jettstarblue is correct on the crankcase breather(bottom nipple)..
Just leave it unhooked, Or if you have a cowl, you can hook-up a
tubing to outside of the plane.. At least thats how it reads on my
manual for my Mag 91 4s I just leave my open..
But if I read your post correct, You have a thrid or forth nipple
counting the muffler?? My mag has only three nipples 1: intake
by needle valve. 2: crankcase breather on the bottom of engine
and the pressure on the muffler.. So if you do have the extra nipple
I guess it could be as Jim said... But... If it does not plug if just
left open.. Why would you want to return "junk" to the engine..
Thta's just my thoughts though..