Prop RPM Fluctuations at high MP and RPM

I am flying an RV-7A and am just under 700 hrs. of great fun. For the past 18 months or so I have had minor RPM fluctuations that are only at takeoff and climb settings. The first takeoff is pretty normal except over the 18 months or so the max RPM does not quite reach my 2650 limit as it used to. (I know I can adjust the max RPM but I'm after the bigger issue right now) After the engine is hot and I do a second takeoff I am getting RPM fluctuations of 50 to 200 RPM at takeoff MP and high RPMs. After I level and pull power and RPM back to cruise things are fine and the flight is normal. This has gradually worsened with time. I have a Hartzell CS HC-C2YR 74" prop and an MT governor which I just had overhauled 2 weeks ago. Lycoming O-360 A1A engine with standard mags. I had a friend with similar issues and his governor overhaul cured the problem. No so with me. I will have oil pressure of 82 psi in climb which is normal for my plane. The prop was bought new and has not been overhauled yet. No leaks that I can see. Engine runs great, clean oil, no ignition problems, and good oil analyses over the years. Hoping someone may have some insight here.

DCalland said:

I am flying an RV-7A and am just under 700 hrs. of great fun. For the past 18 months or so I have had minor RPM fluctuations that are only at takeoff and climb settings. The first takeoff is pretty normal except over the 18 months or so the max RPM does not quite reach my 2650 limit as it used to. (I know I can adjust the max RPM but I'm after the bigger issue right now) After the engine is hot and I do a second takeoff I am getting RPM fluctuations of 50 to 200 RPM at takeoff MP and high RPMs. After I level and pull power and RPM back to cruise things are fine and the flight is normal. This has gradually worsened with time. I have a Hartzell CS HC-C2YR 74" prop and an MT governor which I just had overhauled 2 weeks ago. Lycoming O-360 A1A engine with standard mags. I had a friend with similar issues and his governor overhaul cured the problem. No so with me. I will have oil pressure of 82 psi in climb which is normal for my plane. The prop was bought new and has not been overhauled yet. No leaks that I can see. Engine runs great, clean oil, no ignition problems, and good oil analyses over the years. Hoping someone may have some insight here.

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Does just your gauge change rpm, or do you feel/hear the fluctuations! If it is actually changing rpm, call MT and tell them that and they will put in a heavier spring (this used to be a common issue with the RV-10 with the MT governor). If it?s just the gauge, then your sensor may be failing, or the resistor between your rpm pickup and the pleads is weakening so false pulses are coming through at high rpm.

It's an obvious RPM change that "wakes you up" kind of fluctuation. Speeder spring was replaced.

RPM Fluctuations

Is all the linkage and hardware tight at the governor arm? Is the control cable securely clamped to the bracket where it can?t move? Is the bracket intact? No cracks and securely bolted in place?

Don Broussard
RV9 Slow Rebuild in Progress
57 Pacer

All checked and double checked

Had the same problem see Lyc service instructions 1462A

I have an IO 540 C4B5 on my RV. Type of Lyc shouldn't make a difference.
I had 3000 RPM's overspeeds at full RPM's and fluctuations at lower RPM's.

The steps I took to remedy the problem.

Had the prop resealed twice. No difference
Replaced the governor with a rebuilt one. No difference
Complied with Lycoming Service instruction 1462A Found the cause.

The service instructions had a differental air pressure test. Similar to a compression cylinder test. It failed on the low side. Minimum pressure was 6 psi. I had 3 to 4 psi.

The service instructions show a special made plate. FORGET THAT Ken at LyCon said to just use a rubber tipped air nozzle. It worked great

I totally dismantled the engine and found no visible cause. The engine is in my shop totally rebuilt and waiting reinstallation on my plane. I am keeping my fingers crossed.

Any questions call me. Steve (707) 972-3582