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Im having trouble starting my JS 550 and it backfires? Please help…

Ok here is the deal I am totally new to these kind of skis/motors, I’m going to give as much detail as possible. I just bought a 1984 js 550 a few months ago and when I first took it out to ride it, off the bat I had a problem with the engine, I had to feather the throttle to get it up and going into its higher RPMS, then it ran fine. So I looked further into the problem it had in the lower RPM’s and was told it was a vacuum leak in the case, causing low vacuum to the carb, not getting enough gas??? So with not knowing the past condition of the motor I figured I’d ripped it down to the bottom end. So from taking it down to the case it looked like all it needed was, rings, honing, a pressure jet cleaning, a full gasket kit and lower end seals, witch I was hoping it would take care of the low RPM vacuum problem? HAHA…Nope.
Ok so when I put it back together I did everything that I thought I should do… Using the right sealer 1211 bond (grey in color) applying it all around where the case splits, around the outside of the crank seals, on the base of the cylinders and around the head. So I got the whole motor torqed and back together and back in the ski, all lines hooked up to where they needed to be and now it won’t start… it backfires out of the carb and exhaust…WTF is going on?
I’ve been reading saying the flywheel key may have sheared, it seemed to be fine when I was putting it all back together. So I figured I should time it, I put the front cylinder at TDC and put a mark on the top of the output shaft of the crank and a mark on the case, lining both of the lines up I cranked it over with a timing light like you would do to a car and the line was on it with the mark on the case!? Now if there is another way to time these motors please chime in, let me know. Seems right to me, I took off the little plug on the flywheel housing, im not sure if you can do it that way as well?
Please if anybody has any idea at all, on what the heck is wrong with this motor I would kindly appreciate the feedback, I’m trying to take it out for the weekend.
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