It sounds like a carburetion issue. There are two things I would like you to check. First, check the high speed jets to make sure they are not clogged. To access the jets, both high speed and low, you will have to remove the crescent moon shaped piece that i s held in place with 2 phillips screws(#44 in the diagram). Second, check the dime shaped check valves in the fuel pumps to see if there are creases in them. The creases reduce fuel pump efficiency which certainly could produce your symptoms. You will need to take the carbs off to do this, which also included removing the exhaust. It is quite the ordeal to get the exhaust on and off if you have never done it before. The carbs are directly below the exhaust on the right side of the engine compartment. Here is a diagram of the carbs and what you are looking for. Reference #14 is the high speed jet, Reference # XXXXX is the check valve you need to inspect.
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It sounds like you have carburetion issues or the catalyzer in the exhaust is blown out. You need to remove the carbs, which is a rather involved ordeal, and inspect them. When you take the rear half of the exhaust pipe in the engine compartment off, you need to inspect the catalyzer to see if the center of it is still there. If you see any kind of open hole, the cat has blown out and is lodged farther down inside the exhaust pipe and restricting exhaust flow producing the problem.
As far as the carbs, if it isn't the cat, need to be inspected for restrictions and damage to the fuel pump check valves.
Getting to the carbs on your particular ski is a huge pain. As I eluded to earlier, you have to remove the exhaust sytem in the engine compartment to get to the carbs. It is about a 4 hour job for an experienced mechanic. Honestly it is a bit more than I would reccomend the week end warrior to attempt. I will leave it up to you to evaluate your mechanical expertise to decide whether it is something you want to tackle or not.
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You are most welcome.
Good luck with the repairs!