Engine running bad, 1t good 1b not firing to dead to firing intermittent , cylinders 2 and 3 firing very well

2t good 2b good.    3t good 3b good

4t good 4b intermittent to good to very intermittent 

912uls 172 hours, disconnect p leads and taped off, tests - gen coil charging coil, trigger coil readings, gapped @ 13 thous, used compressed air to blow out coil area, also checked ignition primary and secondary as per test procedure, also spark plugs,  caps, secondary wires tests all in spec,  2 grounds on intake and third ground cleaned, all good plugs, using timing light

Just by chance I ohmed gray wire to coil pack and got 138.5 ohms, I pulled and replaced each spark plug wire when I got to 1b the ohms went back to infinity,  that's when I changed 1b 2 b coil as stated lower in story 

Another startup  1b not firing  to intermittent 

4 b intermittent to intermittent to good

    Reversed vertical 1t intermittent 1b occasional firing.     4t dead 4b dead

     3rd test still Reversed vertical 

1t dead 1b intermittent 4t occasional firing

4b dead

     Vertical plugged back to normal

1t intermittent 1b intermittent 4t no spark

4b intermittent 

      Test run at night no archin , reading then

1t dead 1b occasional spark 2t dead 4t dead


     Installed new coil for 1b - 2b 

1t dead 1b dead d 4t occasional 4b occasional 

On primary wires, ac volts .011 DC volts .08 to


   I'm detailing the best that I can w 2 peeps, prop and wind, I know some of this probably makes no sense,  but trying hard to get this right, any and all help appreciated 

One day the engine started and ran perfectly and all cylinders were firing for about 15 minutes then started running rough, my timing light showed those non firing cylinders again ! So packed cd boxes with ice, no help same problem 



  • Re: 912uls cylinders not firing some intermittent

    by » one month ago

    A few initial thoughts to get things going:

    1. Any recent maintenance?

    2. Is wiring harness in good condition? Any abrasions against engine or airframe?

    3. Are the grounds all good?

    4. Are both grounds to the charging coils intact?

    5. Could there be a fault with the ignition switch?

    I am curious about the 138 ohm measurement. Are you saying that was between the coil primary and the ground? Are you then saying that when you removed the one of the spark plug leads from the spark plug that the 138 ohm disappeared? If so, that suggests that there is some form of galvanic connection between the primary and the secondary of the coil, which should never happen. If for some reason the coil has broken down then you will have intermittent ignition. Quite why this should be the case on an engine with such low hours, I don’t know.

  • Re: 912uls cylinders not firing some intermittent

    by » one month ago

    Hi, everything is unmolested,  all ohm reading good, 11.5 volt engine running on 2 red wires to ignition,  now brand new spark plugs, 4 bottom only sparks once every maybe 30 seconds but passed mag check - then taxi and mag check engine very rough,  all clean gas, bowls dropped perfectly clean, it all ignition related , timing light shows lack of spark, engine 172 hours,very clean under cowling, 4 coils set @ 13 thousands,  4 bottom that spark described above, it's coil mate is firing,  going to try and purchase those ingtechcd (spelling)boxes overseas or if someone can steer me to another vendor 

          Any help appreciated 

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