Engine roughness on climb

How to keep the Cessna 170 flying and airworthy.

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Vertical
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Engine roughness on climb

Post by Vertical »

Over the last 25 hours or so of flying I've been experiencing some engine roughness. Details below. Does anyone have any suggestions? Any help or experience would be greatly appreciated.

Aircraft:
1955 170b with mid time o300A
Averages approx 15hrs/month flight time.
Newer 80" climb prop.
Equipped with single cylinder CHT and EGT
Tempest Spark plugs approx 1 year old, cleaned and gapped >25 hrs ago.
New Slick mags and Harness ~6 years ago. -need to check logs for exact age.

Symptoms:
Minor to moderate engine roughness on VX full throttle climb
Occurs all altitudes, sea-level to 14k
Carb heat off, mixture rich.

Testing during full throttle VX climb, all altitudes:
Lean mixture, and engine smooths
1/4 open carb heat and engine smooths
Full open carb heat and engine runs slightly rougher momentarily with RPM drop as normal, and then engine runs smooth.

Testing while on Ground:
Mag check and carb heat check on the ground at 1600 RPM as well as 300 PRM (throttle out) behaves as normal with normal RPM drops. Runs smooth.

Take Off:
Engine power and rpm smooth and normal during ground roll and initial take-off.

Untested suggestions: -I haven't tried this yet
Lean mag test.-Aggressively lean aircraft in cruise (LOP if possible) and turn off each mag. -If one mag is rough, then look at spark plugs next?

Any other ideas/Suggestions?
Thank you!
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GAHorn
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Re: Engine roughness on climb

Post by GAHorn »

What air filter are you running?
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Vertical
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Re: Engine roughness on climb

Post by Vertical »

Challenger-K/N type
Replaced with new and oiled per instructions in approx Feb 2019. I'd have to dig into the logs, but maybe 40-50 hrs since then.
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Bruce Fenstermacher
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Re: Engine roughness on climb

Post by Bruce Fenstermacher »

Check your primer isn't leaking. When it does your engine runs rich. Notice you say the engine runs fine when you lean the engine either with mixture or carb heat!

Simple check. Disconnect primmer line at intake. Cap the manifold fitting. Fly and see if the issue has gone away.
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Vertical
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Re: Engine roughness on climb

Post by Vertical »

hmmm..doesn't adding carb heat richen the mixture Bruce? That's what's got me stumped, both leaning and richening seam to make it run smoother.

I suppose if the air filter was clogged and adding too much resistance, maybe adding a little carb heat would lean the mixture since it bypasses the filter and I might get more air volume. -just guessing here. But then why does it act fine on take-off, but not on climb....
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Bruce Fenstermacher
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Re: Engine roughness on climb

Post by Bruce Fenstermacher »

Vertical wrote:hmmm..doesn't adding carb heat richen the mixture Bruce?
Yes, yes it does. I don't know why I can't keep this straight.

Putting my inability to remember what carb heat does, it is a simple trouble shooting technique to eliminate the primer from the equation by caping it off. A leaking primer is often overlooked and has baffled more than one trouble shooting team.
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GAHorn
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Re: Engine roughness on climb

Post by GAHorn »

Vertical wrote:hmmm..doesn't adding carb heat richen the mixture Bruce? That's what's got me stumped, both leaning and richening seam to make it run smoother.

I suppose if the air filter was clogged and adding too much resistance, maybe adding a little carb heat would lean the mixture since it bypasses the filter and I might get more air volume. -just guessing here. But then why does it act fine on take-off, but not on climb....
Yeah...that’s how I was thinking, that an overly rich mixture from a blocked air filter and the alternate-air source of carb heat bypassing that filter might explain how leaning in both cases help.
The idea of Bruces is worth looking into.
I’d also ask “when was the last time someone looked up inside the induction system above the carb?” And...
“What kind of static RPM do you see at the beginning of the T.O. Roll?” And....
“AT what density altitude are you operating?”
Vertical wrote:Challenger-K/N type
Replaced with new and oiled per instructions in approx Feb 2019. I'd have to dig into the logs, but maybe 40-50 hrs since then.
Well, here it is a year-and-a-half almost and it should have been serviced at your last annual inspection. When was that?
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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DaveF
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Re: Engine roughness on climb

Post by DaveF »

Do the in-flight magneto check. If one is rough, I'd look at the plugs, as you said, but also definitely the magneto. Have the mags been IRAN-ed since being installed 6 years ago? 6 years at 15 hours a month is 1080 hours.
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Vertical
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Re: Engine roughness on climb

Post by Vertical »

I’ll do some digging in the logs when I get off work.
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johneeb
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Re: Engine roughness on climb

Post by johneeb »

What is the name of the gismo located in the oil sump induction tunnel? I could not find any of the discussion about how the engine runs when that gismo comes loose.
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GAHorn
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Re: Engine roughness on climb

Post by GAHorn »

johneeb wrote:What is the name of the gismo located in the oil sump induction tunnel? I could not find any of the discussion about how the engine runs when that gismo comes loose.
The diverter was a sheet-metal baffle riveted in early sumps but later sumps had it molded in the casing. While I haven’t seen this defined, I believe it’s another difference between the early sump and the later so-called “5-bolt” sump. The diverter is not illustrated in the parts catalog.

I’d guess that if a riveted diverter did come loose It could block an induction tube and create a rough-running engine.

AS for the OP’s question... One thing I’d look at is the carburetor, especially the float and float-valve to be certain it’s not sticking. Was the carb overhauled at any time? Did it incorporate the single-piece venturi and the latest float? (Colored plastic)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Vertical
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Re: Engine roughness on climb

Post by Vertical »

A few updates and clarifications after digging into the logs:

Last annual Dec 2019 -42 hrs since then.
Mag timing checked at that time.
Compressions all over 70 except #5 at 68
New Challenger air filter. -Has not been serviced since that time.

MISC
New mags and harness 292 hrs ago in 2016
Correction: Idle RPM 600

Questions:
Static RPM just before ground roll? ~2300+ on 1A175/DM8040- going off recent memory.
What DA? I did various testing of the issue last weekend while climbing between sea level at approx 55* F and 14k ft at 10*F. I started full rich and leaned as necessary for max climb. I did however need to crack the carb heat while climbing to keep the engine smooth and at optimal climb power, especially at higher altitudes. If I leveled out at any altitude, and reduced to cruise RPM (2450) the engine would run very smooth with or without carb heat. I noticed no difference in characteristics of the roughness at any altitude. At lower altitudes It was always solved quickly by leaning or a small amount of carb heat. At higher altitudes, while leaned, cracking the carb heat would very quickly solve any roughness, and slightly increase RPM. Full carb heat would reduce RPM as normal, and run smooth.

On a recent desert trip, I had similar roughness during aggressive climbs in dry 70-80*F and 4-5k ft altitudes. Lots of other examples in various other conditions with the same symptoms.

I’d also ask “when was the last time someone looked up inside the induction system above the carb? Logs show new carb float, bowl gasket, and installation of AvGlas air box kit in 2011 (497 hrs ago) Before then, it might have been a a couple decades since overhaul.
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gfeher
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Re: Engine roughness on climb

Post by gfeher »

Another question: When it happens while you are climbing, if you don't touch any of the engine controls (throttle, mixture, carb heat) and just level off, does the engine smooth out and run normally?

Related question: Does it ever happen in level flight?
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
Experimental J3 Cub Copy N7GW O-200
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Vertical
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Re: Engine roughness on climb

Post by Vertical »

If I level out it gets smooth, and engine will soon redline (because I’m still full throttle)

It never happens in level flight. Only at steeper VX /aggressive climb attitudes.
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170C
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Re: Engine roughness on climb

Post by 170C »

In other trouble shooting posts checking the induction hoses for possible leaks has been suggested.
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