Engine running rough

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Magisterol

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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
 

TFF

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A friend had a Grumman that would load the plugs up on the ground and would have to be leaned excessively on the run up to clear the plugs. Although compression read ok, better cylinders cleared it up. His was not rough in the air or at least I don’t remember that.
 

Hot Wings

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The exhaust stacks started to get black soothe on them when I run my finger inside. The consensus is that is running rich.
Way out of my comfort zone but:
Could the consensus be wrong and it isn't rich, but oil? Oil soot and rich soot often look similar but oil generally has a more greasy feel than rich.
Running rough only at at high manifold vacuum (low rpm) could be an intake guide that is loose enough to suck that much oil. The valve could seat well enough not to show on a differential pressure test but doesn't seat fully in a dynamic - running - state adding to the roughness.
I'd check the manifold vacuum and maybe even try an automotive style pump up compression check.
 

Pops

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First thing I would do is get rid of the one piece venturi. Send the carb off and have it overhauled and changed to the 2 piece venturi. AD is just a visible check of the venturi at the annual or conditional with the 2 piece.
 

dwalker

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What is the oil use? Agree oil can be getting sucked past the rings or valves.
What do the plugs show? It would be on the unusual side to have all cylinders sucking/ burning oil. It would not be unusual for them all to be carboned up if running rich.
Has the carb been inspected? And I mean, pulled off and completely checked? Carbs can do weird things, like having things stick or get bent or worn.
Has the fuel pressure been checked? Too much fuel pressure on a carb could do exactly what you describe.
Has the cam lift at the valve been checked?
Does the engine run evenly with good power when it hits 2000rpm or does it just seem to run smoother?

I would agree, this needs to be fixed before I would want to go up in it.
I also agree with a standard compression test being done, just make sure you do it right- throttle open, all plugs removed, ignition and fuel disabled, with a battery charger on the battery throughout. One thing about differential compression/leakdown tests is they can give somewhat misleading information.
 

Magisterol

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A friend had a Grumman that would load the plugs up on the ground and would have to be leaned excessively on the run up to clear the plugs. Although compression read ok, better cylinders cleared it up. His was not rough in the air or at least I don’t remember that.
Plugs are clean. A bit of of sooth on them. I cleaned them again and same thing.
 

Magisterol

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Way out of my comfort zone but:
Could the consensus be wrong and it isn't rich, but oil? Oil soot and rich soot often look similar but oil generally has a more greasy feel than rich.
Running rough only at at high manifold vacuum (low rpm) could be an intake guide that is loose enough to suck that much oil. The valve could seat well enough not to show on a differential pressure test but doesn't seat fully in a dynamic - running - state adding to the roughness.
I'd check the manifold vacuum and maybe even try an automotive style pump up compression check.
Plugs are dry. A bit of sooth on them. I cleaned them again and runs the same.
 

Magisterol

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First thing I would do is get rid of the one piece venturi. Send the carb off and have it overhauled and changed to the 2 piece venturi. AD is just a visible check of the venturi at the annual or conditional with the 2 piece.
I did the AD and this is what I have now. The new venture acording with the new AD.
 

Magisterol

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What is the oil use? Agree oil can be getting sucked past the rings or valves.
What do the plugs show? It would be on the unusual side to have all cylinders sucking/ burning oil. It would not be unusual for them all to be carboned up if running rich.
Has the carb been inspected? And I mean, pulled off and completely checked? Carbs can do weird things, like having things stick or get bent or worn.
Has the fuel pressure been checked? Too much fuel pressure on a carb could do exactly what you describe.
Has the cam lift at the valve been checked?
Does the engine run evenly with good power when it hits 2000rpm or does it just seem to run smoother?

I would agree, this needs to be fixed before I would want to go up in it.
I also agree with a standard compression test being done, just make sure you do it right- throttle open, all plugs removed, ignition and fuel disabled, with a battery charger on the battery throughout. One thing about differential compression/leakdown tests is they can give somewhat misleading information.
It doesn’t burn oil. The plugs are dry with a bit of sooth on them. Cleaned them again and does the same. The carb was pulled of and inspected by 2 AME on different occasions and all looks ok. Did the pressure test. It is gravity feed. Engine pulls ok. I don’t see any loos in performance as it climbs and goes the same as before once I lean it. 1700-1800 fpm and 150-155 mph at 2500rpm
 

Magisterol

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I always just thought mags just broke and quit working, but maybe resistance in the wires? Poor connection at the terminals?
There is no difference between L and R mag. It drops on both the same amount. Unless both of them are having the same problem at the same time.
 

Magisterol

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It could be weak spark?
I checked and tested the sparks. And there is no difference in rpm drop when I do the mag check. Unless there are 2 sparkplugs or 2 leads one on each mag and different cylinder. If that is the case, the only thing would be to change the whole ignition system on both sides: mag, leads and plugs.....
 

Dan Thomas

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There is no difference between L and R mag. It drops on both the same amount. Unless both of them are having the same problem at the same time.
No, a spark isn't just a spark. The mag can weaken with hours of use. The mag manufacturers specify a 400-hour inspection (Bendix/TCM) or 500 (Slick). As the points erode and the cam or rubbing block wears, the points open at the wrong time, which weakens the spark due to E-gap shift, and a weak spark can cause untold hassles that sometimes lead mechanics to chase the fuel system all over the map.

I don't think spark is the problem here. Weak spark usually needs richer mixture, not lean. This one sounds like the carb's main jet is loose or the wrong size, maybe. If the float was sinking or the float valve leaking it could get richer, but fuel dribbling out of the carb after shutdown should also be apparent.
 

Magisterol

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Check your primer. If it's not locked, or if the spring-loaded valve pin in it isn't sealing, the engine will suck extra fuel through it, especially at low manifold pressures.
Sorry. Should have said I decommissioned it already and didn’t change anything. The only thing that helped a bit at one point was to lean the idle mixture to the point that pulling the mixture to lean towards shutoff, I don’t get any rise in rpm at idle. The AME said the engine should not run so lean at idle. The idle screw was only half turn out. And even that would clear the roughness completely.
 

Magisterol

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No, a spark isn't just a spark. The mag can weaken with hours of use. The mag manufacturers specify a 400-hour inspection (Bendix/TCM) or 500 (Slick). As the points erode and the cam or rubbing block wears, the points open at the wrong time, which weakens the spark due to E-gap shift, and a weak spark can cause untold hassles that sometimes lead mechanics to chase the fuel system all over the map.

I don't think spark is the problem here. Weak spark usually needs richer mixture, not lean. This one sounds like the carb's main jet is loose or the wrong size, maybe. If the float was sinking or the float valve leaking it could get richer, but fuel dribbling out of the carb after shutdown should also be apparent.
Well, the mags were checked by 2 AMEs and changed the condensers even though they said everything checks out. (Just extra money...). I had the main jet changed at some point and it was running very lean. EGT was pushing 1500F on take off and I couldn’t lean it on cruise. The moment I tried to lean it, the engine would go rough. That is why I changed it back to the old jet (47-773). I think I will change it back to the new jet and enrich the idle mixture and see the difference. This would be the only othe option other than buying another carb and realize it is doing the same 😩
 

Victor Bravo

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Run your same tests and observations with the air filter removed from in front of the carburetor. You could be experiencing a pressure drop across the filter that is making a rich mixture at certain pressures and throttle settings. When you are in flight you have a little ram pressure helping push air through the filter. On the ground you don't.
 

Pops

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Still think its the one piece Venturi. When I had the same problem it would go so rich at times while flying that the engine would almost quit and pulling the mixture out to almost shutoff didn't help. C-172 engine was burning 13 gph. Rings on the pistons got frozen from all the carbon. I grounded the airplane. Worse on the Cont's but some Lyc's has the same problem with the one piece venturi.
 

dwalker

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Run your same tests and observations with the air filter removed from in front of the carburetor. You could be experiencing a pressure drop across the filter that is making a rich mixture at certain pressures and throttle settings. When you are in flight you have a little ram pressure helping push air through the filter. On the ground you don't.
He did say above they had removed the filter.

Ok so if the spark checks out- which I am not convinced of, but lets say the spark checks out.
Not using more than a normal amount to oil is a good sign.

Go do a standard compression test, takes an hour to do with no special tools, just some attention to detail.

If passes compression test check valve clearance and cam lift at the valves. Again, an easy and quick check that may reveal something.

Are you "properly" shutting down the engine by leaning it until it stops then shutting off the mags? If so that is a part of why you are not seeing more soot on the plugs, as the lean condition cleans them to some degree. Your sooty exhaust is a much stronger indicator of how rich you are. I would say that your carb is basically not working, that at idle you have incomplete atomization and/or are running pig rich. Above 2000rpm you have enough intake velocity that closer to proper atomization occurs, the fouled plugs clean up, and everything is normal again. Until you decrease throttle, in which case the overrich condition returns.

One last thought- have you had someone with a set of jet gauges make SURE your jet is actually what it says it is?
Not sure how much a new jet is, but it would be worth it to get a brand spanky new one to put in and see if the problem persists.
 
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