Crank Seal?

Is you polaris a 2 stroke or 4? If its 4 then it wouldn't have crank seals because the engine oil serves as engine oil and tranny oil. All 2 strokes have crank seals because the mixed fuel passes through the ports and goes down around the crank. One way to tell is to do a leak down test. Just do a search and you should find out how to build one yourself if you dont' want to pay your local shop to do it.

Yup, what CR said. If the crank area to tranny area gasket is bad or the last guy that split he case used a screw driver to pry it apart, then tranny oil gets into the crank area and is pulled up into the combution chamber. The gear oil is not made to burn so it leaves a residure that fouls the plug, and if your using regular 90W gear oil, the stench from the white cloud is un mistakable. I was following a guy one day when his crank to tranny seal blew and I never got the stench out of my helmet :| .
I dont think a leak down test would show this though since the piston would have to be below the intake port, then preasureising the crank case (not a good idea) and assuming the reed valves sealed tight.