Prop RMPs ~2650 on Takeoff, but can't go above ~2500 in flight

[AdamJD]

Hi all,

I'm hoping Mooneyspace can work its magic and help my mechanic (@jetdriven) and I solve an RPM mystery. 

I'm having a problem where my RPMs go to ~2670 on takeoff, which is normal or just slightly low for my '67 M20F.  At altitude, I pull the RPMs back to cruise RPM of ~2450 to ~2500. After this, I can no longer get the RPMs above ~2500, no matter how far forward I push the prop. A very few times, if I push hard enough, forcing the knob closer to the firewall, the RPMs would surge up to ~2670 or so.  However, most times when I pushed the knob hard, it won't go above 2500.  

The cable is less than two years old and is a vernier-type control from McFarlane. The cable has cushion between the panel and the knob, so it does not reach the panel at full forward, which is correct. We've thoroughly inspected the cable to governor and the arm is travelling to the stop screw at full forward. The cable is firmly secured to the governor cable bracket and is not slipping.  

The prop governor is a PCU5000 model ATH-1.  The governor expert we talked with doesn't think it's a traditional problem with the governor and thinks if we pull it to bench test it, it will pass, as it seems to be governing oil pressure to the prop ok. Prop hub was replaced in 2012 and has approx 1100 hours.  No signs of any leaks in prop.

This problem didn't exist until after an annual this February where I had all 4 cylinders rehoned, which is obviously not directly related to this issue, but may be indirectly related as things were moved around for the cylinder removal and install.

Has anyone experienced anything similar? If so, what was the cause / outcome?

Thanks,
Adam

[PT20J]

When you pull the prop back to 2500 for cruise, are you using the vernier? If so, maybe it is something with the vernier mechanism.

What happens if you just pull it back to 2500 without using the vernier and then try to push the prop control all the way forward?

[AdamJD]

I don’t use vernier for takeoff. But do use it to pull back in cruise. But after experiencing this issue, I have pushed the prop back to full in cruise without using vernier. I will say that testing on the ground didn’t make a difference vernier or not.

However, not using vernier at all is a good thing to test next flight to rule it out.

[Z W]

Have you tried increasing the manifold pressure and mixture at the same time you increase the prop to full forward? Or are you leaving manifold pressure low while trying to achieve max RPM?

I think I would try that (simulated go-around power settings) at several different altitudes to see if it makes any difference, to help diagnose.

[takair]

While I’ve never heard of it, is it possible the prop itself is sticky and not going to lower pitch?  It does seem more like a governor problem though…

[bcg]

Last time I had my Eddy Current Inspection done, they guy asked me if my prop governor wandered using multi viscosity oil.  He said that with some of the governors, especially as they age, the oil viscosity would get too thin for them to hold higher RPMs.  You might try changing to a straight 50W oil if you're using a multi visc.  I don't know if this is going to help or not, just something I picked up from a prop guy along the way.

[AdamJD]

Appreciate all of the thoughts.  Because I had all 4 cylinders rehoned I've been running straight Phillips mineral oil and running at high MP and lower altitudes, keeping power above 75%.  I usually use Aeroshell W100 when not in breakin mode. I have about 12 hours since the cylinders were put back on.  The problem appeared on the second flight, right after the first 2-hour break-in flight.  However, for the break-in flight, I flew the first hour at 2650, so I didn't pull the prop back.  For the second hour, I flew between 2450 and 2550, so it might still have had a problem and I didn't know it.

I did try a simulated go around at altitude.  Low MP, Low RPM, mixture full rich.  When I firewalled everything, prop surged to 2600 and then back down to 2500.  So the governor regulated back to 2500 instead of keeping it above 2600 (or the prop could be sticking as takair mentioned - not sure how to test that).

[EricJ]

The prop goes to fine pitch if there is insufficient oil pressure, so it's unlikely to be the prop.   If it is at fine pitch on the ground, as evidenced by sufficient rpm at takeoff, then it looks likely that it is going to fine pitch when it is supposed to.

 

[Slick Nick]

Would the fine pitch stops have something to do with this if they're out of adjustment? 

[EricJ]

26 minutes ago, Slick Nick said:

Would the fine pitch stops have something to do with this if they're out of adjustment? 

If it was adjusted too coarse it wouldn't make full rpm on takeoff/climbout.

[AdamJD]

Don’t know how to rule that out, but I checked with my mechanic this afternoon and he doesn’t seem to think it’s the prop. 

[jetdriven]

It’s governing it to 2500 rpm.  The arm hits the stop and the cable has cushion.  It just doesn’t seem to work properly after takeoff 

[Shadrach]

I’m wondering if the governor is rotating in the housing. The PCU 1000 like most governors can be “clocked” within the housing to accommodate a multitude of prop cable mounting positions. It’s conceivable that if the governor was not properly tightened down or has become loose, it is moving slightly..

[Shadrach]

 

Don’t judge my safety wire too harshly. It’s super tight in there.

[AdamJD]

That's interesting.  It doesn't take much movement to make a 150 RPM difference.  We will definitely have to check that out.  @jetdriven, what are your thoughts?  We were wrenching on it a bit and it didn't seem like any of those 6 screws were loose - but we should check them.  Might explain when sometimes you push hard, the RPMs go up to near 2700.  Pushing hard may be rotating the whole thing a hair.

[jetdriven]

Anything is possible for sure.

[47U]

On 3/1/2025 at 8:42 PM, AdamJD said:

Has anyone experienced anything similar?

16 hours ago, bcg said:

He said that with some of the governors, especially as they age, the oil viscosity would get too thin for them to hold higher RPMs. 

It does seem that the issue arrived after the switching to mineral oil for the break in of the cylinders.  Once the cylinders stabilize, will switching back to 100w be the cure?

How old/hours on the ATH-1?

As to the prop, 1100 hrs since the hub was replaced might not be a lot, but 2012 was a long time ago, calendar wise.

[AdamJD]

Same hours on the ATH, as the hub was done with the overhaul in 2012.

[Shadrach]

10 hours ago, AdamJD said:

That's interesting.  It doesn't take much movement to make a 150 RPM difference.  We will definitely have to check that out.  @jetdriven, what are your thoughts?  We were wrenching on it a bit and it didn't seem like any of those 6 screws were loose - but we should check them.  Might explain when sometimes you push hard, the RPMs go up to near 2700.  Pushing hard may be rotating the whole thing a hair.

I think it’s a longshot as well, but should be ruled out..  I think the easiest way to check is to just put a bit of rotational force on one of the stop brackets and see if it moves. 
 

Seems unlikely that break-in oil would have an effect on the governor. I’ve run all different types of weight oil in mine without a problem.

Is the problem duplicatable on the ground?
 

[jetdriven]

If you push on the knob with 20 pounds of force against the stop, it will go to 2680, and when you let go of it, it will come back to 2500. But it appears that this doesn’t  work in the air. 
at sometimes the stop pin on the arm contacts the screw, and when you overdrive it like that, it actually flexes that pin and allows the governor to move another quarter of a degree. It just seems really hard to believe that a quarter degree rotation is 100 RPM roughly.

[AdamJD]

28 minutes ago, jetdriven said:

But it appears that this doesn’t  work in the air. 

That was true on my most recent flight. But the flight before that I got it to surge to ~2650 at least once by pushing hard.