All of a sudden, engine runs rough at low RPM

[3914N]

Hoping some of you can help me diagnose an issue I experienced this morning.

Airplane came out of annual 4 weeks ago with the #3 cylinder overhauled after the discovery of a worn exhaust valve guide. Cylinder walls honed, new piston rings, sump full of mineral oil for the break in. 

I put 5 hours of >75% power flying on it over two weeks. During the last flight the CHTs had seemed to settle down a lot, so I’m hoping the rings on #3 set okay. Immediately after this, airplane was down for 2 weeks for ADS-B install. 

Picked up the airplane this morning and did an engine run on the ground. Start up was normal, but once idling at 1000 RPM the cowl was shaking violently and the engine running extremely rough. Increasing RPM to above 1300 made the engine run smoothly again. A mag check at 1700 showed some roughness, so I did a quick burnout and then got a clean mag check. 

I’m suspecting morning sickness, so I idle at 1500 for a bit waiting for the oil temp to get in the green, then do a few full-power run ups. Once oil temp and CHT are both solidly in the middle of the green, I reduce rpm back through 1300 and the cowl shaking starts again, just as bad as before. Reducing power to idle, and the engine quits!

Never experienced anything like this in the last four years of owning this airplane. And I’m the typical guy with no engine monitor so I don’t have any empirical data to share. Thoughts as to what this could be?

Summary:

• 5 hours into a cylinder break-in with no abnormal observations
• Running the engine after two weeks of no activity, wet outside and temperature colder than normal but not too cold (38F)
• Extremely rough running below 1300 RPM, even with oil temp and CHTs solidly in the green, and dies at idle, neither of which have ever happened before
• Everything sounds and feels normal above 1300 RPM

Edited November 28, 2019 by 3914N

[markgrue]

 

57 minutes ago, 3914N said:

Hoping some of you can help me diagnose an issue I experienced this morning.

Airplane came out of annual 4 weeks ago with the #3 cylinder overhauled after the discovery of a worn exhaust valve guide. Cylinder walls honed, new piston rings, sump full of mineral oil for the break in. 

I put 5 hours of >75% power flying on it over two weeks. During the last flight the CHTs had seemed to settle down a lot, so I’m hoping the rings on #3 set okay. Immediately after this, airplane was down for 2 weeks for ADS-B install. 

Picked up the airplane this morning and did an engine run on the ground. Start up was normal, but once idling at 1000 RPM the cowl was shaking violently and the engine running extremely rough. Increasing RPM to above 1300 made the engine run smoothly again. A mag check at 1700 showed some roughness, so I did a quick burnout and then got a clean mag check. 

I’m suspecting morning sickness, so I idle at 1500 for a bit waiting for the oil temp to get in the green, then do a few full-power run ups. Once oil temp and CHT are both solidly in the middle of the green, I reduce rpm back through 1300 and the cowl shaking starts again, just as bad as before. Reducing power to idle, and the engine quits!

Never experienced anything like this in the last four years of owning this airplane. And I’m the typical guy with no engine monitor so I don’t have any empirical data to share. Thoughts as to what this could be?

Summary:

• 5 hours into a cylinder break-in with no abnormal observations
• Running the engine after two weeks of no activity, wet outside and temperature colder than normal but not too cold (38F)
• Extremely rough running below 1300 RPM, even with oil temp and CHTs solidly in the green, and dies at idle, neither of which have ever happened before
• Everything sounds and feels normal above 1300 RPM

A few things to try:  

Carb heat..  could be carb ice.  

Lean the mixture at 1000 see if it clears up or just wants to die right away.  Does it get worse?

Does it do it on both mags at 1000?

Repeat on a warmer or cooler day and does it to it again?

Perhaps a less humid day?  

 

Mark

 

[3914N]

Thanks Mark. I’ll try again with carb heat and see if It changes anything and will report back. I’ll try messing with the mixture as well. 

I did try different mag settings at 1,000 RPM... no change on L/R/both. 

[Pete M]

The engine quit at idle...no bueno...see a&p. Goes without saying...don't fly it.

[PT20J]

Perhaps a leak in the induction system? Not enough to affect high power settings, but enough to make it too lean at idle. Did the radio shop do anything in the engine compartment?

[Gagarin]

I had the same problem on my 1966 M20C. The problem is related to flooding of the carburetor bowl at idle due to the float valve not closing the fuel port. You can verify this with the engine off and checking for fuel on the floor when the electric pump is turned on for 3 minutes or longer.  It does not happens at high rpm due to the engine higher fuel consumption. 

[3914N]

6 hours ago, Gagarin said:

I had the same problem on my 1966 M20C. The problem is related to flooding of the carburetor bowl at idle due to the float valve not closing the fuel port. You can verify this with the engine off and checking for fuel on the floor when the electric pump is turned on for 3 minutes or longer.  It does not happens at high rpm due to the engine higher fuel consumption. 

Thanks for the insight, I’ll check for this next time I make it out to the airplane. Did you end up having to do a full carb overhaul?

[carusoam]

if it is a mixture issue....

There are a couple of things to adjust... Idle mixture, and regular mixture...

How is your mechanic regarding working on carbs?

When was the carb OH’d last?

The M+S book was posted around here somewhere...  Marvell Shebler...

There are a few things you can do to test if things are set-up properly...

try adjusting the mixture first to see if it is running to rich... pull the knob out looking for improvements...

When killing the engine, pull the mixture slowly, looking for an rpm rise...

PP thoughts only, not a mechanic... all stuff you can find written about on MS...

Best regards,

-a-

[Skates97]

9 hours ago, carusoam said:

The M+S book was posted around here somewhere...  Marvell Shebler...

Here it is, along with the overhaul manual.

MA-4 Carb Manual.pdf marvel-schebler-OH.pdf

[carusoam]

Thanks Richard!

i get an O for two on the spelling...   

Best regards,

-a-

[3914N]

Thanks everyone! I’m going out to the airplane tomorrow morning to see if I can diagnose further and will keep everyone posted. 

[3914N]

Went out to the airplane today, fired it up, and everything was working perfectly. 

The only difference I could think of is that when I experienced the issue a few days ago, I had accidentally run the electric fuel pump prior to start with the throttle full in. I’m wondering if that could have flooded the carb. 

@Gagarin: I ran the electric fuel pump on the ground with the engine off, as you suggested. I was getting drops of fuel out of what I think is the electric pump overflow port on the pilots lower side of the cowl. Drips went away after I turned the pump off. No drips inside the cowling. Is this what you were experiencing as well?

[carusoam]

Fuel leaking from the pump vent is a sign of a leak... seal or pump diaphragm...

Depending on plumbing... that leak may allow air to enter the system... bubbles in the fuel line may be what you are describing...

Either way... get with mechanic to find out what is causing fuel to leak out of the pump’s vent... that is not normal...

Simple verification method... post pic of fuel leaking out of pump vent... and people will verify... yep! That’s fuel leaking out the pump vent... get it OH’d or re-sealed...

As for rough running after a flooded start... flooding the engine usually keeps it from starting... once cleared, the effects of the initial flooding are gone...

PP thoughts only, not a mechanic...

Best regards,

-a-

[n3914n]

Update after talking to my A&P.  Here is his list of suspected causes, in order of most to least likely...

1. Water in the float bowl.  It was raining heavily when I picked up the airplane, and didn't sump the tanks since I was just taxiing across the field.  The theory is that water could have found its way into the carb float bowl, causing rough running only when at low RPM.  This would also explain why the issue couldn't be reproduced today (no rain).
2. Clogged/contaminated carb idle circuit.  Some FOD such as a bit of tank sealant could have slipped past the fuel strainer and clogged something.  The fuel strainer was cleaned only 5 weeks ago at annual, but we are going to pull the strainer and verify it isn't contaminated next time we are out at the airplane.
3. Sticking carb float.  Mechanic thinks this is unlikely since we couldn't reproduce it, and because he considers the carb to be relatively young (OH'd in 2006).
4. Air in the fuel line.  This would be due to the deteriorating o-rings in the electric fuel pump, as @carusoam mentions above.  Mechanic thinks this is unlikely due to the severity of the roughness, and because I've known about this leak for the last few flights that went off without incident.

We ruled out ignition problems since I had passed a mag check at high and low RPM, and we ruled out a stuck valve since the roughness did not smooth out at high CHT.

In the end, my mechanic recommended I continue to fly but to keep an eye on things and perform a idle check at every run-up.  If I have any issue at all in the future, we will know its very likely a carb issue and begin the overhaul process.  And we're going to get the pump overhauled as well.

Thank you @carusoam, @Skates97, and @Gagarin for your insight.  I'll add to this thread if anything else happens.

[carusoam]

What changed your that you are logging in under a similar yet different name/ID?

 

As for water getting to the carb... it has to get past the fuel caps as well... give a good look to what can allow water to enter the tanks...

Two Orings per cap...

Best regards,

-a-

[PT20J]

3 hours ago, n3914n said:

Water in the float bowl.  It was raining heavily when I picked up the airplane, and didn't sump the tanks since I was just taxiing across the field.  The theory is that water could have found its way into the carb float bowl, causing rough running only when at low RPM.  This would also explain why the issue couldn't be reproduced today (no rain).

Water would sink to the bottom of the float bowl. Looking at the docs @Skates97 supplied, the outlet for the main metering system and the idle system both come from the bottom of the bowl, so the water should affect both idle and higher powers. Also, you said still had idle issues after running at higher power for enough time to exhaust any water.

3 hours ago, n3914n said:

1. Clogged/contaminated carb idle circuit.  Some FOD such as a bit of tank sealant could have slipped past the fuel strainer and clogged something.  The fuel strainer was cleaned only 5 weeks ago at annual, but we are going to pull the strainer and verify it isn't contaminated next time we are out at the airplane.

Hard to imagine how something would get past the carb inlet finger screen, the float needle valve, the metering jet and the get stuck in the idle system and then find it’s way out again. 

I had a stuck float once on a Beaver and it wouldn’t idle, but would run well at higher powers. I also had a carburetor come loose on a Beaver (apparently it wasn’t torqued properly) and it created enough of an induction leak that it idled poorly but ran OK at higher powers. 

I would check the security of everything in carburetor/induction system before flying it  just to be sure nothing’s loose. Might be worthwhile to leak check the induction tubes.

Skip

 

[carusoam]

+1 Skips logic..

if running poorly at low power...

and improves at high power...

Expect to find a small air leak into the intake system...

The small leak greatly affects the low power air/fuel mixture...

The same small leak hardly shows up in the high power air/fuel mixture...

Since leaks have a tendency to become unstable and grow...  it is not something to become comfortable with and ignore...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Yetti]

You probably lost a cylinder and then it came back.   Pull the plugs let them tell you a story.   You may have fouled them doing the break in.   I would do lots of ground running and fast taxis before flying it, since you don't have a defined cause.  Then your first flight should be above the airport.