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Weird Engine Roughness…

Piper J3

Piper J3

Well Known Member
My friend’s RV-12 w/ 912ULS @ 500TT has just developed a slight engine roughness noticeable at just below cruise RPM. This is really strange… 5500 RPM cruise is very smooth (prop is dynamic balanced), but when pulling back throttle there is a roughness almost immediately. A reduction of just 150 RPM causes a noticeable vibration. The problem just started yesterday after a 120nm cross-country flight when we began to let down from 5500’ MSL. Same problem on return flight home.

Any ideas?

I’ll ask same question over in Rotax Owner’s forum, and if I get answer, I’ll communicate here….
 
This sounds maybe like the same problem I had earlier this year - premature dog gear wear . There was a band of RPM where something was abnormal. Perhaps 4800 - 5000 rpm. If I adjusted the throttle, it went away. For instance, slightly bumped the power up -- it would go away, but once the aircraft sped up and came to equilibrium again, the vibration would start again.. until I was out of that band.

I wouldn't call it "engine roughness" - the engine seemed to be firing all cylinders.. but there was a noticeable vibration that I could feel .. especially in my feet. Not enough to start to make me look for a place to land..but enough to catch my attention and wish my destination were a bit closer. It almost sounded like a clankety clank vibration..like someone put a few large ball bearings inside the engine and they were rattling around in there.
 
Try a mag check at the rpm where the vibration occurs. A loose spark plug cap can cause intermittent spark at certain frequencies
 
seagull said:
No it doesn’t, I have personally experience with it, 912ULS.
Click to expand...

Perhaps it is different between the ULS and iS models. Mine is the 912iS and I did not get a spark plug cap firmly seated after changing spark plugs during condition inspection. After a couple hours flight time I had the red light glowing on one of the lanes. Upon inspection I discovered the 3B wire dangling.
 
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