M20J Engine Vibration on Approach

[Jack46]

While flying a coupled approach with new GFC 500, noticed an engine vibration when the power was pulled back during approach. Checked mags and mixture, no change in vibration. Increase or decrease throttle will make it smooth out. Same thing on a second approach. Ground checked everything I could, can't find anything wrong. On take off and cruise, smooth as always. The vacuum pump is still in the engine, even though none of the instruments are using vacuum.

Any ideas?

[ohdub]

Maybe a dumb question, but do you have the yellow arc on your tach (only with the McCauley prop I believe). Mine has a yellow arc from 1500-1950 rpm, if in that range there is a vibration.

[carusoam]

Vibrations come in multiple levels...

1) some you hear...

2) some you feel...

3) some you can’t even notice...

4) Some show up in engine data... got a JPI?

Often the cause of vibration is something touching where it shouldn’t...

Engine mounts usually do a good job of keeping vibrating things separate from non-vibrating things...

flat rubber biscuits are problematic...

broken engine mounts are really problematic...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

 

 

[KB4]

I am assuming you had the boost pump ON. 

Hand fly the approach to eliminate the GFC500 servos are not the cause.  Highly doubt it, but easy to eliminate.  VCOA duplicate conditions then test mags.  Let sit on Left and fly her.  Switch Right fly her.  When you get on ground before shut down do another Mag check for a hot P lead. No change then get A&P to check timing.  

[rcwagner]

I have a recently rebuilt engine in a J and it vibrates when in the yellow arc where you are not supposed to run continuously.  I just don't run there and there is no problem.  At just above the yellow arc, with landing gear down and full flaps, I get 90 knots with a 4 to 5 hundred fpm rate of decent on my approaches.  Works good for me.