Just an FYI, those two screws on the front of the carb are the idle mixture screws. They dont have anything to do with the mixture when accelerating or cruising. The metering Rods and jets have to be changed to richen or lean out the mixture.
But, it sounds like timing to me too. You should invest is a decent timing light since too much or too little timing can harm the engine and you cant tell which side of the fence you are on without one.
-Dan
