I have a 98 yz400. As far as I know the engine is stock. I have put a wirtz acc pump cover on the carb and uni air filter. I didnt see much of a difference. I am looking for more bottom end response and power. The bike is also very unpredictable( not smooth through the rev). Any advice or suggestions would be great. One more thing, Any good tricks or tips for keeping it cooler. It likes to overheat on hard hills and rocky areas.