1993 Waverunner 3 revs but doesn't go

I have a 1993 Waverunner 3 650, the last time I took it out it was running fine then it lost power, I cut it off turned it back on and it starts fine, idles fine and goes up to 1/4 throttle, after 1/4 throttle it just revs and doesn't go any faster... it is almost like it kicks into a neutral (i know it doesn't have one but that is how it acts). Do any of you have any idea what the problem might be. I got the ski last summer and took it out maybe 5 times never changed the plugs and never cleaned the carb. Any help is greatly appreciated