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Old 07-07-2019, 06:58 AM
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Piper J3 Piper J3 is offline
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 1,818
Default Misfire "A" Ignition...

I asked this question over in the Rotax 912/914 Technical Forum and didn’t get a response... I have an early S/N RV-12 with 912ULS now with 450TT. Carbs are synched beautifully with electronic CarbMate. Synch is good at idle and just off-idle. L&R EGT’s are very closely matched from idle to WOT.

Problem…. Most of the time the engine runs smooth 1800 – 3000 RPM. Sometimes the engine has a miss at this low speed. When “B” ignition is turned off the miss continues. When “A” ignition is turned off the engine runs smooth. So, problem is definitely with “A” ignition. I have changed sparkplug caps to NGK Spark Plug Resistor Cover - 90 Elbow Type (LB05F) and installed new NGK DCPR8E Spark Plugs gapped to 0.025”. The new spark plug caps improved COM radio noise but didn’t fix misfire. I thought maybe problem is temperature or moisture related but that doesn’t seem to be the case. The misfire almost seems like induction/cross-talk between the high-tension wires. Engine always runs smooth at 4000 + RPM and extremely smooth at 5500 RPM cruise. Either ignition can be shut off at cruise RPM and engine continues to run smooth.

Possible next steps:
  • Replace high-tension ignition wires (7mm copper)
  • Maybe there is problem with magnetic timing sensor?
  • Maybe timing sensor gap is incorrect?
  • Maybe one of the new spark plugs is bad?
I have an oil change coming up in 20 hours so I’m trying to get ducks lined up for when I pull the cowling.

Any suggestions appreciated. Thanks in advance…
Jim Stricker
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 542

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
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