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Lower than expected power on takeoff


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When flying my 1968 G model the other day, I had a situation which seemed unusual. I flew to an airport about an hour away, flying without any problems. After landing, I must have not leaned the mixture as I taxied to the ramp. After engine start for the return trip I leaned, but upon performing runup one of the mags yielded a rougher running engine and a significant RPM drop (around 250 or more). I leaned it out, opened the throttle and brought it to 2000 RPM for a half a minute. Checked again, same drop. Repeated 2000 RPM, same drop. Then I brought it up to full power, and after 30 seconds went back to 1800 RPM and the check succeeded. Fouled plugs cleared, though the CHTs went up to the end of the green and possibly closer to redline. I let it cool down a minute.

On takeoff, however, the airplane produced what felt like less power. It took off and climbed, but the climb at 100 mph was far less fpm than expected, and I would never develop more than what looked like 2450 RPM or so. Maybe 2500, but I'm used to having the prop reach closer to 2700 and requiring me to pull it back to 2500 instead of struggling to produce the full power. 

Right seat from me was an experienced instructor and mechanic, and we worked to troubleshoot this while also deciding whether to turn back or continue -- we could climb at 500-700 fpm, possibly due to updrafts, but with lower airspeed and with noticeably less power than expected. We tried adjusting throttle, mixture, prop to no avail. When we leveled out ten min later, power was still lower and thus airspeed was about 15mph slower than expected. Then about 3min later it felt like power just came back, airspeed increased, etc. The rest of the flight was uneventful.

The only thing that for me would explain this would be that we had run the engine hot enough during our runup that it didn't want to perform after takeoff when quite so hot. But I don't have a logical explanation for how this would happen. Does anyone have thoughts on what could have happened? I have stock engine instruments so I have little insight into what it was doing -- oil temp and pressure we're fine, and EGT -- while hot -- was not unusual.

Edited by Ilya Haykinson
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Hey Ilya,

Are you ready?

This is what engine monitors are for...

Having a mechanic next to you isn’t very helpful when he has no tools/data to work with...

You know you weren’t getting full power.  You have a couple of data points that show that...

But what caused the lack of power...  This is where the data comes in handy...

  • First round... Expect that a spark plug isn’t delivering properly... a guess without data... plugs, wires, mags...
  • Fuel issue... fuel pump, FF, too much, or too little?
  • What plugs do you have Champion massives?
  • A sticky valve can do this as well.  A stuck valve would have been more like an emergency...
  • Air intake leaks are on the list as well...

Did you try a mag test to see if you could identify something about the mags or plugs?  Narrow down the field a bit?

What is your mechanic recommending? Finding out which cylinder isn’t working like the others?

PP thoughts only. Not a mechanic...

Best regards,

-a-

 

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1 hour ago, carusoam said:

Hey Ilya,

Are you ready?

This is what engine monitors are for...

Sigh, I know... Coming soon.

1 hour ago, carusoam said:

Having a mechanic next to you isn’t very helpful when he has no tools/data to work with...

You know you weren’t getting full power.  You have a couple of data points that show that...

But what caused the lack of power...  This is where the data comes in handy...

  • First round... Expect that a spark plug isn’t delivering properly... a guess without data... plugs, wires, mags...

Possible, though I would imagine a spark plug wouldn't "fix" itself suddenly after some number of minutes.

1 hour ago, carusoam said:
  • Fuel issue... fuel pump, FF, too much, or too little?

Fuel flow registered fine. We tried with and without fuel pump.

1 hour ago, carusoam said:
  • What plugs do you have Champion massives?

I don't remember precisely (co-owner bought the plugs and managed their install), but I believe we use fine-wire.

1 hour ago, carusoam said:
  • A sticky valve can do this as well.  A stuck valve would have been more like an emergency...

I would imagine in a stuck-valve situation we'd get an extremely rough engine, with one of the cylinders producing no power. We didn't have any roughness.

1 hour ago, carusoam said:
  • Air intake leaks are on the list as well...

That's a great idea. I can imagine that if we weren't getting all the air in that we wanted, we'd get some power but not all of it. Like, say, if there was something partially blocking the intake that later moved -- power would return immediately. We should have tried alternate air.

1 hour ago, carusoam said:

Did you try a mag test to see if you could identify something about the mags or plugs?  Narrow down the field a bit?

After we cleared the plugs at the end of the runup, the mag check came back clean. We did try a mag check while it was running under-powered, but there was just a normal small drop on each mag. After power returned we tried it again, and then (half an hour later, after landing) we tried it with the engine at idle too. Everything was smooth.

1 hour ago, carusoam said:

What is your mechanic recommending? Finding out which cylinder isn’t working like the others?

PP thoughts only. Not a mechanic...

 

Well, I guess one step is to figure out whether this issue comes back, but I'm going to chat on Monday about his recommendations. 

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Reading a third time...

i can’t get past the part where you can’t get 2700 rpm.

That is a prop control issue.

The governor adjusts the blades using oil pressure and a valve. If the blades are too angled the gov isn’t relieving pressure...

The valve isn’t moving with rpm..?

or the control isn’t going all the way in...

All stuff you can test on the ground...

Time to check log books for anything related to the gov... how old is it? How many hours...? 

 

Real important... problems don’t fix themselves.  They may get some temporary relief, don’t trust the temporary part...

Max power and 2450 rpm on climb-out is not a good place to be... without an engine monitor you won’t see what is happening...

You want to look up and understand things like engine ping, and operating inside the red box, and operating over square...

You mentioned the CHT moving towards redline... that could be a sign of preignition... a side effect of high MP and low rpm...

How deep is your engine ops skill/knowledge?

O360s like to be leaned on the ground.  They have a tendency to collect lead BBs in the lower spark plugs if you don’t... fine wires are helpful to avoiding this...

focus on the gov things first without leaving the ground...  :)

 

PP ideas only.  Not a mechanic...

Best regards,

-a-

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Isn’t your G model carbureted?  I wonder if either your carb heat was still on or possibly you experienced carb ice that resulted in a lack of power?  Then, as it cleared, the power returned.

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Having a drop of 250 RPM on one mag really sounds like an issue with that mag. Have you checked the mag (timing, wires, etc.)? If you have fine wire spark plugs and they are new, I highly doubt it is fouled plugs. I would check the leads for the plugs as well. You may be dealing with multiple issues, but I would certainly figure out what caused the mag drop. 

And as Anthony mentioned, this troubleshooting is a lot easier if you have an engine monitor showing what is going on under the cowling. 

Edited by Marauder
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I had a mag once that did that. It was rebuilt with a bad capacitor. It would always check good on the ground but sometimes it would not quite operate correctly under load.  If  it is a bad cap, it will do it again.  Quick mag check in flight will tell you. 

 

Edited by tony
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As Anthony said the prop control issue could be different than the power issue. I had something similar happen after I had bought my plane and changed the plugs/wires. I thought it was related but it wasn't. It turned out that the cable had slipped up in the clamps (they were too old to hold it in place) and it wasn't travelling as far as it should. A thank you to @M20Doc who got me pointed in the right direction. New clamps and some safety wire fixed it and I haven't had an issue in the 150+ hours since then.

 

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You didn't happen to note your MAP reading when this was happening, did you?

For it to be an engine (as opposed to prop control) problem, the engine would have to be significantly down on power when standing still, maybe 50-60%.  If you were moving fast enough to be airborne, the cruise power schedule shows the engine able to make 2700 RPM even at 45% power.

It's hard to imagine this could be solely a problem with the motor.  Even if you lost one cylinder, in theory you should still be able to make more than enough power to reach 2700 RPM (although it might be rough enough you'd lose a seat cushion). 

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Thanks folks for the suggestions. 

My mechanic thinks that there was some continued fouling in the plugs, which resulted in one cylinder producing no or insufficient power. We might have not sensed this as roughness, but it would be enough to produce less than full power. It also works well with the sudden return of power in flight -- whatever was blocking the spark could have rattled out / melted / whatever which would bring the spark back into life.

The "cable for prop not traveling as far as it should" is a reasonable idea. The cable was certainly working -- we were able to adjust the prop down and back up, just not all the way to 2700.

For the MP -- I don't recall, unfortunately. It did seem a bit low to me at the time, but the airport I was departing from was at 3,000ft, and it was a warmer-than-standard day, so I didn't expect it to read the same as at sea level. 

It could be mags. They were on the plane when we purchased it, and I don't recall how many hours on them at the moment. 

Carb heat was definitely off.

I think as a matter of troubleshooting, I am considering doing a run-up followed by high speed taxi to see if I have a repeat while developing full power and moving through the air (at least below takeoff speeds). If both of those show that everything is behaving as it should (full RPM, normal MP, etc) then the problem was likely transient. If not, I'd pull the plugs and check them, and then repeat. 

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I've definitely fouled fine wire bottom plugs on my carb'd bird when running full rich at low rpm on the ground.  Indeed, I've done it so badly that I've had to pull the plugs and clean to solve the problem.   If you are confident you forgot to lean on the ground for a  long while, then your experience up to the point of clearing the fouled plug makes perfect sense.  What happened after is confusing. I imagine a sizable loss of power on takeoff could make your prop run at 2450 rpm at finest pitch, but a single persistently fouled plug in a cylinder certainly shouldn't rob that much power, and both plugs fouled  on the same cylinder would  be quite dramatic with noise and vibration. 

A data logging engine monitor would have provided a lot of insight as others note.  

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I had a problem with piss poor climb and low airspeed. Turned out the carb heat butterfly broke and flapped open. Look inside the intake below your prop, it should be closed. Mine flipped open . . . It was bad enough that ATL Center asked if I was really a Mooney.

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13 hours ago, Ilya Haykinson said:

Carb heat was definitely off.

I agree with @Hank.  Get a good visual check of the airbox.  A loose flapper could definitely explain the power loss and mysterious recovery.  Anyone who has taken off with the carb heat on :ph34r:, knows takeoff performance can be very anemic.

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2 hours ago, Mooneymite said:

I agree with @Hank.  Get a good visual check of the airbox.  A loose flapper could definitely explain the power loss and mysterious recovery.  Anyone who has taken off with the carb heat on :ph34r:, knows takeoff performance can be very anemic.

I pushed mine closed, took off to test, had normal climb to about 500' and was happy. Then it opened and climb rate went from ~1000 fpm to maybe 200 fpm. Didn't look at MP, too shocked by loss of climb. 

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