Pull the Fuel Line off and turn it on for a couple of seconds to check how much fuel you have flowing, you wanna see a heavy flow of gasoline. If that's good then.... Next I would say check your Float to make sure it's adjusted properly It could be out of adjustment and causing the carburetor bowl from filling up all of the way and the engine is sucking the bowl dry before the bowl can re fill.
Another thing to look for is you might have to small of a Pilot Jet or a clogged pilot jet. Does it fire up after you have pumped the throttle a few times?
I'm going with the fact that it is probably just sucking the bowl dry before it can refill. Look up on YouTube or something on how to Adjust the float level, I Would explain it but it's kind of a difficult to type out and give you an idea of how to do so.
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