I changed the spark plug - cleaned the K & N air filter - changed the coolant & changed the oil & the K & N oil filter on my 400. It runs fine now ever since I did all the maintenance on it. I just need to buy a new exhaust shield. The old one just fell off somewhere. The only thing I don't like is having to loosen the exhaust bolts & move the exhaust up to get to the oil filter that was just stupid of Yamaha to do that. I might buy an aftermarket exhaust one day. If I do will I have to change the jetting since I have a K & N filter also on it.