To adjust the pilot air screw turn your idle up to 1,100 rpm, then screw it in until the engine starts to flatten the exhaust tone and the rpm's begin to drop. Then back it out until you have the best engine sound and the rpm's are at their maximum. Then drop your idle back to where it belongs.

I really do not believe it is a carburetor problem with the pilot air screw creating the flooding in the rear cylinder. If it is a carburetor problem it will be with the float level being too high and overflowing into the rear intake port.

If you are lucky a simple float adjustment will solve the problem. Not usually the case unless someone fiddled with the float. About one time in ten you can replace the float valve and reset the float level and have it work another few years.

Now for the bad news, which I suspect is the case given the age of the carburetor. These carburetors are essentially garbage after a few years as the float valve vibrates in the carburetor body and eats away the cast pot metal float valve seat. The float valve seat is not replaceable. Pull the carburetor off, remove the float and float valve and look for a shiny ring around the seat where the valve has been vibrating. If you see that your choices are to replace the carburetor body with another that will last from three to ten years, or go with a Mikuni replacement carburetor that is fully rebuildable. The cost will be about the same either way.

 

Did you check to see if the other screw was set the same? 4+ turns on the A/F screw seems kind of excessive to me, may have to up your pilot jet size?

Idle drop method to set A/F screws.

Try to find out what kit was installed, what mains, pilots, needle setting, ect.