ORIGINAL: Ed Cregger
Either your muffler pressure nipple is clogged, your fuel tank vent line is not permitting enough air to enter the fuel tank as the fuel burns off (check for crimps), or the fuel is bubbling in the fuel tank when the engine is running and is causing the mixture to go lean. It could be something else, but I would check for these things first.
Ed Cregger
Thank you. I've checked all that except the muffler nipple. But I will add it all to my list of "stuff to check"