About a month ago I noticed that my 2003 YZ450f would lurch pretty bad when I held the throttle open. I thought it might be the clutch slipping so I put new springs and plates in. The old ones looked fine but since I had neww ones in hand, I put them , (soaked them in oil for about 30 minutes first). The lurching is still there and I suspect that maybe it's not the clutch at all. (Carb??) When i pop the clutch there's no slippage, problem mostly occurs when I hold the throttle open. (Should note that my header pipe is pinched pretty bad and I'm waiting for a new one). Any ideas?