engine mis-fire during idle

Peter Ha

Peter Ha

Pre-takeoff checklist
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Nov 18, 2019
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Leadpan
Hey folks,
My C150 engine runs fine in higher rpm but mis-fire every so often when close to idle.
The AP says might be valve, magnetos points, or spark plugs. I do have high time engine (1500hrs) with one cylinder just replaced during annual.
Anyone else experience something similar?
 
Just leave it alone, 90% of the 0-200s do that. I have about 50 guys that have a an explanation for it. no one have a problem with it
 
I accepted similar (leave it alone) advice like this many years ago. It was a question I had about the spark plug caps on an inverted Rotax engine installation in a UL that I was building. I was concerned about the possibility of the caps coming loose during take off rolls on rough strips.

Sure enough, I had just lifted off, barely cleared the power lines at the end of the backyard strip, and I faced a choice. I could land straight ahead in the trees, or jog left into an open field with a barbed wire fence across the middle. I jogged left and set it down hard. Missed the fence, but destroyed the undercarriage. Nothing like your butt strapped to a seat pan skidding along the ground at 35 or 40 mph because a two cylinder two stroke won't make any power when it's running on one cylinder.

That's been over twenty years ago, and from that day forward I never again listened to anybody telling me that something is normal, or to just leave it alone. Your mileage may vary.

For several years I owned a 150 with an O-200, and it never had mis-fire issues, or anything resembling for the 500 hrs I flew it, unless we want to talk about the valve that it stuck. It leaked oil, made ice, and stuck a valve, but otherwise, it started and ran just fine. Listen to your AP.
 
Peter Ha said:
I do have high time engine (1500hrs) with one cylinder just replaced during annual.
Click to expand...


How many hours or years since those magnetos were off for internal insepction? Were the sparkplugs cleaned and pressure tested for spark at the last annual?

Ignition is responsible for around 90% of engine performance issues.
 
Absent any other problems, and as long as the engine continues to idle, after said "mis-fire", and doesn't quit, I'd tend to agree with Tom.
 
wilkersk said:
Absent any other problems, and as long as the engine continues to idle, after said "mis-fire", and doesn't quit, I'd tend to agree with Tom.
Click to expand...
So, if you have this funny cough sometimes that still hasn't cleared up after three months, is everything still fine?
 
Dan Thomas said:
So, if you have this funny cough sometimes that still hasn't cleared up after three months, is everything still fine?
Click to expand...
I don't think that is all to it either..
There is a lot we don't know about the engine.
 
Dan Thomas said:
How many hours or years since those magnetos were off for internal insepction? Were the sparkplugs cleaned and pressure tested for spark at the last annual?

Ignition is responsible for around 90% of engine performance issues.
Click to expand...

Suggest the OP take a progressive approach to troubleshooting. Start with the simplest step and work the way up. Clean, check and regap the plugs, then test the ignition leads, magneto caps (arcing), mag timing, induction manifold leaks, those sorts of things before going to the effort of pulling the mags themselves.

The OP has a new cylinder installed, which may warrant attention. Any time there's one change...
 
GRG55 said:
Suggest the OP take a progressive approach to troubleshooting. Start with the simplest step and work the way up. Clean, check and regap the plugs, then test the ignition leads, magneto caps (arcing), mag timing, induction manifold leaks, those sorts of things before going to the effort of pulling the mags themselves.

The OP has a new cylinder installed, which may warrant attention. Any time there's one change...
Click to expand...
Very seldom we will find one discrepancy.
 
Perhaps an overly simple question... is the engine aggressively leaned at idle and taxi? Also when was the last time the mags were overhauled and timing checked?
 
NordicDave said:
Also when was the last time the mags were overhauled and timing checked?
Click to expand...
I already asked that in post #5. My question is based on years of experience in correcting such performance problems by checking the logs to see how long things have been neglected, and tackling those first. Sometimes one find that mags that haven't been off in 1500 hours and 20 years are far beyond economical repair and have to be replaced with new mags. Real savings, that, and real risk of mag failure in flight. I tell you, you don't want to have corroded magneto bearings crumble and let broken bits of bearings and gears trash the whole accessory drivetrain and kill both mags and wreck the engine.

These are 40- and 50-year-old machines based on 1930s technology. They're not like new cars that run for years without anything more than oil changes. If you expect your airplane to run flawlessy without the required inspections and repairs, you'll suffer.
 
Dan Thomas said:
So, if you have this funny cough sometimes that still hasn't cleared up after three months, is everything still fine?
Click to expand...

It depends. Sometimes a "funny cough" is just a "funny cough". Sometimes, its a polite way of not saying "********"!
 
Tom-D said:
How much would you spend to correct the occasion skip?
we already have read that it runs well at power.
Click to expand...
I'd pay whatever it costs to fix it. My landings are usually at idle power.
 
Salty said:
Let me follow your logic. Because it might be the worst, he’s better off ignoring it. Okey dokey then.
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You must be able to figure out when the thread is ignoring you.
 
Doc Holliday said:
But your first post you stated to do nothing. If this is normal, as you allege in your first post, how can there be multiple discrepancies? o_O
Click to expand...
That's actually humorous, How naive can you be. :)
 
Tom-D said:
Then a total overhaul isn't out of the question.
Lucky you.
Click to expand...
Do you think it may be that an overhaul may be needed, in this case?
Why is hesitation at idle potentially symptomatic of an overhaul being required?
If a total overhaul is truly needed, it should be done.
 
Tom-D said:
Changing the topic won't help get your answer.
Click to expand...

OK, so quit trying to change the topic. ;)

The discussion: You made this post:

Tom-D said:
Just leave it alone, 90% of the 0-200s do that. I have about 50 guys that have a an explanation for it. no one have a problem with it
Click to expand...

Then later, you made this post in response to post 12.

Tom-D said:
Very seldom we will find one discrepancy.
Click to expand...

So on one hand you are telling the OP not to do anything, your words "Just leave it alone". But when someone said to start trouble shooting, you state "Very seldom we will find one discrepancy", which reading what you wrote means there could be several things wrong.

So by your logic, you know there is a problem, perhaps multiple problems, but you advocate the OP not to do anything and just accept the problem? o_O
 
Last edited:
And this ridiculous thread, tit for tat, silly little spat...........
Another reason why you should never come to a site like this for technical support, advice, or information.
 
Art Rose said:
And this ridiculous thread, tit for tat, silly little spat...........
Another reason why you should never come to a site like this for technical support, advice, or information.
Click to expand...

What makes it difficult when some here post really bad "advise" that could result in potential really bad problems later on.
 
Dan Thomas said:
So, if you have this funny cough sometimes that still hasn't cleared up after three months, is everything still fine?
Click to expand...
My best friend has had a funny little cough for a decade. After a few months he went to a doc. No answer. After another year, he went to another doc--no answer. After three years he went to another doc--no answer. Finally, after five years he went to another doc. It was a symptom of a bit of harmless reflux.

So, in answer to the question: Yes, sometimes.
 
Maybe OP could check the idle-mixture setting. Having the idle-mixture slightly rich might be preferable to worrying about an occasional rumble when throttle is closed.
 
The OP brought us a symptom that disturbs him. There is no way I would tell him to just keep flying it, since I'm not looking at the airplane and only have some hunches that the mags are getting tired and should be looked at before it's too late. It might be just a bad plug, but bad plugs don't often make trouble at idle, where the cylinder pressures are low, unless the mag is delivering weak spark.

If it bugs him he should first consult a mechanic, not a bunch of guys trying to isolate the problem over the internet, based on a few words. He might be looking for the "it's OK" so he can stop worrying, but what if....?
 
wilkersk said:
Maybe OP could check the idle-mixture setting. Having the idle-mixture slightly rich might be preferable to worrying about an occasional rumble when throttle is closed.
Click to expand...

He can check that himself. Warm up the engine, run it to 1200 RPM or so for a few seconds, return to idle, when it stabilizes start slowly pulling the mixture out. The tach should rise 25-50 RPM just before the engine dies. More rise and it's too rich. Less and it's lean.
 
Peter Ha said:
Hey folks,
My C150 engine runs fine in higher rpm but mis-fire every so often when close to idle.
The AP says might be valve, magnetos points, or spark plugs. I do have high time engine (1500hrs) with one cylinder just replaced during annual.
Anyone else experience something similar?
Click to expand...

OK, let's take this head-on:

Valve: Easy to check. leak-down test-does it leak? Borescope: is it burnt?

Magneto Points: A little more work, but, take the cap off, pull the mag, look at the points. Are they burnt? Is the gap set correctly?

Spark Plugs: Clean and test. How do they look? What is the internal resistance under load (spark plug tester)? Any cracked insulators?

All easy things to check. And, by the way, stuff your A&P should be doing at least every annual. But, if there were problems with any one of these things you mentioned, I'd think you'll have different symptoms. Backfiring when reducing throttle usually points to an over-lean condition. Since you're saying it happens at idle, I'd suspect Idle-mixture. And, you can chase your tail a bit getting that perfect for every day. Hence my suggestion that you might prefer a slightly rich idle-mixture setting.
 
Dan Thomas said:
I already asked that in post #5. My question is based on years of experience in correcting such performance problems by checking the logs to see how long things have been neglected, and tackling those first. Sometimes one find that mags that haven't been off in 1500 hours and 20 years are far beyond economical repair and have to be replaced with new mags. Real savings, that, and real risk of mag failure in flight. I tell you, you don't want to have corroded magneto bearings crumble and let broken bits of bearings and gears trash the whole accessory drivetrain and kill both mags and wreck the engine.

These are 40- and 50-year-old machines based on 1930s technology. They're not like new cars that run for years without anything more than oil changes. If you expect your airplane to run flawlessy without the required inspections and repairs, you'll suffer.
Click to expand...

Good post. We agree.
 
wilkersk said:
OK, let's take this head-on:

Valve: Easy to check. leak-down test-does it leak? Borescope: is it burnt?

Magneto Points: A little more work, but, take the cap off, pull the mag, look at the points. Are they burnt? Is the gap set correctly?

Spark Plugs: Clean and test. How do they look? What is the internal resistance under load (spark plug tester)? Any cracked insulators?

All easy things to check. And, by the way, stuff your A&P should be doing at least every annual. But, if there were problems with any one of these things you mentioned, I'd think you'll have different symptoms. Backfiring when reducing throttle usually points to an over-lean condition. Since you're saying it happens at idle, I'd suspect Idle-mixture. And, you can chase your tail a bit getting that perfect for every day. Hence my suggestion that you might prefer a slightly rich idle-mixture setting.
Click to expand...
We don't set points gap. We set the points to open at the E-gap, which gets them opening at the rotor position that gets the hottest spark. Anything else is less than ideal.
 
I'm surprised no-one has suggested an induction leak. An imperfect cylinder replacement could have caused this.
 
I flew a 150 for years and never had such an issue. I might disregard it, though had my engine recently caught fire I think I'd be a bit more cautious.
 
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