Idle engine RPM is much higher after landing than before takeoff

[Nokomis449]

Just had Penn Yan rebuild my O-360A1D on an M20G.  Also had an EDM900 installed.  Still working through some engine/install squawks, but this particular one puzzles me.  The engine seems to run great and fires up in a blade or two on a cold start (SoS).  Engine will idle smoothly at about 750-800 RPM.  Taxi out and runup are typical - idle check is still 750-800.  After about 25 hours the break-in seems to be typical of what I was told to expect.  The puzzler is that on approach and landing, especially in the flare, it becomes evident that pulling back to "idle" still gets me about 1200 RPM, leading to a long, LONG float before touch down.  Tonight it would NOT go below about 1200 even after roll out, and the slightly uphill taxi was at about 12kts without braking.  Possibly related is that the prop governor only allows about 2575 RPM both static and during takeoff roll.  Scheduled to go back to the shop next week, but curious to know why the idle RPM changes.  Also possibly related, I've told the IA that the prop cable seems to be harder to pull/push than before, but he checked it and said it's fine.  I've checked it visually and the cable doesn't appear to be binding/have too tight turns (per the IA). 

Any ideas?

[markgrue]

At 1200 rpm the prop governor is not doing anything.  The prop is on the low pitch stop. When you do an idle mixture check what is the rpm rise.  In other words when the engine is idleing after warmed up and you pull the mixture out slowly how much does the rpm rise before it dies?  It should be about 50 rpm.  If it is more it is too rich.  If it is less then it is too lean at idle.  Does the throttle hit the stops at both ends.  When the engine is cold it should not run smoothly at idle until it has warmed up just a little bit.  If it does then it is probably very rich at idle.  Just a few things to look at.

 

Mark

[Nokomis449]

I'll double-check those the next time I'm out there.  I mentioned the governor mainly it definitely needs adjusting to get to 2700rpm.  Throttle cable does hit both stops.  Gear horn sounds at about half throttle but that's just a simple adjustment under the panel, I can't imagine it's connected to any of the other issues.  I'll also shut down the engine and check that the throttle cable is making it all the way to the idle stop next time I can't get it below 1,000rpm.

[RLCarter]

3 minutes ago, Nokomis449 said:

I'll double-check those the next time I'm out there.  I mentioned the governor mainly it definitely needs adjusting to get to 2700rpm.  Throttle cable does hit both stops.  Gear horn sounds at about half throttle but that's just a simple adjustment under the panel, I can't imagine it's connected to any of the other issues.  I'll also shut down the engine and check that the throttle cable is making it all the way to the idle stop next time I can't get it below 1,000rpm.

Make the changes when its up to temp, also its the Arm that the cable attaches to that needs to go stop to stop not the cable

[Nokomis449]

Just now, RLCarter said:

also its the Arm that the cable attaches to that needs to go stop to stop not the cable

Right, that's what I was referring to.  I checked the arm on the carb and it was moving stop to stop but I'll put eyes on it again.  Both the throttle and the mixture control "feel" different, but maybe that's expected when going from a 2500 hr engine to 0 SMOH.

[carusoam]

Very often... at the time of OH, the control cables get renewed...

If they are old, and feel different... check the routing to see if something didn’t get put back the same...

Check and see how old they are... and why they aren’t operating the way you expect...

If you have the vernier adjustment on your throttle...  don’t forget to unwind it until it hits the stop.... (all the way out)

The nice thing about 1200 rpm during landing.... it will be the softest landing ever... if it actually finishes flying... 

Engine set-up after install is key... sounds like a few steps got left out...  or a couple of things aren’t working as expected...

Min rpm isn’t right (prop stops)

Max rpm isn’t right (gov)

Check the rpm rise as the mixture is pulled slowly for idle cut-off...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Floyd]

Check for an induction  leak.

[PT20J]

You mentioned checking the throttle and mixture cables can drive the associated control arms to the stops on the carburetor. Did you also check that the prop control can drive the governor arm to the stop with about a 1/8" cushion before the control hits the panel? The prop governor should not affect the rpm during a static run up. The prop should be sitting on the low pitch stop and the rpm (assuming full engine power) should be within 100 rpm below redline. Often it will be about 2650. As you begin the takeoff roll the airflow will want to speed the prop up and a properly adjusted governor will stop it at 2700. Also, it's always a good idea to check that the tach is accurate. I assume that your A&P has an optical tach for that purpose.

Skip

[Nokomis449]

Good things to ponder.  I haven't personally checked the prop control travel since I know he has to adjust it anyway.  Don't know if he used an optical tach (I wasn't around his shop when he did the annual/engine removal and install), but he installed an EDM900 in the process.  The factory tach (newly replaced about 10 years/1000 hrs ago) is still in the panel and both it and the EDM tach agree on the RPM's.
I'll look into the possibility of an induction leak, but the engine runs very smooth at all RPM's, including low RPM's.  It just likes to land with about 450 more RPM's at idle than it takes off with, and never produces more than 2600 RPM's at full power on the takeoff roll, and after wheels up it rarely hits more than about 2550 even though MP will hit about 28.  Not sure the 2600 RPM limit and the additional idle RPM's at landing are even related.

My long-time friend and IA passed away this year and this is the first time in many years that someone different turned a wrench on the Mooney, so my "go to" expert isn't around to coach me.  The new IA has some pretty large shoes to fill, and he's only about 5' 8".

[carusoam]

Sorry to hear about your loss...

Happy to think about the opportunity to get data out of you monitor...

post it to Savvy...

Click the share button...

copy link here...

Best regards,

-a-

[Shiny moose]

 visually make sure your carb heat is working properly and its also closing and opening completly, not loose. No leaks in exhaust or intake, recheck them for signs of leaks. Recheck those carb stops. Mag timing?? 

Idle and mixture settings only when engine is to temp, not just warm, no more than 50RPM increase ( 25 better) when mixture its pullled. Dont forget to clear between adjustments

then you can move on to prop take off  RPM 

My .02

[takair]

Your baseline idle RPM seems high.  I think it should be 550RPM....maybe 650....but 750 is high.  As others suggested, mixture and idle should be set once the engine is warm....so perhaps after landing.  As Shiny Moose said, check also the carb heat.  

[Skates97]

As Skip said make sure to check the prop control and that it is getting full travel. I had an instance where the cable had slipped in the clamps and no longer allowed the governor to hit the stop resulting in not getting to 2,700rpm.

The reading on the 900 "should be" accurate as it is taking its reading from the mag, not a cable like the factory tach. While you're at it pull the factory tach and the other gauges you don't need anymore now that you have the 900, it is primary. All you need to pull the factory tach is a cap that gets screwed on and safety wired. $32.50 from AS. 

https://www.aircraftspruce.com/catalog/inpages/tachcap.php?clickkey=13626

Also as Rob said get that idle down closer to 500. Between 500-700 but as close to 500 as you can get it and still have it run smoothly. The higher your idle the longer your landings. Just be aware that the first few will be different with a low rpm if you are used to the extra power, you have to be more careful to keep from planting it on the runway. The nice thing is that you will need much less runway to land with the lower idle. 

Also as Rob said it will idle different after warm up and a trip around the pattern so do that and them make your fine tuning adjustments. 

[skykrawler]

A 750 to 800 rpm idle is too high IMHO.  Needs to be 550 to 650.

I replaced the original control cable rod ends with spherical bearing joints.  The hole had elongated on original cheap ball/socket joint (which was 36 years old) would bind sometimes and prevent the controls from reaching the closed stop.  The was a problem on both the mixture and the throttle.   The parts are available in a kit, but you can buy them from separately from Spruce.

[Guest]

Your idle speed is way too high.  From the maintenance manual 550 RPM.

Clarence

[Shiny moose]

On 12/27/2020 at 5:52 AM, skykrawler said:

A 750 to 800 rpm idle is too high IMHO.  Needs to be 550 to 650.

I replaced the original control cable rod ends with spherical bearing joints.  The hole had elongated on original cheap ball/socket joint (which was 36 years old) would bind sometimes and prevent the controls from reaching the closed stop.  The was a problem on both the mixture and the throttle.   The parts are available in a kit, but you can buy them from separately from Spruce.

Did you find PN 550017-005 at Spruce? Seems to be a Mooney PN actullay Nose gear spacer?

[skykrawler]

25 minutes ago, Shiny moose said:

Did you find PN 550017-005 at Spruce? Seems to be a Mooney PN actullay Nose gear spacer?

No.....A&P friend had some of various sizes.  Same spacers used for something on a 757.

[brndiar]

On 12/27/2020 at 1:27 PM, M20Doc said:

Your idle speed is way too high.  From the maintenance manual 550 RPM.

Clarence

Hi, 

how do you translate "rolls" and "lags" in point #3 & #4 ? 

Not sure what they mean by that.

thanks, m

 

[brndiar]

My thread to Carb. Idle Mix setting.

Carburetor Idle Mixture setting (practically) - Vintage Mooneys (pre-J models) - Mooneyspace.com - A community for Mooney aircraft owners and enthusiasts

lg,

m

[Guest]

5 hours ago, brndiar said:

Hi, 

how do you translate "rolls" and "lags" in point #3 & #4 ? 

Not sure what they mean by that.

thanks, m

 

It must be Texan speak?  
 

With the engine warmed up and idling around 650-700 RPM, pull the mixture to ICO.  You should see a rise of 30 RPM.  If the mixture is too rich the rise will be greater, if it’s too lean there will be less of no rise.  Turn the idle mixture needle valve counter clockwise to enrichen, and turn clockwise to lean the mixture.  
 

Once idling smoothly you can adjust the idle speed lower toward the 550 RPM if desired, low idle RPM produces lower thrust and shorter landings.

Clarence