I have an old 5hp B&S, MTD lawn mower engine that hunts. It's not real noticeable, but having repaired a number of them, it kind of drives me nuts. I cleaned the carb and blew out all the openings. I replaced the gas line, spark plug and air cleaning element. It doesn't hunt as much as it did before doing the cleaning job. The needle valve adjustment does make a difference, but I still can't smooth it out. I'm beginning to wonder if the needle valve is just worn out?
Thanks

I had a couple of mowers doing the same thing,does this mower have a idle speed screw,if not I put a screw in the hole provided with a spring on it to have tension on the screw.I screw the screw in just enough to let it touch the throttle to stop the surging.My guess it has a worn throttle shaft.This works for me.Hope this helps.

Jerry

Thanks for the info. I suspect that you're right considering the age of the mower. It runs good enough to cut the lawn and so I'm not going to spend any more money on it.