my freind bought a 98 zr500 and when we test drove it , there was no bottom end and backfiring out the exhaust. we found the pistons were very worn ,they actually had a ridge along the crown.we put in new pistons, cleaned the carbs, checked the reeds everything was ok . the machine has a very bad hesitaion on the bottom end .if you pin the trottle it almost dies before it revs up.the engine seems to idle fine so i dont think the crank seals are bad ,also the old pistons showed no signs of running lean. one thing we did notice was that the mag side carb has a lot less vacum to it if you put your hand over the bell .we checked the fuel pump and pulse line and they both hold a vacum so the only thing i could think of is a leaking crank seal; on the mag side. any ideas. once the engine is reved up it seems ok but we havent driven it yet under load. could a bad stator cause this?