The ignition modules are mounted on the top of my 912 ULS engine as described in my Kitfox 6 build manual. Although this is a 2001 vintage engine, at present I only have 59 hours of operation on it. On a previous recent flight the engine started fine and ran smoothly with no missing during run up mag check or during flight. A few days later on 08Oct22 the engine started and warmed up normally but during a run up mag check at 4000 rpm the engine began missing randomly. It seemed minor at first so I took off and while in the pattern it happened again at around 4500 rpm (with the mag switch set on “BOTH”). After landing another mag check was performed and the random missing seemed to be the worst when the ignition switch was set on “L”. During the mag check at 4000 rpm the engine rpm dropped around 150 rpm (from the “BOTH” setting) for both the “R” and “L” switch setting, which seemed normal. I checked the continuity of the kill wires from the ignition modules “A” and “B” and they were both functional. I also confirmed that the Ignition module “A” is connected to the ignition switch “L” terminal. I assume that when the ignition switch is set on “L” the “A” module is turned on and the “B” module is turned off via the kill wire grounding that module. The ignition switch was removed and checked for proper continuity at its various settings and it checked out as functional.

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I can’t be sure yet but I suspect that the current missing is due to an ignition issue of some type since it came on so suddenly and it seems more isolated to the “L” side setting on the ignition switch. Any suggestions?


In order to rule out any potential fuel supply issues a subsequent check of the fuel level in each carb fuel bowl checked out as normal and a look at all the spark plugs showed the normal tan color as well. Both carbs have recently been overhauled and have since been performing very well. After reinstalling the plugs I reran the engine in order to check the fuel pressure and it was within the normal range. I even changed the fuel pump from the original Pierbug pump to the latest Rotax pump p/n (thinking that the old pump may be defective somehow). I also checked the gas tanks and header tank for water with none found. None of these checks showed any potential cause of the engine missing due to fuel supply anomalies.


Note: A previous carb issue a number of flight hours earlier was experienced that caused an engine missing (stumble) due to air leaks at the carb to manifold rubber interface (resulting in extreme lean operation). This was remedied and the sound of that engine stumble was different (more severe) than the missing described above.


A follow on operational check for spark in cylinder #2 with a strobe light displayed a flashing strobe for both the top and bottom plugs. None of the other plugs were checked for operational spark at this time. Should I do this test on all the other plugs while running the engine?