Ski-doo 1980 377cc and 503cc both bogging?

[Ski-Doo Man]

I got a 1980 Citation 377cc free over the summer. Cleaned it up and took it on the trails... would bog down and backfire out the carb at middle throttle and if I feathered it, would go like a bat outta hell at high speed. Brought it back home, and the next day was playing in the back yard and it started to bog down, and now is smoking like Cheech when I try to throttle up, and won't move, tries to stall. I bought a blizzard 503cc, and have been bombing down the trails for the past 2 weeks with no problems, and just yesterday and today it started doing the exact same thing! Weird thing is, that when I open the hood of the blizzard, it will gain some power again...
Changed the plugs, cleaned the carbs, new fuel lines, new fuel, etc... still no change...
What the heck is going on here?
Are there gaskets for the exhaust on these where the springs hold the muffler to the manifold? Could those be bad and causing no backpressure?