RPM Stumbles

[mkrakoff]

Hey all, I've been troubleshooting an issue (with Savvy) that has been happening. So, after start everything seems fine, I taxi (very lean) and do a run up (rich enough to be smooth and then re-lean). While taxiing to the runway or waiting for departure I've noticed intermittent ( a few times a minute?) small RPM drops and it is accompanied by a very minor audible and engine stumble. I'll attach a video of one the I was able to capture while full rich waiting to take the runway. It doesn't seem to be associated with being cold bc it occurs after landing on taxi in as well (I pulled over and waited after my last flight to check). Thoughts? I submitted a flight for analysis to Savvy that included an in flight mag check which seemed uneventful. I'll pass along results from that when I get them.

 

 

 

 

[mkrakoff]

Quick follow up with savvy's response:

Based on what we have to work with so far, and your description of the mild roughness/burble at idle which sounds like Lycoming lopeness I'd suggest looking for induction leakage in #4. At low MAP, both at idle and landing we have the largest differential between MAP and ambient atmospheric pressure so any induction leakage is going to be most symptomatic at this time. Lycoming are famous for leakage in the sniffle valve at idle, so besides typical induction plumbing leakage, be sure to check the cylinder drains and sniffle valves. The sniffle valve has a plastic ball in the line that can become gummy preventing the normal sucking of the engine cylinders from pulling the ball up and sealing it and then allowing air to leak in from the drain. If its a pretty rough idle, one easy test for the sniffle valve, is to pinch off the drain line where it exits the cowling after the valve and see if it improves the idle. Of course this won't fix a hole in the line above the valve (but past the cylinder drain) that could be leaking - so be sure to exhaust all possible sources of leakage doing a soapy water test. 

Can anyone define the term "Lycoming Lopeness"? Google couldn't
 

Edited November 10, 2020 by mkrakoff

[PT20J]

I recall Don Maxwell once describing sniffle valve leaks as giving Lycoming M20Js a characteristic idle roughness. I don’t remember if he described it as a “lope” and I don’t recall where I read it. The M20J sniffle is a Mooney-made part. Mine didn’t seal perfectly even after cleaning it. It’s worth checking to see if it’s stuck. Also, IO-360s can leak at the gaskets where the intake tubes attach to the sump and I’d check the bolt torque where the tubes attach to the heads.

Skip

[mkrakoff]

2 hours ago, mkrakoff said:

al induction plumbing leakage, be sure to check the cylinder drains and sniffle valves. The sniffle valve has a plastic ball in the line that can become gummy preventing the normal sucking of the engine cylinders from pulling the ball up and sealing it and then allowing air to leak in from the drain. If its a pretty rough idle, one easy test fo

Thanks Skip. It's always fun working out the kinks.  

[N201MKTurbo]

It could be the sniffle valve, but you may just have your idle mixture too rich.

[mkrakoff]

4 minutes ago, N201MKTurbo said:

It could be the sniffle valve, but you may just have your idle mixture too rich.

I try to taxi very lean (that video was at the runway hold after richening the mixture) and it is happening in both situations, so I'm thinking it's more likely something else.

Edited November 10, 2020 by mkrakoff

[N201MKTurbo]

16 minutes ago, mkrakoff said:

I try to taxi very lean (that video was at the runway hold after richening the mixture) and it is happening in both situations, so I'm thinking it's more likely something else.

When I say your idle mixture, I don't mean where you have the mixture knob set, I'm referring to the adjustment on your fuel servo. The fact that you have to taxi with the mixture knob out indicates that it is adjusted too rich.

[mkrakoff]

9 minutes ago, N201MKTurbo said:

When I say your idle mixture, I don't mean where you have the mixture knob set, I'm referring to the adjustment on your fuel servo. The fact that you have to taxi with the mixture knob out indicates that it is adjusted too rich.

Gotcha. However, I don't just taxi with it out, I taxi lean enough that i have to enrichen it to add power (just rich of rough). I've replicated the RPM drop at all mixture knob positions. 

Edited November 10, 2020 by mkrakoff

[N201MKTurbo]

Just now, mkrakoff said:

Gotcha. However, I don't just taxi with it out, I taxi lean enough that i have to enrichen it to add power (just rich of rough). I've replicated the RPM drop at all mixture knob positions. 

Have you tried it on one mag or the other? When the points wear to where they barely open, they kind of do that.

If it was the sniffle valve, it would be mixture dependent. An induction leak changes your mixture. 

[mkrakoff]

50 minutes ago, N201MKTurbo said:

Have you tried it on one mag or the other? When the points wear to where they barely open, they kind of do that.

If it was the sniffle valve, it would be mixture dependent. An induction leak changes your mixture. 

I haven't tried that yet! I will. I have my A&P taking a look tomorrow.

[carusoam]

mkr,

Did you have JPI data to share?

I see you shared something with Savvy...

It would be good if we could see it as well...

The video is clearly showing the result... we weren’t questioning that...  (your judgement is good)

If the sniffle valve is allowing some air in.... or an air leak doing the same) it might be possible to use more /richer mixture to affect the stumble...

Something you can experiment with to define if more or less fuel changes the mixture enough to cover for the extra air leaking in... or exacerbate it even more...

Shared JPI data is really cool... some people around here can read them very quickly and point out some things that can be really helpful...

A stumble that noticeable is most likely a fuel ratio challenge... if it were a spark issue... it wouldn’t be as big... unless it is missing one spark already, and the second one is dropping off line...

JPI data will show it as it happens... where it happens... how it happens...

Its like a puzzle... kinda fun to solve... 

Best regards,

-a-

[mkrakoff]

I'll try an post the data when I get a chance. So far, my mech has found a frayed ignition wire on the #4 Cylinder, which is the same one that Savvy called out. Should have more info later.

[Yetti]

When was your Fuel Servo last rebuilt?  Flow Divider?   At mininum do a baby jar test and clean your injectors.    Texas carburetors and Injectors in Justin Texas would be a place to redo everything.

[bmcconnaha]

mine does the exact same... im having an IO390 built for it at the moment from lycoming, so not sure how far ill get in diagnosing it tho 

[mhrivnak]

@mkrakoff did you ever get this resolved? What fixed it?

[mkrakoff]

Yup! Looks like it could have been a number of things BUT after discussing it and completing my annual with Henry Weber MSC it is resolved. Here are a few of the corrective actions we took during the annual that may have solved it:

Alternate air door had low tension. - re-tensioned

Sniffer valve was “cleaned” (they sprayed it with a cleaner that should help it operate properly at lower rpm’s)

Replaced #4 intake gasket and seal (looking at my original post savvy called out #4)

hope that helps at least get you looking in the right direction if you have the same issue  

 

[Guest]

The ball in the sniffle valve can become sticky over time, a short soak in the ultrasonic cleaner helps, or a new valve.  Lycoming have their own versions.

Clarence

[PT20J]

44 minutes ago, M20Doc said:

The ball in the sniffle valve can become sticky over time, a short soak in the ultrasonic cleaner helps, or a new valve.  Lycoming have their own versions.

Clarence

I think the Lycoming valve is probably better. But Mooney made its own right angle one to clear the muffler on the J. It’s a 90 degree fitting with a plastic ball and a pin to hold the ball in. When we replaced the engine, I soaked it and cleaned it thoroughly and it never sealed perfectly.

[Guest]

You could screw a 90* elbow into the bottom of the engine, and then screw the Lycoming valve into it, simulating the Mooney ball style valve.

Lycoming p/n LW75444

Clarence

[201Steve]

Lycoming much cheaper than the “Mooney” part, also. 

[PT20J]

1 hour ago, M20Doc said:

You could screw a 90* elbow into the bottom of the engine, and then screw the Lycoming valve into it, simulating the Mooney ball style valve.

Lycoming p/n LW75444

Clarence

Maybe. Have you tried that? There’s not much clearance there.