I just installed a 102.4mm piston (and bored XR650L cylinder) on an otherwise stock XR600. It makes for about 660cc, so about a 10% big bore which is not outrageously big. I'm running a stock muffler, stock cam, stock air box (with foam filter), etc. I made no other changes than the piston/cylinder and redoing the valves. I was running a #65 pilot with the standard bore, which was slightly rich but ran well. With the new big bore it would barely idle and I could bottom the idle mixture screw with the engine still running. I tried a #62, a #60, and a #58, all of which would still idle with the mixture screw bottomed. I verified that the choke is off. Big bores generally require leaner jetting, but 4 steps leaner on the pilot jet seems a lot. Does anyone have a suggestion?