You want to go check over in the 1/2A forum for this engine, sounds like your tank to backplate gasket is bad, you;ll have to remove the engine from its mount to get to the backplate screws, they may only need to be tightened...if it runs for seconds then you are burning off the prime and your reed is gummed up or stuck,,, the valve ,,close it and then turn it out 2 and a half turns to start it......you may be able to take it apart and clean it or it may need a rebuild kit...Rog