My lawnmower has a 3 1/2 hp Briggs vertical pull.. Won't start on it's own, but will start if I take the aircleaner off & squirt gas into the carb..I put a new diaphram into the carb..after it starts, it'll run for under a minute,I adjust the needle to where it runs best.. then backfires & quits. It won't start on it's own after it quits, I have to reprime it again..
John

 

Anyone, with a remedy??? would a partially sheared key do that, or is this purely a gas related problem??

 

With a vertical pull starter, this tell me that it is older since these haven't been made since the mid- to late-eighties and with such you may have an engine that is just plain old, old. The first test I'd make is a compression test then an inline spark test and then go from there. Post back with the engine's model, type and code numbers as well as the results from the two suggested tests.