"Rough" range

[wadeg]

Anyone with a 912s note a little bit of vibration from the engine in about the 2400 rpm range? Mine is pneumatically balanced right on and idles very smooth. Moving through 2400 or so, it has a 'rumble' to it. Then when you get beyond, it really smooths out. At 5000, I was amazed at how smooth it was. Put my hand on a carb and it is barely vibrating at all! Is this just perhaps a harmonic thingy going on? Not real worried. My Aeronca Chief rumbles and shakes at about the same spot (of course different RPM setting but still in the 1/3 throttle setting area).

[RandyL]

Wade,
Hopefully others can answer whether the periodic resonance you're feeling is normal. Since you're local though I thought I'd remind you about our chapter's prop balance program...


Once you get your bird flying and a few hours on it let's get it balanced. I need to get more information from DSS Micro as to tricks and tips for balancing the 9-series installations which I plan to do at OSH next week. I've now done over 70 Lycomings with good results. .

[84KF]

I too get a very slight "shake" at low rpm, but I can't remember the exact numbers at this time.
I notice it in flight when setting up for landing. Usually downwind @ 3500-4000 rpm is silk smooth, and a reduction to say 2500 or so sets up a (slight) noticeable vibration so I just keep the rpms a bit lower or higher and it goes away.

I've checked for carb throttle arm travel synchronization (equal travel through out throttle range) and a few other thing but all seems normal.

I did just recently adjust the throttle cables where they are attached to the carb body. I found that there were slight "bows" in the cables when the throttle control was pulled all back in the idle position (the bows) wanting to "push" the throttle arms forward.
I've included a picture of the " after" adjustment. Forgot to take a "before" picture. The cable housing was only about 1 inch or less from the arm in the idle position before the adjustment. (see attachment) No test flight yet.

I also stuffed a bunch of Aeroshell #5 grease into the throttle assembly at the panel. There was to much metal to metal friction before... now it's nice and smooth and the twist lock still works fine. (see attachment)