help! can't seem to get my cub going ran fine two weeks ago,went to start it no go, maybe 20 sec died.went to start it again turns over fine check spark had it. check fuel had that put my finger over spark plug hole seems ok but won't have a gauge till tues when i return to work. but pulled the head every thing looks fine.valves move up and down nice.put it back togather and all i get is a big back fire out of the exhaust once in a while not all the time.pulled the carb every thing looks ok.check the ignition switch did not see anyting wrong,but had a new one so i installed that.still no changei even change the points & condeser, anybody have a answer