom

 

[kutsuda]

sounds like your cable is sticking or you have an air leak it seems 99% of the time when this happens people have an airleak around the carb most of the time its where the carb slides on to the intake manifold as stated above you can get some good gasket maker/sealant i recomend permatex black or the high temp red rtv take carb off and right where carb slides on smear it neatly around making sure not to get it to close to the intake area replace carb wait to dry then try it if it still revs high check idle screw also another way you could identify a airleak is after engine is warmed up turn choke on engine should die immediately if it doesnt you most likely have an airleak somewhere

 

[Easy Rider]

A quick way to check if its an air leak it start the motor and spray carb cleaner around the carb. If the engine starts to rev higher than its an air leak. This is a good way to pinpoint where the leak is coming from.