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912 uls ignition

  • Karl Haagsma
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3 weeks 2 days ago #17376 by Karl Haagsma
Karl Haagsma created the topic: 912 uls ignition
ALCON: I hope you can help me. In short, I have a 912 uls that used to run great, but not so much anymore. Functional problem is that the engine does not seem to fire on #3 cylinder at lower rpms. I was wondering why things were so rough and with significant vibration. Carbs are synced. Replaced spark plugs with similar results. EGT over 2500rpm is consistent with left bank of cylinders, so once up to significant RPM’s # 3 seems to be working and vibrations are attenuated. Once down to lower rpm’s it appears there is no spark on #3 as plugs are quite wet after low speed operation.. Does anyone have any good explanation? am I looking at an ignition module replacement? Please help.

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  • David HEAL
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3 weeks 2 days ago #17379 by David HEAL
David HEAL replied the topic: 912 uls ignition
Could be a poor spark plug connector / wire on #3. You can remove and reset the spark plug wire into the spark plug connector -- good info as to how to do this on this site.

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  • Karl Haagsma
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3 weeks 1 day ago #17381 by Karl Haagsma
Karl Haagsma replied the topic: 912 uls ignition
Copy all. Thanks for your input. Will try and report.. Karl.

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2 weeks 4 hours ago #17454 by Karl Haagsma
Karl Haagsma replied the topic: 912 uls ignition
This from Dean at Lockwood Aviation:

Hi Karl,
It’s good that you wrote. The probability of both ignition systems, failing on the same cylinder, in the exact same mode, is almost as remote a probability as self-reproducing life springing up by accident. If you had pursued the problem as an ignition issue, you’d have been chasing your tail for a long time.
From what you describe, the problem is most likely to be the idle circuit in your 1/3 side carburetor. It might also be the float valve. When you did the carburetor synchronization, did you do it at or above 2500 rpm? If so, what does the vacuum look like on that carburetor when you pull back below 2500 rpm and the engine gets rough? When the engine is at 2500 rpm, the vacuum should be about 11 inches, give or take an inch. When the engine is at idle (1800 or below) the vacuum should be about 15 inches, give or take an inch.

Upon further investigation, I think he might be correct. Will attempt to verify and report.

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