C150 vibration at high rpm

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bobkiksass

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bobkiksass
Hi,

I have a c150 with o200 and it seems to have developed a bit of vibration at high rpm noticeable in the instruments, mirror and pedals.

The prop was overhauled/balanced a few years ago and the engine was overhauled a few years ago. The engine feels very smooth on the ground, and at full power on takeoff. When in the air with the engine loaded up it seems to have a bit of vibration at 2400-2600 rpm. Any advice on what to look for, it just doesn't feel quite right. I had a mechanic take a look at it on the ground, and he said the engine feels very smooth on the ground when ran up to 2000 rpm. In the air it feels different ? Any thoughts of what to look for ?
 
FWIW:
1) Have you noticed any change in the magneto RPM drops during the pre-flight checks? Ignition system can be one source - bad spark plug, bad ignition lead, arcing inside the ignition harness magneto cap, magneto timing, internal mag problem. These are relatively easy and comparatively inexpensive to check so not a bad place to start. For example, if they have not been done for a while, pulling, checking, cleaning, re-gapping, rotating and reinstalling the spark plugs isn't difficult and might be a good first step. Same thing with checking the mag timing - not a difficult thing to do.

2) Engine needs fuel, air and spark to run - if the ignition checks out then it's carb (throttle and mixture controls set and working properly?) and air intake (filter) next.

3) Has the prop been dressed recently? Dressing out knicks can, over time, result in an imbalance one blade to the other - unfortunately it requires a dynamic balancing tool to measure, and unless there is something obvious on the prop blades visually, best left until you've checked the stuff above first.
 
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My 150 had the same vibration. I had the rubber engine mount shock absorbers changed out and most of the vibration went away.
 
The air intake spider has become loose from the bottom of the case and is vibrating causing the float level to drop and the mixture to go lean.

Replace the two stat-o-seals that the spider mounts on and reinstall the spider properly and the vibration will stop.
 
are you sure it's the engine? not airspeed related? slow it down and go into a steep full power climb, still have the issue?, I flew a Dakota that owner was convinced had an engine vibration issue, turned out to be a broken mount on a wheel pant.
 
I am chasing a similar gremlin in my C150/O-200. I'll be watching this thread. So far I've replaced engine mounts and balanced prop. Still get unusual vibration around 2400-2500.
Jim
 
Airframe related perhaps? Engine RPMs causing something recently loosened to vibrate at that frequency...guessing.
 
I had an unusual buzzing sound. I was sure something was wrong. My buddy & I went up and we could hear it. It was diffuse but was coming from somewhere in front of the windscreen. He had me slow down. It stopped! Then pitch down, it started again. Pitch up, it stopped. We traced it to loose windscreen sealant on the outside, directly in front of me. It would start vibrating at high enough airspeed. Not suggesting this is your problem, but experiment with pitch changes in that RPM range to see if you can cause a change.
 
Tom-D said:
The air intake spider has become loose from the bottom of the case and is vibrating causing the float level to drop and the mixture to go lean.

Replace the two stat-o-seals that the spider mounts on and reinstall the spider properly and the vibration will stop.
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Couldn't you troubleshoot this by leaning or enrichening the mixture at cruise when the problem develops?
 
Mike5250 said:
Couldn't you troubleshoot this by leaning or enrichening the mixture at cruise when the problem develops?
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It is easier to simply see if the carb is ridged on the case. and that it has the proper stat-0-seal mounting
 
Check the clocking of the prop to TDC. I've seen that to be the problem on a Lycoming powered C-172 that was almost torn down for re-major due to vibration. Now can someone tell me why it is critical though I accept that it is?
 
nrpetersen said:
Check the clocking of the prop to TDC. I've seen that to be the problem on a Lycoming powered C-172 that was almost torn down for re-major due to vibration. Now can someone tell me why it is critical though I accept that it is?
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An engine can have a side-to-side rocking motion as opposing pistons reach the ends of their travels. I think the non-counterweighted cranks are a bit worse this way. If the prop is near level at TDC/BDC it can damp some of that, and if vertical it can't.

Vibration can have some weird effects. The Cessna 185's alternators (with the old flexible alternator mounts) will tend to toss their drive belts. The cure is to re-clock the three-blade prop 180 degrees. I cannot figure that one out.
 
The 0-200 / Cessna 150 doesn't care where you put the prop, it is not a dynamic balanced crank. I always place it at 10/4 o'clock with #1 @ TDC. But it seems to make no difference where ya put it.
 
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