I just bought a 2009 Troy Bilt Pony & it needed a tune up. I bought it because it was being sold for $250.00 so I figured with it being in good shape for the price I'd try fixing the issues. I've replaced the air filter, fuel filter, oil & spark plug. I have it running rough while it idles. Is there any good things to do to eliminate this issue. What additives might work & what else should I try.