My 400ex is not idling, I have to rev the engine to keep the motor running. Have a good plug,and new
throttle cable. Need help what could it be.

 

I guess that you have tried to turn the idle screw in some? If this does not work, you will need to take the spring off and cut off just a little bit. I had to do this after putting a 440 in mine to get it to idle.

 

I am not an expert here but my 450ES would do the same thing. It could be your carbureator (sp?). There could be some dirt in there or some old gas sat in there for a while. Anyway, something may be blocking the gas flow in there. You may want to have that checked out. Please get someone who knows what they are doing to check it out.

 

i had the sam problem today when i started my 440ex for the first time since rebuild, turned ou to be the throttle cable but yours sounds like its just an adjustment problem, you can adjust the idle with the little screw underneath the carburetor or adjust it at the thumb throttle where the cable goes into the housing, how much was the new throttle cable? i have to order a new one on monday