I had no real idea what was happening. The engine started just fine and the run up test was fine (I do my run-ups at 1800 RPM). At first I thought it could be a sticky valve, however, I pulled the prop through all 4 compression strokes on the engine and I had good compression all around. We took the cowl off of the plane to see if anything looked off - sensors disconnected, exhaust crack, etc.. Nothing.
I went to start up the airplane again, and it started right up and hummed along just fine. Run-up was fine too. Was this a fluke? Did the issue work itself out? Knowing what could happen again, we attempted another takeoff, but I planned to put her down on the long runway if anything sounded off. Sure enough, the engine made popping noises when I firewalled the throttle.
I taxied back to the ramp and did some static RPM run-ups. Anytime the engine was over 80% power, the popping noises would occur. I also started looking at my engine monitors. The EGT for the #3 cylinder was VERY high compared to the other cylinders, indicating a lean condition. Ah ha! I must have a clogged injector on my #3 cylinder. The lean mixture was causing the EGT's to be high. Also, the mixture must of been so lean under high engine load that the combustion stage was still taking place when the exhaust valve opened, causing the popping noise...or perhaps it was unburned fuel burning in the exhaust? (Experts?)
After discussion the issue with a few people, everyone decided it was safe to fly her back home (20 minutes) with a reduced power setting, but keeping a close eye on the engine.
Here are some screenshots of my engine in different phases of flight:
72% power, low CHT, high EGT.
51% power, higher CHT (in comparison), high EGT
The engine monitor graph shows the landing cycle, where the #3 EGT comes back in line with the other EGT's, but the CHT is still elevated:
When I removed the injectors, there was a visible piece of crud in the #3 injector. I removed all of them and cleaned them with Hoppes Gun Cleaner, followed by letting them sit in a combination of hot water and soap in an ultrasonic cleaner. I reinstalled all of them and the engine runs perfectly now.
My lesson learned from this is to keep an eye on the EGT's on the engine run-up.
