Unable to make maximum RPM a few hours post overhaul...

[Tito22]

Subtitle: The overhaul from hell...

Just got my M20J back from overhaul. Mind you this was after two test flights. The first flight, oil leaking from governor, we pulled it and overhauled it. Put it back on, second test flight was leaking from gasket. Replaced that gasket, third time was the charm and no leaks and I brought the Mooney back home.

 

Next weekend, I flew it about 8 hours, to and from Omaha and Oklahoma - performed great with no issues on the flight. After landing, taxi back, and shutdown was uneventful.

 

Skip forward a week and I go fly again. Engine start, taxi, runup was uneventful. Mag check was good. On takeoff roll engine sounded weird and like it wasn't making full power. I look down and see ~2200-2300 RPM. I abort the takeoff. A Couple of more static runs and it seems to be governed just above 2000 RPM.

 

I pull the cowl, check the cable and it seems to be making it to the low RPM stop just fine. I was sure it was the governor since it was having issues.

I have the governor removed and sent to the place that did the overhaul. They said it bench tested good...

 

Has anyone seen anything like this before? Looks like I'm pulling the prop to send it to the shop to see if there's something wrong with the prop. It seems that with enough RPM there is something leaking into the prop hub to increase the oil pressure to increase the pitch on the blades.

 

Attached videos show the runup. When I took this video, it looks like #2 cylinder dropped off (not sure if that was due to over boosting it or something else) but fuel flow and manifold pressure all looked good. #2 seems to keep good ignition the rest of the runs. I didn't want to run it too much over square. I also haven't checked the accuracy of the tachometer but it seems to be sounding like the prop is digging in and causing rough running.
 

[Andy95W]

@Cody Stallings is our resident propeller expert.

[Shadrach]

Was this a particularly high DA day? I’ve never checked to see what static RPM looks like at 24 inches of manifold pressure. Is that the best you could do?

[Bolter]

1 hour ago, Shadrach said:

Was this a particularly high DA day? I’ve never checked to see what static RPM looks like at 24 inches of manifold pressure. Is that the best you could do?

Maybe I have it wrong, but on a hot or cold day, you should always get full RPM as the governor will pitch the blades until you reach full speed.  Assuming you are not hitting the stops in the prop (not the governor limit), in which case he would have not reached full RPM in the past.  

[Z W]

Engines run on fuel, air, and spark.

Your fuel flow looks adequate in the video, maybe even a little high. Your fuel system does need to be set up and tuned properly, and I wonder if that was done post-overhaul. You might verify it's reading proper during a full power run-up as well. There will be a spec for how many GPH you should be seeing at max power.

For air, verify nothing is blocking your filter or intake. I had a similar problem and it was air intake related. I spent a lot of time and money chasing the problem before we figured it out. Lack of air means the engine can't make full power and won't spin the prop to full RPM. This could also be caused in theory by a high density altitude takeoff, especially without properly leaning the mixture, but I'm not familiar with your engine and whether you should be seeing this at your density altitude. Your naturally aspirated engine should be leaned for best power for the density altitude.

For spark, you would start with a mag check during run up, verify nothing is abnormal. Especially watch your #2 cylinder that was a little slower than the rest to come online during your run-up, but frankly, that didn't look abnormal or concerning to me. It came right up as you gave it some more power.

What does the engine do at a full power static run up? Those are allowed, no need to be concerned about "over square".

Hope this helps and good luck. Getting the plane dialed back in after major engine work can involve working through some gremlins.

[EricJ]

Can't tell much from the vid, but the rough running appears to be due to the #2 cyl dropping out for a bit.    It came back hotter, so it's possible you had two fouled plugs for a bit and at least one of them cleared when you adjusted power.

 

[Stephan Kablitz]

Dear Tito22, your M20J should put through about 17 gph at full power, full rich. In your video I see fuel flow below 10 gph or do I get the display wrong?

[Tito22]

Thanks so much guys for the responses. I’m going out to hangar now, gonna try and measure my prop blade angle to see if that somehow got out of whack. Also going to borescope the exhaust and intake. I am worried I’m going down the wrong rabbit hole with the propeller idea. 
 

@Z W the only time I tried to go full power was during the initial takeoff that only got me up to 2200-2300 rpm and a sound of engine not working well. The fuel system is a thought. I was concerned about going too high power with low rpm so I only got to about half-2/3 throttle before I wimped out. Fuel flow seemed about as I expect for not WOT. 
 

I’m at 1200’ and it’s hot but not a really high DA day, about the same as I did on other days with test runs and were in OK so not much elevation difference from the other airports. 
 

I think what really threw me into not thinking it was an engine thing is the fact that this happened between sorties, it wasn’t like a rapid change in flight in engine operation which I would have assumed would have happened…

 

[EricJ]

10 minutes ago, Tito22 said:

I think what really threw me into not thinking it was an engine thing is the fact that this happened between sorties, it wasn’t like a rapid change in flight in engine operation which I would have assumed would have happened…

Plug fouling can happen during taxi after a flight, when the power is very low and the engine is cooling down.   Leaning on the ground and keeping the rpm at or above 1000 rpm helps, but it can still happen.    I don't know if that's what your issue is/was, but there are things like that that can come up suddenly.

 

[Z W]

1 hour ago, Tito22 said:

Thanks so much guys for the responses. I’m going out to hangar now, gonna try and measure my prop blade angle to see if that somehow got out of whack. Also going to borescope the exhaust and intake. I am worried I’m going down the wrong rabbit hole with the propeller idea. 
 

@Z W the only time I tried to go full power was during the initial takeoff that only got me up to 2200-2300 rpm and a sound of engine not working well. The fuel system is a thought. I was concerned about going too high power with low rpm so I only got to about half-2/3 throttle before I wimped out. Fuel flow seemed about as I expect for not WOT. 
 

I’m at 1200’ and it’s hot but not a really high DA day, about the same as I did on other days with test runs and were in OK so not much elevation difference from the other airports. 
 

I think what really threw me into not thinking it was an engine thing is the fact that this happened between sorties, it wasn’t like a rapid change in flight in engine operation which I would have assumed would have happened…

 

When I was looking into this, there was no real way for the propellor blade angle to "get out of whack". It's all bolted together.

If your engine just didn't sound good, but you didn't push it to full power, you might want to just do some more testing on the ground. Run it up to full power. Report back on maximum MP, RPM, and fuel flow while at full throttle. Do some high speed taxis down the runway, pulling power before rotation speed, and let us know what it's doing. Bonus if you can download the data from an engine monitor and post it here.

You have a fuel flow instrument in the panel. Let us know what it says during full power run up and high speed taxi down the runway. We'll be able to check it against your factory service manual for your engine and let you know if it's correct.

At 1200' density altitude you should be making full power, no question.

Make sure you sump your tanks well so you can rule out something like water in the fuel, which can cause stumbling and momentary loss of power on the takeoff roll.

[Tito22]

Hey all, thanks for the replies. I looked at the ignition and power theories after the suggestions here. I realized I could download the engine data and that helped out a bit. #2 cylinder is definitely off... I can't tell if it's just the "weakest" cylinder and loses power due to getting bogged down by a higher pitch RPM or if the cylinder is lagging at higher power settings and that's preventing full power. Previous flight had much closer EGT and CHT spreads, this attempted flight had #2 about 25 degrees cooler and CHT about 50 degrees higher than other cylinders. 

Before pulling the prop, I pulled #2 plugs and the bottom one was a little dirty and had a close gap. We re-gapped and tested, good spark all around. I'll try running it before I pull anything else. Guess I shouldn't have been quick to assume it was the governor again after all the problems it gave us... 

 

Thanks again and I'll post back with results...

 

21 Aug.pdf

[Shadrach]

9 hours ago, Stephan Kablitz said:

Dear Tito22, your M20J should put through about 17 gph at full power, full rich. In your video I see fuel flow below 10 gph or do I get the display wrong?

17gph is lean for full power and full rich..

[Shadrach]

22 hours ago, Bolter said:

Maybe I have it wrong, but on a hot or cold day, you should always get full RPM as the governor will pitch the blades until you reach full speed.  Assuming you are not hitting the stops in the prop (not the governor limit), in which case he would have not reached full RPM in the past.  

The blades should be against the fine pitch stop (inside the hub) unless the governor sends pressure to the hub to increase pitch. 

[TCC]

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Why is that plug wet?

[Pinecone]

On 8/21/2025 at 11:07 PM, Shadrach said:

17gph is lean for full power and full rich..

Agreed.  Rule if thumb is around 1 GPH per 10 HP.  So around 20 GPH would be what to expect.

[Hank]

10 minutes ago, Pinecone said:

Rule if thumb is around 1 GPH per 10 HP.  So around 20 GPH would be what to expect.

The Performance Tables in your POH should have Sea Level, full power fuel flow. The one for my C does. Should look something like this. I have 180 hp, and max fuel flow is 18.2 gph.

[Pinecone]

Just now, Hank said:

The Performance Tables in your POH should have Sea Level, full power fuel flow. The one for my C does. Should look something like this. I have 180 hp, and max fuel flow is 18.2 gph.

 

Yeap, 18 GPH is what I would expect on a 180 HP engine

[Bartman]

Best I remember I see 18.7gph on takeoff but that was summer temps and DA so may be a little higher in cooler air. 17gph would be low, but would not explain not making full RPM. 

[PT20J]

According to the IO-360 graph in the Operator’s Manual, an IO-360-A should burn a little over 15 gph at full power at best power mixture. It burns more during takeoff because it needs additional fuel for cooling. Mine burns a little over 18 gph.