Welcome to the forums. You can check the change clutch by measuring the thickness of the clutch discs. You can check the centrifugal clutch by measuring the thickness of the linings on the shoes and also measuring the inside diameter of the drum.
When you installed the change clutch, did you make sure the clutch discs were held tightly together in the basket? Sometimes when you install the discs and plates, one of the plates will hang in the basket and it will keep the clutch springs from holding the discs and plates tightly together. If this happens, the ATV will usually jerk like it's wanting to go, similar to what you described.
One more question, at any time, did you mess with the clutch free play adjustment on the crankcase cover? If so, you might need to adjust it again. To adjust, loosen the lock nut and slowly turn the adjusting screw counterclockwise until you feel resistance. Then, turn the adjusting screw clockwise 1/4 turn and hold the adjusting screw while tightening the lock nut. If you didn't mess with the clutch free play adjustment, it should be OK.
I would begin by pulling the crankcase cover back off and checking to make sure the clutch disc and plates are held tightly together in the basket. If they are loose, take them out and reinstall them. If they're tight, I would measure the linings on the shoes of the centrifugal clutch and also measure the inside diameter of the drum.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.