My 620 is having some issues with timing and it needs adjusted. It needs more advance, BUT both the adjuster on bottom of distributor and on top of distributor are all the way advanced, if i move it retarded AT ALL it doesn't run so Its right on the threshold of running now. Also even if I had more adjustment my dist is millimeters away from hitting motor where it wouldn't be able to advance at all.. At low RPM the CARB backfires HARD, at higher rpm the exhaust backfires slightly. What do I need to do to get more adjustment? I tried jumping the plug-wires one spot advanced then retarded the dist but then it wont even turn over!! . PLEASE HELP>