Engine running rough

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Magisterol

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Jul 22, 2021
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What I would do. A shop that specializes in aircraft carburetors.


When the AD came out to replace the 2 piece Venturi with the one piece, I sent my carb to a large carb shop. The carb came back and was put on. The Cont 0-300 on the straight tail C-172 would go rich in flight and almost quit with the one piece venturi. Pulling the mixture almost to shut-off made no difference. Fuel burn was about 13 gph. About that time my neighbor IA changed the venturi on a later model Champ with the LYC- 235 engine. I watched the take-off when the owner left and he wasn't out of sight when he turned around and tried to make it to the airport. Engine went rich and almost quit. The IA grounded the airplane and called the local FADO about what to do. AD says to change to the one piece and its unairworthy with the one piece venturi. They told him that they just got the notice that the 2 piece venturi can be used with a visible inspection at each annual for any looseness.
I called my carb overhaul co and my carb had been mailed back to me. Said that the fix was a different spray bar with the one piece venturi. I told them I will try it. Carb going rich was not quite as bad and the fuel burn was down to 11 gph, but still afraid to fly it. I shipped the carb back and told them to put the 2 piece venturi back in the carb and at least i can fly it until the annual inspection. They said they just got the notice about the inspection of the 2 piece venturi.
Received the carb back and the engine run like it should with normal fuel burn. All of that extreme rich running froze the rings solid in the piston grooves with carbon. Ended up having to have all 6 cylinders honed and pistons cleaned and new rings installed.
Well, it makes sense. It started going wonky after the mechanic changed the venturi. I will see what he says. It was running fine before except for skipping a beat from time to time. The AME said it might be it the reason.... I guess not. I will see tomorrow.
 

dwalker

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It is not the P-51 replica. It is a poor guy RV. I like it more. But I would say I am bias..😊
I know exactly what it is, when I was in High school- pre-internet- Kitplanes magazine had several articles about someone building one and bucking all the rivets. The Busby Mustang 1 FV racer is likely the best looking airplane ever penned and built, but the 2-seater Mustang 2 is a close second.
 

Pops

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I agree. Very nice looking airplane. Friend of mine in IA built one and flew it for several years.
 

Map

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Dec 29, 2020
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Check for obstructions anywhere in the induction system, such as a gasket that has shifted inside a duct and blocks some of the airflow.
 

Magisterol

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Jul 22, 2021
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90
Check for obstructions anywhere in the induction system, such as a gasket that has shifted inside a duct and blocks some of the airflow.
I am heading to the field in about 2hrs. I was thinking about it. Not much hope though, because is after the carb and the mixture is already set, but shouldn’t hurt. My hope is in the jet, somehow not atomizing properly. I heard some people sand the jets to have a smooth surface, which apparently is detrimental to atomization. Will see.
 

Dan Thomas

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Sep 17, 2008
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I am heading to the field in about 2hrs. I was thinking about it. Not much hope though, because is after the carb and the mixture is already set, but shouldn’t hurt. My hope is in the jet, somehow not atomizing properly. I heard some people sand the jets to have a smooth surface, which apparently is detrimental to atomization. Will see.
The jet doesn't atomize anything. It meters the fuel. The nozzle does the atomizing. The jet is buried deep inside the carb.

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Talon38

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Jun 13, 2021
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Hello guys. I have a problem with my engine and so far all the AME couldn’t find the culprit. For a few month the engine started to run rough when I bring the power back below 2500 rpm. It goes away if I lean it as long as I stay above 2000 rpm and it is a bit better if I bring it all the way to idle. It is worst if the carb heat is on and it doesn’t make a difference on L or R mag. It drops within the range on single mag ops but no difference between L and R. The engine is an O-320 E2A with a 8.5:1 pistons, MA-4SPA, 10-3678-32 carb with single piece venturi, blue float with 47-773 jet. Now for whatever reason, it started to go rough even on the ground when I do mag check at 1700-1800 rpm. So I fave to lean all the time even on takeoff. The exhaust stacks started to get black soothe on them when I run my finger inside. The consensus is that is running rich. I tried flying without the air filter and it does the same. Either too much fuel or not enough air. Thing is, it was running fine before. Other than borr someone else’s car and try it, would be to buy another carb, but nobody is sure if it is the carb or something else, because on the ground when it runs rough, it is not blowing black smoke. Any ideas? Thank you
 

Talon38

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Jun 13, 2021
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If / since this is a standard configuration and if this is not maintenance needed because of maintenance (i.e. the venturi work) I would suspect the fuel level running high in the bowl. So look for trash in the needle seat and valve area, warn float stop or pivot pin. Hole in float (heavy from fuel.) Composite floats mess up too. Was the the float adjustment inadvertently altered during the venturi work? Idle mixture set properly? If your prop is not balanced and / or your engine shock mounts are not up to snuff, vibration will wreak havoc on carb, baffles, linkages, cowlings, pilot etc.
I started working on aircraft in the Navy in 1969, A&P in 1975, Evergreen Helicopters, Piedmont General Aviation, Piedmont Airlines (USAIR, USAIRWAYS, American) I retired June 2018. I did a lot of rotor and propeller balancing, it really pays off.
 
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Talon38

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Jun 13, 2021
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Slight RPM rise on shutdown? Effect of leaning at 2000 RPM, increase in RPM, EGT?
Marvel-Schebler certified repair station with test stand.
Service Bulletin: SB-22. 2.
Reasons:
To notify operators of the availability of an improved fuel nozzle to provide better vaporization and distribution of the fuel/air mixture to the cylinders.
3. Background: In some instances, operators have reported high and/or split cylinder head temperatures (CHT) and/or high and/or split exhaust gas temperatures (EGT). A new carburetor nozzle has been developed to improve this condition.
4. Compliance: If you are experiencing these symptoms and your aircraft is equipped with a MSA carburetor model MA-4SPA™, part number 10-3678-32 carburetor, we recommend replacing the current fuel nozzle 47-773 with nozzle part number 47-862 or replacing the carburetor with a new or overhaul exchange MSA carburetor part number 5-6200.
 
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Magisterol

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Jul 22, 2021
Messages
90
If / since this is a standard configuration and if this is not maintenance needed because of maintenance (i.e. the venturi work) I would suspect the fuel level running high in the bowl. So look for trash in the needle seat and valve area, warn float stop or pivot pin. Hole in float (heavy from fuel.) Composite floats mess up too. Was the the float adjustment inadvertently altered during the venturi work? Idle mixture set properly? If your prop is not balanced and / or your engine shock mounts are not up to snuff, vibration will wreak havoc on carb, baffles, linkages, cowlings, pilot etc.
I started working on aircraft in the Navy in 1969, A&P in 1975, Evergreen Helicopters, Piedmont General Aviation, Piedmont Airlines (USAIR, USAIRWAYS, American) I retired June 2018. I did a lot of rotor and propeller balancing, it really pays off.
I will tel the AME about it. I dropped the carb today to a flying school AME. He wasn’t there but I will talk to him on Monday. Will see what he found.
 

Magisterol

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Jul 22, 2021
Messages
90
Slight RPM rise on shutdown? Effect of leaning at 2000 RPM, increase in RPM, EGT?
Marvel-Schebler certified repair station with test stand.
Service Bulletin: SB-22. 2.
Reasons:
To notify operators of the availability of an improved fuel nozzle to provide better vaporization and distribution of the fuel/air mixture to the cylinders.
3. Background: In some instances, operators have reported high and/or split cylinder head temperatures (CHT) and/or high and/or split exhaust gas temperatures (EGT). A new carburetor nozzle has been developed to improve this condition.
4. Compliance: If you are experiencing these symptoms and your aircraft is equipped with a MSA carburetor model MA-4SPA™, part number 10-3678-32 carburetor, we recommend replacing the current fuel nozzle 47-773 with nozzle part number 47-862 or replacing the carburetor with a new or overhaul exchange MSA carburetor part number 5-6200.
I have the bulletin. There is no rpm rise on shutdown. The idles mixture might be too lean. Other than that, the engine I keep it lean from take off to landing, because otherwise the roughness is getting to bad. Funny thing is that on the ground the vibration from the roughness is not that bad. This is another thing that I don’t understand. Why the roughness is worst in the air than on the ground. Mind you, on the ground I cannot go higher than 2000rpm...
 

Magisterol

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Jul 22, 2021
Messages
90
It comes all apart.
Ok. I thought the jet and nozzle is one piece that mounts inside the carb body. So the nozzle and the jet can be separated from each other? Didn’t look like that to me, but I guess you might be right. Well is in the mechanic shop now. I will talk to him on Monday.
 

TFF

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Apr 28, 2010
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In field replacement of the Venturi can be a problem of parts matching. Old carb with CNC new part. On a bench, they know the problems and how to match them. Gone are the days of having a basket full of parts and the tech could go through all of them and match them to the best case.

I think you have more than one problem or something else got damaged working on the carb. It’s been too long since I have taken one apart. Can the mixture metering valve be put in backwards? I would think about if the fuel
can draw through accelerator pump somehow. Something not seated. No backfires lately? Mags are timed right?
 

Magisterol

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Joined
Jul 22, 2021
Messages
90
In field replacement of the Venturi can be a problem of parts matching. Old carb with CNC new part. On a bench, they know the problems and how to match them. Gone are the days of having a basket full of parts and the tech could go through all of them and match them to the best case.

I think you have more than one problem or something else got damaged working on the carb. It’s been too long since I have taken one apart. Can the mixture metering valve be put in backwards? I would think about if the fuel
can draw through accelerator pump somehow. Something not seated. No backfires lately? Mags are timed right?
He check the accelerator pump and he said the valve was not seating properly so he changed the seal. No changes. I don’t know if the mixture valve was put differently. There was no difference how the engine was working before and after the last AME took it apart. Now lately I had backfires bringing the power back. Usually while descending, but I assumed it was because the prop is driving the engine. I was thinking about mags timing and asking the mechanic to redo It, but it wasn’t high on my priorities because I couldn’t find a reason for them to move from the position they are on. Wouldn’t they have to rotate on the engine block? I guess I will ask him to check the timing.
 
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