Always keep in mind that all Y-S 4 srokes use the crankcase as a super charger. Under the valve cover is used as a plenum. So the .91 has two plenums. The air chamber or box on the back and then under the valve cover too.

Any leaks in these will cause you many heart aches.

If you take the airchamber off the back, you cannot get it back on properly without raising the jug and putting it back on then set the jug back down with the plenum going onto the carb out let.

Good luck,

jim