Smoking like crazy, could be your inner oil seals gone bad. The 2 lines that go to the center of your engine ar to lube the cener bearings. There is seals on either side. If one, or both are bad, then it can injest ALOT of oil, and that wil make the sled run realy bad. One way to test, drail all the oil form the oil tank or just plug the lines, and run some premix. Easier to run a line form the fuel pump to a Jerry can with mixed in it, bypassing the tank. Drain all the straight gas out of the carbs too. Then, fire it up and see if it runs better. Only do this on the stand, and for a short bit. Your center bearing wil only have a little bit of oil in it, and if it is leaking there, it will suck it out. But, if you can now rev up on the stand past 4500, you know the problem.