Poor Engine Power (RPM) Problems - M20J

[bennergh]

I have a 1977 M20J, S/N 0031 and have been experiencing a on-going problem with engine RPM/power problems. The issue seems to reveal itself in several ways. First, I've never seen 2700 RPM or above and second, I've noticed a slight surging on climb-out when the engine is cooler. The engine will mostly only perform 2200 - 2400 RPM static.

I've sent off  the Prop & Govenor and the Throttle Body; both were returned 'No Problem Found". I've repaired the, Muffler because of a metal obstruction in the center of it. I've replace the valve lifters and checked the tolorances. Checked the timing and Cam Shaft alignment - still to no avail.

I seem to have run out of options, except for maybe suspecting some quasi-alignment that may exist with the Prop/Airframe?

My A/P spoke with a tech rep at Lycoming who told him not to rebuild the engine. That he knew someone who had the same problem and rebuilt his engine, only to realize that the same problem existed afterward!

At this point, both my A/P mechanic and I are extremely puzzled about where to proceed. 

[Magnum]

I don't think it is the engine. IMHO you need approx. 60-70% HP to get 2200-2400 RPM static, if the prop is in fine pitch. I doubt that the engine is producing so little power, as it would be hard to get your mooney off the ground.

If your prop is not in fine pitch, you won't have full power available and you are at the limit for max. manifold pressure for continous operation: At 2200 RPM Lycoming limits man. pressure to 27", at 2400 RPM it is 28". The engine has to work very hard, temperatures go up and detonation is more lilely. Think about driving your car with low RPM in high gear and full throttle.

I am not a prop expert, but suggest to check the blade angle in fine pitch first and then make sure that no oil pressure is getting to the prop to keep it on fine pitch (I think a plate can be installed instead of the prop governor for the job). Run the engine and check if you get max RPM (but be careful to not go over 2700).

[takair]

As Magnum said, there is a low pitch stop in the prop.  If you have never seen 2700 RPM, this may be set to too coarse of a pitch.  A local prop shop should be able to help there.  Before you do too much, be sure that your tach is calibrated.  A handheld device can help there. There is also a possibility that your prop is overfilled with grease and is not allowing the mechanism to move full range.  Happens sometimes on Hartzells when over filled.  This might also account for the surging since the grease would be stiff until the oil warms it up. 

[Shadrach]

I agree with Magnus, this appears to be a prop issue. Has the plane always been this way? What do you're prop logs show? Also, consider ordering one of these:

http://www.google.com/products/catalog?hl=en&safe=off&client=safari&rls=en&q=Hangar+9+tach&bav=on.2,or.r_gc.r_pw.&biw=1305&bih=653&um=1&ie=UTF-8&tbm=shop&cid=3183190388327094138&sa=X&ei=aK38TZKxDMX30gGJz7zYAw&ved=0CFgQ8wIwAA#ps-sellers

I keep one in the plane and check RPM regularly on climb out. It is consistently between 2700-2730 depending on conditions. 

If one were trying to prevent oil form from going to the prop hub, it seems it would be far easier to temporarily block the prop line at the front of the engine then to install a plate designed for fixed pitch applications... 

 

[bennergh]

Thank you for all the great advice. I've ordered a prop RPM gauge (thanks for the tag!) and will explore the Prop and Prop governor for its stop position.

Pardon my naiveté, but can you shoot the prop RPM gauge through the windshield? Seem almost too comvenient to be true - Ha!

 

Thanks again!

[Shadrach]

 "can you shoot the prop RPM gauge through the windshield?"

 

Absolutely, The instructions will tell you to make sure the sun is at you're back, but I've never had trouble with mine getting a reading.

You did not mention when the prop was last worked on, and what was done. Also, has the plane been like this since you've owned it?

[bennergh]

I've owned this plane for a little over a year and this problem has been on-going. The previous owner had a gear up landing and the prop was replaced. The prop apears to be in excellent condition and both the prop and prop governor were sent off, put on the bench and tested to be fine.

Whether or not the prop and governor are properly aligned is my (new) next asignment!

Thanks,

~ Greg

[Clarence]

Hi Greg,

 

I would not fly the plane until you resolve this.  You need to eliminate a tachometer problem, the tach check should confrim this.  If it is OK then you have to eliminte the prop as the cause.  Ask the prop shop to confirm the blade angles to which they set the prop. 

Also check that the governor is on the high speed stop screw and that the cable functions freely.

If all this is OK see if you can borrow another prop and do a run up.  If it does the same thing you have an engine problem. 

I've seen and flown a Mooney with a mis timed camshaft, it made RPM but no torque, therefore no climb performance.  I suspect a prop problem which is limiting RPM

 

Clarence

[bennergh]

Thanks Clarence!

No. I don't intend to fly this thing until everything is resolved. I've taken a trip or 10 down the runway to test the performance, only to taxi back, scratching my head.

It seems that it's a 50/50 chance that either the prop isn't feathering properly or the engine simply isn't producing HP.

~ Greg

[danb35]

Don't really have anything to add other than to notice that we're practically neighbors, but hopefully something here will be helpful...

Rather than buying an optical tach, there are apps available for Windows laptops, Windows Mobile PDAs, and even the iPhone/Pad/Pod Touch which will determine the engine RPM from the sound it makes.  The iPhone app is $9, but the Windows ones are free at http://www.tunelab-world.com/rpmsound.html.  Or, if you decided that you wanted an electronic tach in your plane, this might be a good time.  The point is, of course, to confirm that you're getting a good RPM reading, and that you really do have a problem.

Assuming that your tach is anywhere near accurate, my guess would be the prop is the more likely source of the problem, just because it seems that it would be a simpler explanation (fine pitch stops set too coarse).  When you apply takeoff power, does the RPM rise smoothly to your max RPM (and what is max on your takeoff roll), or does it overshoot and then settle back down?  If the latter, it would indicate that the governor is acting to reduce the RPM.

Do you have an engine monitor?  It could answer a lot of questions that might help diagnose the problem.

[takair]

One other thing I forgot in my earlier post.  Do you know if your governor ever comes on line.  When you do your static run up and ease back on the prop control, does it immediately begin to come off of the static RPM, or is there a long delay?  In your case, I would expect that you could pull out on the contorl significantly before it begins to regulate.  If it immediately comes off of static RPM, that would indicate that your governor is already "on line".

[bennergh]

When I do my static run-up and exercise the prop there is a delay of no less than 2 seconds - depending on how fast I move the prop control of course. The prop exercise seems by sound to go pretty deep in pitch. I'm not sure what the RPM loss is at this point, but it seems fairly significant - maybe 300 RPMs? ... puzzling.

Thank you,

~ Greg

[bennergh]

... an additional note: I always move the prop control (during run-up) a bit slowly. I'm never very heavy handed with the prop control.

~ Greg

[jetdriven]

You wont be able to develop 2700 RPM until around 80 knots with the McCauley prop. 

[bennergh]

I was finally was able to get to the source of my RPM (Power) problems! Took my airplane to Triad Aviation in Burlington, NC and they ended up replacing the Magneto Impulse Coupler and the Fuel Divider. It seems the fuel divider was flooding one of the cylinders. I found (thanks Shadrach) that my RPM gauge is about 120 RPMs low as indicated using the handy 'Micro Digital Tach' which appears to be pretty consistent and accurate!

[Hank]

Thanks for reporting back on the cause of your problem and its solution!

[Shadrach]

Thanks for the info. I never would have suspected the flow divider. With regards to the impulse coupling, were you having starting issues prior to replacement?  

[bennergh]

Absolutely ... starting was most often a chore, my last 4 start cycles (since repair) were prompt and smooth.

Before repair, a slight surging was also an issue while fire-walled on the runway.

[takair]

I'm glad they were able to find the problem without too much trouble.  Do you know how they narrowed to the flow divider.  Did they measure flow into jars? 

The tach explains alot of it.

[bennergh]

I don't have a clue what their sequence to solution was. The flow divider that they put in was used and looks a little grungy - I'm not complaining as long as it works!

We had to lower the idle fuel flow this afternoon - A/P at my field (CQW) adjusted - it purred like a kitten at idle. Then I took it up and everything flew smooth. Just hoping that it starts and runs the same tomorrow when the engine's cold - ha!

~ Greg