5 hp B&S horizontal shaft, siphon carb. ,first the troybilt tiller was ran on varnished fuel-exhaust valve was stuck . no compression do to stuck open valve. Fuel tank had loose rust in it. rebuilt carb and and cleaned fuel tank out . replaced head gasket after getting stuck valve loose.cleaned head etc. replaced air filter also, started tiller, tilled a while with it ok- next day started again and it ran 15 second and died-wont start. broke pull rope fix it . tried choking no good. back fires thru carb if choked .have spark and fuel and compression. what happen?