Thanks for the suggestions. I'd had this problem on another plane with a Pitts muffler and had to plug one of the outlets. However, this one was "trouble-free" until recently. I'm loathe to mess with the idle as the engine responds perfectly when I go from idle to full throttle quickly. I'll try plugging one outlet first and see how that goes. I'll let you know what works as, when I did a search, I could not find anything helpful.