Don’t worry about idle, they don’t have to,go back to the first settings, change the plugand as others said, throw a carb kit at it. the pumpers will stretch. The engine surge you mention may be an air leak, you may want to pull the motor off, take off the pipe and carb and dunk it in a tub to check for bubbles. I once had a leak around the spark plug, thin copper gasket and life was good. If its running at all you’ve got enough compression to get it on the track running, but the best idea may be sending it to a shop. you would be surprised how inexpensive builders can be to fix minor problems. Don Grainer and his crew at L.A.D. have seen about a billion yamaha’s (including mine) and I’m sure they could help. Hope you get it running.