I just rebuilt my 20hp briggs 461707-0145-E3 engine along with the carburetor that came on it. This engine is for my LT-1500 craftsman lawn tractor built in 2000. I could not get it to run unless I poured small amount of gasoline in carb but it ran nicely until the fuel burnt out. I found that a gasket was backwards in the fuel pump assembly. I corrected that and it cranks up now but backfires terribly, pop pop bang! What could be wrong? I noticed with the new flywheel key in place the flywheel could be moved back and forth a small amount. before I torqued it down would this cause the backfire? Do you think the carb could be off a little or would the carb cause a backfire? It seems strange that it runs nicely when I pour gas in the carb and get no backfire. There are no loose wires that I can see. Can any one tell me what to check next. Your help is greatly appreciated.