I was cleaning up my 10 Y.O.'s 70 last night and adjusting the chain, and noticed a slight Pop on throttle down or down shifting. I tweaked the mixture screw to find start and end of highest idle, then set 1/2 way between. This setting is about 1 3/4 turns out, and it helped, but still has a little pop. Any ideas?? Only mod was to cut rear 1/3 of air filter ball off.