The threads on the idle a/f mixture screw seems to be damaged. There is nothing wrong with the screw itself, the damage is on the carburetor. If I turn the idle a/f mixture screw all the way in, I can only turn it back out 2 rounds before it's completely loose. I'm supposed to turn it out about 2.5 rounds, but that can't be done now. This is on the rear right cylinder. Any suggestions on what I can do, except buy a new carb?