Low RPM Misfire M20F

[tylerkocon]

Hey everyone. I own a M20F that I just recently purchased. The only thing I have found with the airplane the bothers me is a slight misfire at low rpm settings. The mag check is really good and all the EGT's are very similar up at cruise. I have tried running the engine hot on the ground to burn off any carbon deposits on the spark plugs but the plugs seem to be fine. Anything about 1300rpm or so there is no misfire at all. The engine run perfectly. The engine is the IO-360-A1A. 

Does anybody have any ideas on where to start?

 

Thanks

[N201MKTurbo]

It is probably too rich. Have you tried leaning it out to see if it smooths out? If it does have your mechanic lean the idle mixture.

[N201MKTurbo]

That's the J'est F I have ever seen.

[tylerkocon]

Yes we tried leaning and it did not help. And yes! It has every speed mod you can put on it. It was an old Mod Works bird. Love it! 

[N201MKTurbo]

Do you have an engine monitor? If so do you see any of the EGTs going wild when it gets rough? Have you tried a mag check when it is rough?

Edited May 20, 2018 by N201MKTurbo

[Yetti]

Could be an exhaust valve.  how many hours on the mags?  When was it timed last?  What was the timing set at?

[tylerkocon]

100hrs since new mags, 100hrs since IRAN, 600hrs since new engine. It's not even that the engine is running rough...its more of a small vibration/misfire. I have flown rentals that run way way more rough haha. I might go take a video tomorrow. 

 

I do have a JPI 700 and there is nothing unusual with the EGT's. I might go do a run up this evening and play with the mixture again and just double check she isnt idling too rich

[Marauder]

100hrs since new mags, 100hrs since IRAN, 600hrs since new engine. It's not even that the engine is running rough...its more of a small vibration/misfire. I have flown rentals that run way way more rough haha. I might go take a video tomorrow. 
 
I do have a JPI 700 and there is nothing unusual with the EGT's. I might go do a run up this evening and play with the mixture again and just double check she isnt idling too rich

You may not see anything on the JPI unless the data is collected at 1 second intervals.


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[Oldguy]

13 hours ago, Marauder said:

You may not see anything on the JPI unless the data is collected at 1 second intervals.
 

 

And then upload it to Savvyanalysis.com if you want, and get a good look at how things correlate and what is actually going on in the engine. 

[RobertGary1]

If you advance the throttle does the the RPM increase more than a couple hundred RPM? If so you're too rich. Adding power for taxi should require a temporary increase in mixture or you're already too rich.

-Robert

Edited May 21, 2018 by RobertGary1

[Stephen]

22 hours ago, N201MKTurbo said:

That's the J'est F I have ever seen.

I thought they are all "J" looking "F's"    

[Brian Scranton]

Do you see any fluctuation in fuel pressure? Does this happen at all when you pull the power back to land or only when the engine is cold?

 

[MARZ]

Intake Leak?  The offending cylinder(s) will have a lower egt at idle than the rest, and will drop faster from high idle to idle.

[jetdriven]

I think the leaking cylinders will have a higher egt at idle. They are leaner. 

[N201MKTurbo]

7 minutes ago, jetdriven said:

I think the leaking cylinders will have a higher egt at idle. They are leaner. 

Why would they be leaner?

The fuel injectors spray the same amount of fuel no matter what. Any leak in the intake system will increase the manifold pressure for all cylinders.

Edited May 22, 2018 by N201MKTurbo

[tylerkocon]

I am meeting with my mechanic tonight and will have some for info for you guys. My #1 EGT runs cooler than the rest but by no more than like 80 degrees. I have not noticed the issue when landing just when taxiing around bellow 1100rpm or so. I do not think it is a fuel leak because i have not noticed any blue staining from the cowling but will do some more investigating with the cowling off. 

We are probably going to start with compression and when we run it up I will get some video to share.

 

Thanks for the help!

[bradp]

And when you sweep the mixture to idle cutoff you are getting a 50 rpm bump prior to it’s shutting down, correct? 

I’m trying to get a sense of whether this sounds like missing or loping.  

Slick mags with 100 hrs have a high chance of losing egap and delivering an inefficient spark.  What kind of plugs do you have ? 

[jetdriven]

2 hours ago, N201MKTurbo said:

Why would they be leaner?

The fuel injectors spray the same amount of fuel no matter what. Any leak in the intake system will increase the manifold pressure for all cylinders.

Depends on the leak. If the leak is near the fuel servo, it’s centrally located and yes, it’s simply unmetered air to all cylinders. But one cylinder leaking at the head to intake pipe, that leak is specific to that cylinder. That cylinder gets the same fuel as all the others but has more air. Hence leaner. A guy at our local airport had to replace two cylinders on his Cherokee 180 and didn’t know why. The broken primer lines on both cylinders made them overheat and temper the rings. 

[N201MKTurbo]

11 minutes ago, jetdriven said:

Depends on the leak. If the leak is near the fuel servo, it’s centrally located and yes, it’s simply unmetered air to all cylinders. But one cylinder leaking at the head to intake pipe, that leak is specific to that cylinder. That cylinder gets the same fuel as all the others but has more air. Hence leaner. A guy at our local airport had to replace two cylinders on his Cherokee 180 and didn’t know why. The broken primer lines on both cylinders made them overheat and temper the rings. 

That's a carbureted engine, it is a whole different animal.

[tylerkocon]

Yep! They are 100hr slick mags. RPM drop is about 75 on both mags at 1700rpm. They are tempest plugs. When we take them out to check compression I'll snap some photos. Annual is due in November so if they're looking old and have a bad gap I might buy a new set now. 

We had the cowling off recently and I remember taking a good look at the fuel sender and injectors and did not see any staining. I'll pull the bottom cowling off as well and have my A&P take a look at each cylinders intake manifold. 

He flew the plane with me last week and was thinking an exhaust vulvae may need to be bored, but he wanted to look at a few other things first. 

 

Thanks for the help everyone, I'll have more updates this evening. 

[bradp]

$15 endoscope / borescope / dental camera can tell you a lot about the condition of your exhaust valves prior to any potential need for work.  

Usually valve related failure modes while at idle are more commonly associated with morning sickness.  There’s a saavy algorithm called FEVA that @kortopates could probably elaborate on for prediction of exhaust valve failure.  

[jetdriven]

7 hours ago, N201MKTurbo said:

That's a carbureted engine, it is a whole different animal.

Not wirh respect to vacuum leaks near the head. That cylinder specifically will run leaner. It takes in unmetered air. One guy on here had the hex plug on the head near the intake pile fall out. Ask him how it ran. 

Edited May 23, 2018 by jetdriven

[N201MKTurbo]

11 minutes ago, jetdriven said:

Not wirh respect to vacuum leaks near the head. That cylinder specifically will run leaner. It takes in unmetered air. One guy on here had the hex plug on the head near the intake pile fall out. Ask him how it ran. 

Wfhatever

[jetdriven]

Well, explain the mechanism a vacuum leak makes it run richer?  Beucause I’m a mechanic and i’m not following you here. 

I can see how, on a turbo engine, a boost leak makes the whole engine run richer. But a NA engine has intake vacuum. 

Here’s some more info.  From: https://www.savvyanalysis.com/articles/in-flight-diagnostics

 

 the principle behind this test: During the high-MP test, the induction manifold pressure is very close to outside ambient pressure, so any induction leak will have little or no effect on engine operation. During the low-MP test, the manifold pressure is significantly lower than outside ambient (by about 10 inches), so any induction leak will cause the affected cylinder (or cylinders) to run substantially leaner than the others, resulting in a smaller drop in EGT than the others.

Edited May 23, 2018 by jetdriven

[Guest]

If you’re looking for induction or exhaust leaks, why not just connect the outlet side of a clean vacuum cleaner to the tail pipe, close the throttle and turn on the vacuum cleaner.  Now spray soapy water on all of the flange gaskets and intake pipe O rings or couplings.

Clarence