There's a thousand things that it could be, but it sounds like some kind of fuel related issue to me. Since you have spark, and air, then the only other thing it can be is a gas delivery issue..so it's either too little or too much gas being delivered to the engine. Did you pull a spark plug to see if it was wet? That would indicate a flooding condition. A dry plug would indicate a vapor lock/fuel starving condition. This is all just backyard mechanics thinking of course and I'm sure that others here can help you better than me... but that would be my first place to look guess. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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