Vintage Dirt Bike Q & A

I recently purchased a restored 79 250 elsinore on Ebay. It has a brand new 36mm mikuni carb on it. From the throttle to the carb slide it appears to be working properly. The bike starts first kick but screams to the moon. It will only run wide open. The throttle has no control over it. I've raised and lowered the clip on the needle. Only way to stop it is the kill button. The idle screw also has no effect. I checked the reeds which seem perfect. Please throw some ideas my way...I'm baffled .

Check to see if the throttle slide is in backward. The idle stop cut out needs to be on the same side as the idle stop screw. Otherwise, there is a little peg that sticks out of the carb body to line up with a groove down the length of the slide and that little peg will hit the idle stop cutout and cause just what you describe.

Otherwise, my next step would be to pull out the old pressure test kit and find the enormous air leak that is otherwise probably the cause.

Good luck,
Jon

What MRmickelson said. If slide properly in, you have an air leak. I've had them so bad they will diesel at 7,500 rpms even with the kill switch grounding. Have to kick in gear and pop clutch to save the engine. Look for the obvious first: check both sides of carb boot to reeds/cylinder; base gasket; stator side seal.