engine miss mystery

Walt - I'm not sure which wires you cut; maybe a more detailed explanation might help.  Past that, you might want to trace both of your "P" leads and look for signs where they might be shorting out.  If it happens only under power (run ups and taxi conditions are fine) then it very likely plugs or plug wires.  If it happens during run up, maybe an un-cowled night run up with somebody standing clear will be able see sparks (if it is in the wiring). A really long shot - water in the fuel?

I have cut back the HT coil leads 1/4" at the plug caps in the past. a few are too short to cut back any more. I have new 7mm wire and caps to replace all that coming. 

My concern is it is missing on both ignition sides which eliminates a single wire or plug or even an ignition module. In my mind it points to a problem with a cylinder but I can't make sense about that either.

Walt,

Before you make any further adjustments/start paying out $$ do what Darel suggested "....water in the fuel?".

Small amounts of water may produce symptoms like you describe. Checking is a simple no cost/loss action that will eliminate this as the cause.

Does your aircraft have wing/fuselage fuel tanks and where is/are the fuel caps located?

Do you:                       ;

      Get your fuel from the same supplier most of the time?

      Use a filter funnel (with water trap) when fueling your aircraft?

      Ever use an ethanol blend fuel (eg E10)?

Is your aircraft housed in a hanger or parked outside?

If in a hanger, does it "rain" condensation from time to time.?

If outside, has there been significant rainfall over the period of concerne?

Walt,

I have had the same problem on my 912ULS , with misses over the past several months. I took it to Lockwood and had all the rubber replaced including fuel lines, spark plugs replaced, fuel pump replaced, etc. When I left Sebring I still had the problem. Then, a few weeks ago I flew to Leesburg for a pancake breakfast, early in the COOL morning, and I had no misses. However, when I waited in line to takeoff and flew home near noon, I had many misses. My flying neighbors and I suspected I was getting pre-ignition due to overheated cylinders, as I observed high Coolant temperatures. I cut additional holes in the cowling to provide more air to the cylinders and with lower coolant temperatures the missing problem disappeared!

Jack

Hi Sean, Darrel,

The airplane is a RV-12 with the fuel tank in the fuselage. The fuel is E10 from the same Costco for the past 4 years. I do 4 random tests on when I bring fuel in to confirm the ethanol percentage, This last lot was 8%. I bring 37 gallons to my hangar and pump it into the airplane at the hangar. My pump is electric and filters through a micron filter and water trap. We have had recent rains but no condensation in the hangar. The hanger does not leak and the airplane fuel cap is sealed. The tank vent is under the fuselage. The plane is not flown in rain. You can see the pump system in my signature line.

I just went out and checked the fuel collator and no signs of water, that doesn't mean it wasn't there before. I would expect water in the fuel to make the engine stumble and and effect multiple cylinders, my symptom feels like a single cylinder short pulse.