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Old 01-23-2021, 05:32 PM
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Piper J3 Piper J3 is offline
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,379

I still believe the rough running syndrome when doing initial ignition check at 4000 RPM is carb related and not ignition. Capacitive Discharge Ignition (CDI) is a very hot spark and will operate plugs that are in less than pristine condition.

What I think you’re looking at is carbs running rich most likely from taxing where float bowl level rises when the floats are jumping around. Carb float bowl on an airplane engine is a very violent place. This is especially true on Rotax 4-cyl where carbs are mounted at extreme distance from engine centerline. As the engine shakes the carbs mounted at the cylinder heads move significantly and float bowl level is hard to control.

All that said… When you taxi out and get ready to perform 4000 RPM ignition drop test, the float level is most likely very high and maybe even overflowing the bowl. As you run for a period of time at this higher RPM two things happen – excess fuel in float bowl is consumed establishing a correct fuel level and the engine begins to run smoother because the mixture is leaning and becoming more correct.

This also explains why RPM ignition drop test performed in the air shows no problem. Float level is easily maintained when engine is running high RPM in cruise flight. Engine runs smooth at high RPM and any burp of higher float level is consumed instantly at 5 gallon/hour flow rate.

Bottom line is everything is Hunky-Dory.
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 650

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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