I have been searching the forums and "how to's" for adjusting the air flow Screw and cant find my problem anywhere..........maybe You can help.
here is the issue. stock jetting in carb,...but it only runs Smooth when the rich mixture screw is CLOSED. I can maybe open it a 1/2 turn before the motor slows down. Whats going on ???
Air leak, to rich , to lean or carb issue inside ??
I have removed and meticulously cleaned the carb. It has been apart before.

 

What bike? (Year and model) Stock jetting in a stock carb? By the sounds of it, your idle jet is too large but can't say for sure till you give more info. YD

 

I agree with Yankee Dog, but it may be that the needle tip on your mixture screw is broken as well. Happens when you over tighten it. That would prevent the fuel from being metered properly. Or your slow jet came lose and fell out. (Happens more than you'd think)

 

Check your float level. If it's too high it will make the bike run too rich. Low speed jet should be a 42 or 45 on an EVO.

 

Sounds like it may be running rich to begin with. I would check the air cleaner. Is it an aftermarket air cleaner? Is the air cleaner dirty? A dirty air cleaner may obstruct air flow causing a rich condition thus not allowing for the adjustment of the idle air screw.