Ok so I just picked up a 650sx and kinda got it running. Took it to the river for a spin. My 14 yr old son had to ride it because I broke my collar bone. So he took off from the ramp and sounded/look good. As he hit it to build some speed it looked ok, when it hit the upper part of the rpm it kinda fell off and didn't sound right. After several trips back to the ramp and many adjustments to both the low and high speed Jets in 1/8 of a turned at a time in both directions from stock ( low 1.25 out high 5/8 out) it didn't seem to make a whole lot of difference. When we got it home I got to thinking that maybe it's not getting enough fuel so I took the return line off and started it up little bit of fuel started trickling out of the return line after about 30 seconds. So I decided to plug the return line with my finger and hit the throttle and it sounded like a new machine it revved up nice and sounded very crisp. So I plan on rebuilding the external fuel pump but I think I saw on one of these threads where they were recommending putting a jet in the return line to limit how much is return to the fuel tank. This ski has the stock cdk carb on it. I don't know if that is the carb that they were referring to. If it is what size jet do they use? In the mean time I ordered a fuel pump rebuild kit but I'm not sure that's going to fix it