I had the very same issue with my 1993 SPX.
I chased the problem for about three weeks. There are several things you can check. First, do you still have the grey fuel lines? If so, they can break down and spread gunk throughout the entire fuel system. If the grey lines are still on the machine, buy some 1/4 inch automotive fuel line and replace it all. Then your going to be faced with having to remove and clean the carbs. If the grey lines are not on the machine, try switching your fuel selector to RES and see if the problem persists. If that fixes it, you have a plugged fuel filter. If it doesn't you may have a plugged fuel selector. Pull it off and spray some carb cleaner through it in both positions. The carbs also have internal filters. The internal filters may be plugged. You may also have a bad check valve inside the carb. You have to pull the plate, diaphragm and check valve assembly. On the back side of the check valve assembly, there is a clear piece of plastic that covers a hole in the valve body. If that little piece of plastic is torn, bent or has a hole that's your problem. I just found aa bad check valve in my ski. I'm not sure if this is what is wrong with yours, but thats what I found.
I took it from the beginning. Started at the tank and worked through the entire system. Get a manual (they help a lot) the forum is also a great place to get some help. There is a LOT of knowlege floating around here. Best of luck to you!
