Slowest M20F ever?

[Ronnie Pool]

Hello everyone, I am new to the Mooney forum and recently purchased a 1970 M20F that I am rehabing after sitting for a long period of time (I am an A&P IA with limited Mooney experience) as well as having been worked on by many others without significant flight time afterwards. This plane has the J model windshield and speed mods. I am only seeing about 126kts true airspeed at altitude regardless of prop and power settings. I’ve checked and rechecked the gear rigging and it is within tolerance, however I will be mounting a camera to check for any flex or droop from the doors in flight. I am in the process of locating throw boards to check the rigging as she does seem a bit right wing heavy and needs a slight touch of right rudder to hold true. I’m reaching out for any suggestions as well as experience as to what could be the most likely cause for loosing so much speed. I’ve heard they will not max out on speed without being rigged perfectly. Without the boards I’m limited, but everything appears to be in good order rigging wise with the flaps and ailerons.

 

 

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[KSMooniac]

Are you sure the pitot static system is tight, and the airspeed indicator is not off? How are you determining true air speed? I'd suggest a 3 leg steady state test, and use one of the TAS calculators from the net to determine reality, and compare to your ASI.

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[Ronnie Pool]

The plane just had its pitot static check a few months ago and true airspeed is coming from a G5.


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[Ronnie Pool]

As well, ground speed seems to confirm.




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[N201MKTurbo]

That’s crazy slow. It should be 20 KTS faster. It is hard to imagine what you could do to it to make it that slow. It should go faster than that with the wheels down.

[AndreiC]

Hold on, am I wrong? It seems that at 9500 feet your G5 shows a TAS of 156 kts, which is great!

[DCarlton]

With the windshield and cowl, my '67F cruises consistently around 153 kts at 8500-9500.  That's a good looking F.  Good luck finding the issue.  

[Ronnie Pool]

Hold on, am I wrong? It seems that at 9500 feet your G5 shows a TAS of 156 kts, which is great!

AndreiC, the G5 was set to MPH on that picture unfortunately….


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[KSMooniac]

Go do this at 3-4k and 7-9k:  True Airspeed Calculator VIA GPS

Just because you have a digital readout doesn't mean it is accurate.

After that, then you likely need to check the mag timing, and maybe go deeper and measure the rocker arm/valve lift and see if your cam has lost it's profile.  That could very much be in play if it has been sitting.

Great looking Mooney!  

[AndreiC]

8 minutes ago, Ronnie Pool said:

AndreiC, the G5 was set to MPH on that picture unfortunately….


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I see, sorry. As others have said, something is seriously wrong -- it should be cruising much faster. My impression was that despite the longer body, Es and Fs were very similar in speed, and I consistently see 145kts in my E without the speed mods you have.

I would still do a 3-way run. It's easy: go out on a smooth day, and you fly compass headings going first straight N until the speed completely stabilizes (takes a couple of minutes; refer to the GPS ground speed, not any other speed; this will be the speed you'll record); then turn due E and read the speed after it stabilizes, and finally same thing due S. Record these three speeds and put them in the calculator here:

http://www.csgnetwork.com/tasgpscalc.html

It will give you your TAS.

[Ronnie Pool]

Go do this at 3-4k and 7-9k:  True Airspeed Calculator VIA GPS
Just because you have a digital readout doesn't mean it is accurate.
After that, then you likely need to check the mag timing, and maybe go deeper and measure the rocker arm/valve lift and see if your cam has lost it's profile.  That could very much be in play if it has been sitting.
Great looking Mooney!  

I’ve checked the mag timing and the engine has 20 hrs on the rebuild. Seems to be making adequate power and only slight mag drops.


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[KSMooniac]

Was the overhaul completed this year, or years ago?  

[Ronnie Pool]

Was the overhaul completed this year, or years ago?  

It was completed in 2022.


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[EricJ]

11 minutes ago, Ronnie Pool said:

I’ve checked the mag timing and the engine has 20 hrs on the rebuild. Seems to be making adequate power and only slight mag drops.


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It looks like your prop and throttle controls are full forward at 9500 feet and it's only making 2500 rpm.    That doesn't look right.   Your mixture is pretty far forward, too, for that altitude.    I'm wondering if you don't have an engine rigging problem.

[KSMooniac]

It might be worth measuring the valve lift then.  Lycoming cams suck.  I lost 1/2 a lobe (#1/#2 intake) 9 years ago and that triggered my overhaul.  It could be that the internal timing is off as well (assembled incorrectly), but I don't know how to easily check that.

[Ronnie Pool]

It looks like your prop and throttle controls are full forward at 9500 feet and it's only making 2500 rpm.    That doesn't look right.   Your mixture is pretty far forward, too, for that altitude.    I'm wondering if you don't have an engine rigging problem.

Could be. Since that picture was taken, I re rigged the prop lever as it was indeed rigged wrong with the lever bottoming out on the plastic interior piece. I had the mixture in a bit more as I’m breaking in the chrome cylinders and trying to keep the temps reasonable.


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[Ronnie Pool]

It might be worth measuring the valve lift then.  Lycoming cams suck.  I lost 1/2 a lobe (#1/#2 intake) 9 years ago and that triggered my overhaul.  It could be that the internal timing is off as well (assembled incorrectly), but I don't know how to easily check that.

I’ll check the lifters. Haven’t opened it up since it was so recently overhauled. Compressions are all at 78/80. I have been finding a lot of items installed incorrectly, but haven’t discovered any issues with the engine yet. I’ve been babying it a little until the new chromed cylinders settle in though…


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[AndreiC]

I don't know if this is ok to do in the breaking-in phase, but if this was not an issue I would try a static RPM check on the ground. Good way to see if engine makes full power. The type certificate should say what you should be seeing.

[KSMooniac]

Did the same bunch that installed things incorrectly also overhaul the engine?  Compressions have nothing to do with any timing discrepancies of course.  I thought even chrome jugs should be run hard to seat rings?  When I break in after cylinder work, I fly max power down low and just fly around local airports in case I need to land quickly.

[Ronnie Pool]

I don't know if this is ok to do in the breaking-in phase, but if this was not an issue I would try a static RPM check on the ground. Good way to see if engine makes full power. The type certificate should say what you should be seeing.

Static RPM check is good and per the book. Did one on the annual a few months ago.


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[MikeOH]

Your first picture at 3,000 feet shows only 23" MP.  Was that with the throttle fully forward?  If so, seems like you should see closer to 26" MP

[Ronnie Pool]

Did the same bunch that installed things incorrectly also overhaul the engine?  Compressions have nothing to do with any timing discrepancies of course.  I thought even chrome jugs should be run hard to seat rings?  When I break in after cylinder work, I fly max power down low and just fly around local airports in case I need to land quickly.

Negative on that. Twin Aviation in Georgia rebuilt the engine and although the price for rebuild seems impossibly low ($18k in 2022) they seem to have done a great job and all related documents for parts seem to add up. According to the previous owner, a mechanic did some work on the aircraft a while back and there was some angst between the two of them afterwards. I’ve tore the plane apart for the annual and found multiple items such as the nose gear doors being installed with the hinge facing wrong causing the doors to try and overlap, one gear was drooping more than the other when closed, and the cowl flaps cable improperly secured causing them to not open or close completely to name a few. It’s been an ongoing process of fly and investigate. I am building my knowledge at this time but may have to break down and see a service center at some point. As an A&P IA I hate doing that as I loose the experience.


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[Ronnie Pool]

Your first picture at 3,000 feet shows only 23" MP.  Was that with the throttle fully forward?  If so, seems like you should see closer to 26" MP

The throttle was not full forward. I have flown it at higher settings with the same speeds showing.


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[Ronnie Pool]

Your first picture at 3,000 feet shows only 23" MP.  Was that with the throttle fully forward?  If so, seems like you should see closer to 26" MP

The below pic is in MPH. But it
Shows the affect of higher manifold pressure.


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[MikeOH]

4 minutes ago, Ronnie Pool said:

The throttle was not full forward. I have flown it at higher settings with the same speeds showing.


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Not sure I'm following...are you saying that if you pushed the throttle full forward in that photo (i.e. 3,000 feet) the plane would not have gone any faster???