You need to put pressure into the pulse line fitting and block off the intake and exhaust to properly check the crank seals. But if it is floding with oil it is the crank seals. You can try installing a shut-off valve on the oil line but it is only a bandaid, not an actual fix.
Oh, and if I...
Very solse but not back pressure. Back pressure has nothing to do with it, on a 2-stroke it is the sonic wave that scavenges and keeps the air/fuel in the cylinder and the pipe along with the heat of the pipe changes that sonic wave.
So no, don't use 1211 for cases. You want to use ThreeBond 1184, which is the same as HondaBond, YamaBond KawaChem and Permatex MotoSeal. It is used on all 2-stroke cases in the industry and from the factory on most machines. Even the automotive industry has gone to it for aluminum to aluminum...
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Heat, lots of heat on the aluminum housing. Heat and then boil PB Blaster into it, then let it cool. Do this a few times. On the last cooling cycle then get it really really hot and try removing the stud.
Weld a stainless nut on it with a TIG welder and unscrew is the easiest method but few...
Throttle closed, at idle so it is still under adjustment of the idle screw. When you change the idle screw you have to reset the oil pump alignment mark.
The PWC chart lists them both for your model. They are two completely different impellers so the pitch will not be equivalent.
What is wrong with your stock impeller?