O-200 rough idle

[Pooria]

Hi all,

I was about to buy a block of time on someone's C150 with an O-200 engine. During the runup test, I noticed a rough idle (with and without carb heat on).

Otherwise, the engine performed really well on higher RPMs (mag checks were fantastic). For the couple of touches and gos', the engine was fine during the short final (when set to idle). I would assume because the propeller was windmilling more RPM than what's on idle on the ground!

So, is this roughness a sign of concern as a renter? Will it likely fail me on short finals? is this a sign of bad maintenance and is it a red flag? is God giving the necessary sign to stay away fro this one?

 

[rotormatic1]

Write up the discrepancy and have the company address the concern.

 

[Pooria]

Write up the discrepancy and have the company address the concern.

It is an individual owner with "open pilot" policy on its insurance - that's how I can rent some time on his airplane!

I told him the issue, and he brushed it off, saying: "that's ok since the engine is not stalling! Just make sure to keep the engine above 900RPM on the ground!"

So, given that he is not going to fix that anytime soon, should I continue flying the plane or just walk away and rent from someone/somewhere else?

 

[rotormatic1]

Walk away…

 

[Pinecone]

Was this with the mixture leaned?

 

[Pooria]

Was this with the mixture leaned?

Initially full rich and then I tried with lean as well. Same rough idle.

Field elevation is 650’

 

[GeorgeC]

Does it accelerate smoothly on the takeoff roll? Did it try to die on you on the landing rollout? Were the OAT and dewpoint at the time conducive for carb ice?

If it were my plane, I'd be checking the easy stuff first: idle rpm, idle mixture, the plugs, the mags... did you try a mag check when leaned? At 1700 rpm? 1000 rpm? Idle? What does it take to make it die?

You should be doing ground ops at ~1000 rpm and aggressively leaned anyway to minimize lead fouling, but the plane should also idle correctly. If the owner's response is a complacent "it's always been like that/don't do that", walk away.

https://www.pilotsofamerica.com/com...tal-engine-stumble.130180/page-2#post-3032226
https://www.pilotsofamerica.com/community/threads/o200-rough-failed-runup.83667/

 

[guest user]

Since you know it's not going to be addressed by the owner, it comes down to you.

How comfortable are you with what you have?

How comfortable are you, in the event of an unintended return to earth, having to explain to the FAA why you thought the plane was airworthy even knowing about the rough idle?

As a note "Guys on the internet said..." is not a defense. "The owner said..." is not a defense.

I am not trying to say either way, but I am trying to make sure you realize that if it does break and you do have to talk to the NTSB / FAA, it is going to come down to just you and your ability to say "I felt the aircraft was airworthy when I took off".

 

[texasclouds]

How rough? Like a lope? Glug glug glug, or more like brrrr, brrr, brap brap, brrrr, brap? Or is it riiiingg ta ting ting ting ting riiiing ta ting? Does the panel shake? Have you flown many other C150s to compare it to? How long ago was the last 100hr inspection? What were the compressions?

 

[Dan Thomas]

Make sure the primer is in and locked. An unlocked primer will allow the low manifold pressure to suck extra fuel into the carb throat and cause roughness.

What RPM is it idling at?

Poor compression on one or more cylinders can cause rough idle. Burned valves or something. And a weak magneto will do it, too. I wouldn't fly it until it's fixed.

 

[GeorgeC]

Make sure the primer is in and locked. An unlocked primer will allow the low manifold pressure to suck extra fuel into the carb throat and cause roughness.

Slight tangent... but presumably that also happens while starting? I prime, then (after waiting a bit for the fuel to evaporate) start cranking with the primer still open, in case it needs more. Should I lean the mixture to compensate for the open primer?

 

[Mongoose Aviator]

During the runup test, I noticed a rough idle (with and without carb heat on).

For the couple of touches and gos', the engine was fine during the short final (when set to idle).

Why would you try to test the aircraft in the air if the runup shows an issue?
The purpose of the runup afaik is to stop you from taking an aircraft into the air if the runup shows an issue.

@Pooria - You felt it worth the trouble to create an account and post here. You know in your gut this is a bad idea. You are looking for confirmation this is a bad idea.

This is a bad idea.

It is an individual owner with "open pilot" policy on its insurance - that's how I can rent some time on his airplane!

"Open Pilot" does NOT mean the aircraft is available for rental from an insurance point of view. In fact, it is very possible (likely even) that the insurance policy is for private use only and specifically excludes insurance coverage for commercial or rental operations.

So it is "possible" you could be considering renting an airplane with maintenance issues and which possible would provide you with zero insurance coverage during the flight.

This is a really bad idea.

 

[Dan Thomas]

Slight tangent... but presumably that also happens while starting? I prime, then (after waiting a bit for the fuel to evaporate) start cranking with the primer still open, in case it needs more. Should I lean the mixture to compensate for the open primer?

No, just close the primer. If you lean to allow for primer feeding, you'll forget all about it being unlocked.

 

[Pooria]

First and foremost, I must thank you all. I'm from Canada and found this American forum extremely helpful.

A couple of clarification points:

- The individual that I will be renting from (if I decide to go with ...) is a CFI with commercial insurance on the plane. Thanks for mentioning that @Mongoose Aviator , I ended up double-checking with him.
- I was not the PIC and was trusting his judgment as he had more time (on the plane and overall) compared to me. Just the idle vibrations seemed rough to me.
- I think I have to define what I am calling rough idle: What I am considering rough idle is some extra vibrations that I felt (at idle) inside the cabin compared to ~1000 RPM - no hesitation from the engine or feeling that the engine is going to quit.
- My baseline of comparison: comparing this C150 (O-200) with couple of C172s that I frequently fly with. The owner/CFI is arguing, "there is no problem at idle; it is just I am comparing it to a totally different engine which generally runs much smoother than a typical O-200"
- In order to validate his claim, I have booked a flight with a very well-maintained C150 at a local flight school for tomorrow, and going to confirm if the same said vibrations (which I call rough idle) are felt in this other plane.

I have always been renting from our local flight school (with a hectic and unaccommodating schedule) and learned to trust their maintenance crew and their interpretations of operational conditions of their fleet. In a sense, I always had some more knowledgeable people to look after me. Now I am stepping out of my comfort zone, and I have to make sure that what I am feeling is actually correct and that I am not jumping for no good reason.

So long question short: O-200 with some noticeable (not even moderate, I have to say) cabin vibration (at idle) is considered ok or is it a point of concern? The engine does not hesitate and/or it doesn't feel like quitting when at idle.

 

[Mongoose Aviator]

In order to validate his claim, I have booked a flight with a very well-maintained C150 at a local flight school for tomorrow, and going to confirm if the same said vibrations (which I call rough idle) are felt in this other plane.

That sounds like a practical and smart approach.

 

[brcase]

That sounds like a practical and smart approach.

I agree,

O-200's do tend to run a bit rougher when much under 1000RPM on the ground.
You should not really notice the roughness at idle in flight, because it will be idling at a higher RPM.
My feeling is if the Runup is normal and it runs normal at 1000RPM or more, it is probably fine. But I am not the one flying it.

But comparing it to another 150 sounds like a great plan.

Brian

 

[Dan Thomas]

I have always been renting from our local flight school (with a hectic and unaccommodating schedule) and learned to trust their maintenance crew and their interpretations of operational conditions of their fleet. In a sense, I always had some more knowledgeable people to look after me. Now I am stepping out of my comfort zone, and I have to make sure that what I am feeling is actually correct and that I am not jumping for no good reason.

As a Canadian AME, I can tell you that way too many airplanes have poorly-maintained magnetos and spark plugs and a lot of other stuff. I can tell you that the O-200 has an oddball intake manifold mount that I usually found incorrectly assembled, shaking the carb more than it can take, causing fuel to slosh out of the bowl vent into the carb throat. I can tell you that the small Continentals have a bad habit of fouling their bottom sparkplugs. And I can tell you that you should pay attention to funny sounds and vibrations; I've had two engines fail that were trying to tell me that they were hurting.

So compare it to the other 150 and see what you think. But you might also ask the owner of the shaky engine when the last time the magnetos were out for internal inspection and servicing. How many hours and years ago? Ask the mechanic who maintains it if he knows about the special Lock-O-Seal washers that are supposed to be in that manifold mount as per the O-200 overhaul manual. What were the compression numbers at the last inspection? Does that propeller track straight? The hard questions.

Pilots put up with far too much crap from their engines.

 

[2-Bit Speed]

Engines put up with far too much crap from their pilots.

FIFY

 

[Lndwarrior]

An aircraft is either normal or abnormal.

If it's abnormal, don't fly it until it is normal again.

A rough idle is not normal, regardless of others opinions. Which, of course, is my opinion. Which you should probably ignore because it is "others" opinions.

 

[Magman]

There is an AD pertaining to the 1 piece Venturi ( 98?) and also a Service Bulletin

regarding the Discharge Nozzle from about the same time.

The “ One - piece Venturi” often created problems when installed. This would be

apparent right after installation. Some folks opted to address the AD by keeping

the “ Two - piece Venturi “ to avoid difficulties. This could be done via Recurring

Inspection of the item. In this case the issue may present later. Often Techs

mistake compliance via Inspection of the two- piece as a Terminating Action.

That is not the case.

The Discharge Nozzle has had “ improvements” that also created issues. I’m

dealing with one right now.


It’s possible another culprit may be to blame as well.

I have found numerous cases of the Left Lower Engine Shock Mount being

deformed. This renders the Shock portion ineffective and transmits shakes and

impacts.

Sorry ; I don’t know WHY it always LEFT LOWER.

 

[Dan Thomas]

Sorry ; I don’t know WHY it always LEFT LOWER.

Because the lower mounts take the weight of the engine. The top mounts are just in tension. The lower left gets compressed even more due to propeller torque reaction.

 

[GeorgeC]

So long question short: O-200 with some noticeable (not even moderate, I have to say) cabin vibration (at idle) is considered ok or is it a point of concern? The engine does not hesitate and/or it doesn't feel like quitting when at idle.

Mine's smoother at 900 than it is at 525. A few minutes after start is smoother than right after start, especially when it's cold. Does it make the idle and static rpm limits in the TCDS?

 

[Pinecone]

Most engines are smoother at around 1000 RPM or higher versus full idle.

 

[Magman]

Makes sense Dan.

It’s not easy to tell if “ rough” is an engine issue or associated vibration.

Often replacing or just rotating mounts can makes things smoother.

An exhaust stack hitting the cowl or structure will sound bad also.

 

[Grum.Man]

Is this your first time flying an O-200? A conical mount O-200 is going to feel rough compared to a dynafocal mount 320 or similar. During flight it will never see idle rpm so I wouldn't be too concerned about it if it feels fine everywhere else in the rpm range. You could do an idle mag check. If it's a sticking valve running on one mag will really make it shake.

 

[Pilawt]

Make sure the primer is in and locked. An unlocked primer will allow the low manifold pressure to suck extra fuel into the carb throat and cause roughness.

Or a locked primer with bad o-rings.

… As i discovered to my chagrin a couple of weeks ago when my Citabria’s engine quit on rollout after landing at a busy tower-controlled field. Fortunately I had enough energy to turn off at an intersection and clear the runway with a stopped prop.

i got it restarted, but it wouldn’t run below 1200 rpm. It took a lot of throttle-jockeying to taxi back to the hangar.

 

[Dan Thomas]

Is this your first time flying an O-200? A conical mount O-200 is going to feel rough compared to a dynafocal mount 320 or similar.

The O-200 doesn't have the conical mounts. Those were used on the older, smaller Continentals. The O-200's mounts are actually fairly soft, which is why the intake manifold (spider) has to also be soft-mounted to avoid shaking the carb so hard that it sloshes fuel out the bowl vent and into the carb throat. Those engine mounts allow a lot more wiggle than the old conical mounts, which hardly move at all.

O-200 mounts:



A-65 thru (most) C-90 mounts:



Maybe you've seen the conversion to adapt the O-200 to replace a smaller Continental?

 

[Magman]

Rotating the mounts 180 degrees may prolong their useful life.

Continental used to recommend this for some of their engines.

When they “ squish” out the result is a hard contact with only the

piece of “ hose” providing a cushioning effect. I have found a couple

broken bolts ( left lower) that I attribute to this.

Sagging, squished and improperly positioned shock mounts are high on the

“ findings list” for first time inspections. Keep in mind folks ARE flying them in this

condition.

Many maintenance items that are deferred will have worse consequences later.

If addressed early the only items needing replacement will be the rubber cushions.

These can be purchased individually from ACS and other sources for a

fairly reasonable price when I last checked.

The steel components will deteriorate if not properly positioned and will attack

the engine mounting lugs later.

A hoist is helpful but not essential to do these tasks.

 

[Dan Thomas]

When they “ squish” out the result is a hard contact with only the

piece of “ hose” providing a cushioning effect. I have found a couple

broken bolts ( left lower) that I attribute to this.

Sagging, squished and improperly positioned shock mounts are high on the

“ findings list” for first time inspections. Keep in mind folks ARE flying them in this

condition.

Then some of the parts are missing.

There is a steel tube for each mount there. It fits between the outer washers of the shockmounts to limit the squish and to preload the bolts to prevent bolt movement in the engine's airframe mount. That movement can break the mount or the bolt. The short hoses go over the steel tubes to prevent metal-to-metal contact between the tube and engine mounting lug.

From the O-200 overhaul manual:



Yes, there are a lot of poorly-repaired airplanes out there. Hangar fairies are often responsible.

 

[Magman]

No ; all of the parts were present, BUT!

The large washers with the lip ID can migrate out of the Mount Arm counterbore

and render the installation ineffective . The large ID washers are only seated by

the rubber donuts that can get squished. This results in wear of the aluminum

engine and the steel “ washer” if not addressed.

I have also found the steel tube missing . Hangar fairies?


In my little world there are 2 aircraft that come to mind with shock mount issues ;

the 150 & 182. With the latter the lower forward units are in compression and

likely carry about 80 of the engine / prop weight. If addressed early enough

swapping upper and lower shock mounts will restore alignment and minimize

damage .


A broken bolt on a 150 may not be readily apparent. Putting a wrench and turning

the bolt may be required. BOTH ends are supposed to turn together!