Excess vibration and wandering RPM (M20G)

[The_Journey]

I started to have some issues a couple of flights ago. When climbing I'm getting some more intense vibrations than usual. Probably to anyone else that hasn't flown in it as much as me wouldn't notice it but on my most recent flight its started to get rougher on climb.

And simultaneously I started having issues in cruise. In cruise I can have my power set and will have some RPM wandering. I usually cruise at 17" MP and 2400 RPM and about 50 degF rich of peak. The plane will vibrate while at 2400 RPM. It's just a slightly stronger vibration than standard but after about 3 seconds the RPM will increase about ~25-50 RPM (hard to tell on my old dial gauges) and the vibration will smooth out. After another 3 or so seconds the RPM will settle back down to 2400 RPM and begin to vibrate again. Cycle keeps repeating.

My prop hub was leaking grease when I took it in for my annual end of last year and they did a reseal on my prop hub. I then took it back to Maxwell a couple of weeks ago when I started to have these issues and one of my blades was slightly loose. They send it back to the prop shop and they warrantied the issue and tightened things up but I'm still having the same vibration problems.

Sounds like maybe a governor issue? I've only owned the plane a year and a half so I'm still learning a lot. It's been really frustrating trying to schedule check rides with no DPE's available and having to send my plane back and forth to the shop.

Any advice would be really helpful. Planning to take the plane back to Maxwell this week for them to look at it agian.

[Fly Boomer]

7 hours ago, The_Journey said:

Any advice would be really helpful.

Not an A&P but, if it were mine, I would be thinking about the ignition system.

[N201MKTurbo]

Have you flown it on one mag at a time and see what happens? People seem loathe to do that, but that is why we have dual ignition, so you can eventually fly on one. Fly on each one for a few minutes, unless one is horrible. Most likely, one is smooth and the other is really bad.

If that turns out to be the case, leave it on the smooth one, don’t put it back to both.

You should do this on the way to Maxwells. You can’t legally take off with a flakey mag unless you have a ferry permit. But you can complete your flight.

[Utah20Gflyer]

 Unfortunately it could be a lot of different things so you’ll just have to systematically eliminate things until you find what the problems is.  
 

In addition to the ignition system check for intake leaks - make sure everything is tight and there aren’t any blue stains on the drop tubes.  
 

If you think the prop is good to go after the warranty work you might consider getting the prop dynamically balanced. Could be that after being taken apart and put back together it’s out of balance to some degree.  If it’s not the issue your engine will still run smoother after you find the actual problem. 
 

If you have an engine monitor are any of your cylinders or EGTs running abnormally hot or cold?  
 

Does the engine run smoothly and maintain RPMs at WOT and 2400 rpm’s?  
 

How long since the plugs were cleaned?   If it’s been a while it might be worth pulling the plugs to clean and test them.  
 

You could have an issue with the prop governor, but I wouldn’t send that out for repair until checking some of these other low impact things first.   

[A64Pilot]

By all means check the mags, too easy to do, but as the prop has been worked on twice now that’s where my suspicion would be.

Was it balanced after this work?

Usually intake leaks show up as rough idle as that’s when vacuum is highest and the leak is worst, often it’s a popping thru the exhaust at idle and or slight rpm surge

[Utah20Gflyer]

5 hours ago, A64Pilot said:

By all means check the mags, too easy to do, but as the prop has been worked on twice now that’s where my suspicion would be.

Was it balanced after this work?

Usually intake leaks show up as rough idle as that’s when vacuum is highest and the leak is worst, often it’s a popping thru the exhaust at idle and or slight rpm surge

I recently had an issue with engine roughness at WOT unless I kept the engine much richer that normal.  Turned out to be two intake leaks one of which was pretty significant.   Essentially one cylinder was really lean and I had to dump a lot of extra fuel into the system to get that one cylinder rich enough to run smoothly.  It’s worth checking, good news is it’s a really cheap fix if it’s some old intake seals or couplings.  I think I spent 30 bucks fixing the issue.