I recently bought a 455 and it runs great.....once it gets past the hunting. The engine hunts on cold starting. It will do this for about 20 seconds and then smooths out and runs fine. It will also not respond to the throttle when it is hunting. Once it is warm you can turn it off and restart it without hunting. This has no other effect on the tractor running at all as it runs great and doesn't smoke but the hunting is really annoying. I have replace the fuel sending unit, all the rubber fuel lines, cleaned the tank and tested the fuel pump according to the manual and it checks out fine. It seems like I have a fuel delivery issue or air leak in the fuel line that I can't find. Any suggestions?