What model do you have? 2000? 3000? A/C, F/C?
I have an '80 3000 F/C. It runs real nice, after cleaning the carb. I just put a set of IX plugs in it too. It made a noticeable difference in smoothness. Runs much better now.
BTW, you should expect a little better than 50 MPH top end. I may be able to hit 55 or so on hard pack, or snow covered road.
Still, it's a blast to ride. I just went through the entire sled... bearings, slides, all new shocks, and cleaned the carb. Runs really good, and has been more reliable than my newer (2000) sleds!
