http://parts.yamaha-motor.com/yds/pa...g/consumer.asp
MJ - The link above is the online yamaha parts catalog. Maybe ask Iboats guy to ID which part he is talking about.
either way . . . It's a multiple choice question: :)
1. either the cap is worn out
or
2. the float is busted
or
3. the vent is plugged up causing enough pressure to "pop" it out.
Now it's just process of elimination. You might try running the engine at the dock with the cowling off and watch to see if the top "pops". ;)
You can always use Plan B and rip the oil injection right off and use premix.