If the N&S leak gas or the crankseals are leaking oil, into the lower cases, It may be what is called "hydrolocking". Commonly when people flip the ski and get water into the lower cases. It will take maybe 1-2 minutes of cranking (at intervals to prevent starter over-heating) to get the engine to puke out any liquid. If a bearing has gone south, it may be mechanically locking, when the *plugs are in and affecting the piston's. It also could be the RV gear has stripped. It could also be a broken ring getting caught in the rave. Suggest you spin and wait for any liquid first. Then pull raves, you can at least look at the pistons. Then pull RV cover off as a unit. Ect Ect