Oil tank on engine

I have a 1986 150h pro v Yamah

I have a 1986 150h pro v Yamaha engine on the back of my 373v Ranger. The Plug that is uesd for the cap on the oil tank on the engine keeps poping up and allowing oil to flow out all over the engine and in to the water.Any ideals why this is happening. It has to be building up pressure or has more oil coming in to it then should be to make the plug push out. But why? Any ideals are welcome.

u have 2 check valves on that

u have 2 check valves on that motor
might check those
also look at the plug there should be a tiny pin-hole for a vent

"the cap that keeps popping ou

"the cap that keeps popping out is very special. oil is over flowing because you have A PROPLEM .
that cap is a sender that turns the pump on and off through the cdi via a ribbon cable to oil pump on main tank. you see what is going on here is the pump wont shut off when the little tank is full. ALSO on the end of that cap is a nipple there is also a foam gasket that seals that cap which is also a filter make shure it does not come off and plug the hole you will loose your engine , also cap has arrow bottle has arrow must be instaled with foam gasket on end and line up arrows before seating. there nis a small hose that comes off the oil bottle and goes to the air cover that is your vent, should not be plugged"