CS RPM Drift

My rpm was steady for the first 30 hours of flight. Within the past 3 hours of flight, I am seeing my RPM drift by about 50 to 100 rpm during flight.

This is not a gauge error as I can feel the engine surging. See the plot below. The bottom plot is MAP which is only changing by 0.1 inch.

I have a new (35 hrs) Lyc YIO-360 M1B and MT P-860-4 governor with a Hartzel C2YR-1BFP/F74972 prop.

When I cycle my prop at run-up, it always seems to be slow to respond compared to my other spam can experiences.
If I move the throttle somewhat quickly in flight, the rpm will overshoot the set speed and then come back to the setting in about 1 second.

All my fittings are tight with no leaks. Is this rpm response normal?

Restriction in the oil lines to/from the governor? Also, might want to check to make sure the MT governor has the most current revision, since some had surging type issues, which MT can resolve. Have you spoken to Juergen at MT USA yet by chance? I'd call them.

I spoke to Jeurgen today. He said I have the latest updated governor.

He thought that maybe being a new engine that some trash or metal may have gotten into it and suggested I send it in at the 50 hour mark for an inspection.

He said it was ok to fly with the fluctuations for 15 more hours.

Taildrgr said:

I spoke to Jeurgen today. He said I have the latest updated governor.

He thought that maybe being a new engine that some trash or metal may have gotten into it and suggested I send it in at the 50 hour mark for an inspection.

He said it was ok to fly with the fluctuations for 15 more hours.

Click to expand...

Why would you not want to get to the issue now? Don't you wonder where the "trash" is coming/came from, and wonder what else it might do? Seems like depending on a benign symptom and ignoring the unknown threat. Maybe it is nothing, maybe it is the beginning of something.

Prop surge

Great time to go thru all oil filters & screens. Look for trouble.

Don B

RV 9 Rebuild in Progress

We had this issue on a couple of Mt governors and they changed out a spring for a stronger spring (don't ask me what spring, because I have no clue). Does it happen at all RPM's or just a certain range?

The rpm fluctuations occur at all settings except when set to high rpm such as takeoff and landing.

One thing I did not check is the prop cable attachment stop nut to see if the cable has worked loose.

I have checked the oil screens and filters at 10 and 25 hours and had the oil analyzed each time.
No metal found other than a few flakes in the filter at the first oil change. It was normal for a new engine.

Note that the fluctuations started only a few hours ago which is about 5 hours since the last oil change. So if I don't find any other mechanical reason, I plan to pull the governor, do a transfer bearing leakdown test, and send the governor in for inspection.

Thanks for all your replies.

Taildrgr said:

The rpm fluctuations occur at all settings except when set to high rpm such as takeoff and landing.

One thing I did not check is the prop cable attachment stop nut to see if the cable has worked loose.

I have checked the oil screens and filters at 10 and 25 hours and had the oil analyzed each time.
No metal found other than a few flakes in the filter at the first oil change. It was normal for a new engine.

Note that the fluctuations started only a few hours ago which is about 5 hours since the last oil change. So if I don't find any other mechanical reason, I plan to pull the governor, do a transfer bearing leakdown test, and send the governor in for inspection.

Thanks for all your replies.

Click to expand...

It sounds like you are doing your due diligence homework. Train us in your findings, Bob. One perspective is that doing it now is the same amount of work as doing it later, and you will be safer in the process. Let's just write off this effort to standard development investment for phase I. That is what Phase I is for, safety.

I believe I have found the RPM drift problem.

I removed the sniffle valve and found it to be stuck open.

I put a plug in the sniffle valve port and made two test flights. The RPM is back to steady.

The open sniffle valve was acting like an air leak.

After soaking the valve in acetone for a week, it is still quite sticky so I will just leave the plug in the intake sump.

Thanks Bob!

Good to hear you got to be bottom of it. Rhonda Barrett told be once what sniffle valve brand to use. I forgot. Hopefully, it won't bite. Pretty bad for such an expensive little part.