Lycoming IO-540 rough engine

[Jorge Martins]

I am flying a PA32-300 with a Lycoming IO-540 that gets rough engine when leaning.
The engine runs properly on the ground during taxi and engine run-up, however when cruising and setting the cruise power with the proper leaning, the engine gets really rough. If I am doing a 65% cruise and even If I lean the mixture to the 75% cruise range, it still gets rough. For example, for a 65% cruise the fuel flow should be 15GPH, but the engine starts to get rough even at 20GPH. It goes normally only above 22GPH. The cylinder compressions were tested and the injection system replaced, but the problem continues. Any idea on what this could be?

[A and C]

My guess is that you have a partly blocked injector nozzle.

Run the engine at the power setting you are having the problem with an ( with the cowlings off) check the temps of the exhaust pipe on each cylinder with a laser thermometer.

This should locate the cylinder with the problem, then get the problem injector ultrasonicly cleaned.

[FerrypilotDK]

That was a good hint A & C and the likely cause. We took the injector lines off, had small glasses and ran the boost pump, comparing the amount of fuel in each glass to find the same fault.

One that really drove people crazy, was a broken inner exhaust valve spring. Fine at idle, then a growing vibration to a real shudder, dropping away and then coming back in a pattern. If your vibration or "rough" running has this sort of pattern, and the injectors check out, you could check this as well.

Good luck