n3914n

Update after talking to my A&P. Here is his list of suspected causes, in order of most to least likely... Water in the float bowl. It was raining heavily when I picked up the airplane, and didn't sump the tanks since I was just taxiing across the field. The theory is that water could have found its way into the carb float bowl, causing rough running only when at low RPM. This would also explain why the issue couldn't be reproduced today (no rain). Clogged/contaminated carb idle circuit. Some FOD such as a bit of tank sealant could have slipped past the fuel strainer and clogged something. The fuel strainer was cleaned only 5 weeks ago at annual, but we are going to pull the strainer and verify it isn't contaminated next time we are out at the airplane. Sticking carb float. Mechanic thinks this is unlikely since we couldn't reproduce it, and because he considers the carb to be relatively young (OH'd in 2006). Air in the fuel line. This would be due to the deteriorating o-rings in the electric fuel pump, as @carusoam mentions above. Mechanic thinks this is unlikely due to the severity of the roughness, and because I've known about this leak for the last few flights that went off without incident. We ruled out ignition problems since I had passed a mag check at high and low RPM, and we ruled out a stuck valve since the roughness did not smooth out at high CHT. In the end, my mechanic recommended I continue to fly but to keep an eye on things and perform a idle check at every run-up. If I have any issue at all in the future, we will know its very likely a carb issue and begin the overhaul process. And we're going to get the pump overhauled as well. Thank you @carusoam, @Skates97, and @Gagarin for your insight. I'll add to this thread if anything else happens.