I am having acceleration problems with my 1992 TRX 350 4x4. I just bought it and when I rode it the other day the first 10 minutes were great but then it lost all acceleration, especially in the upper gears. It will rev and accelerate somewhat in first gear and a bit in second gear but there is no accelertation response in 3rd or 4th gear. I checked the air box and everything is clean in there, no blockages. I replaced the sparkplug and changed the oil. Please any help would be great.