Engine running rough

[Magisterol]

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.

I did a flight with it with no difference. it Was last Saturday.

 

[Magisterol]

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.

Well this is the AD that Lycoming says you have to do. This is why the AMEs are scrat their heads. They say it is a simple O-320 with nothing fancy on it and can’t figure out what is wrong.

 

[Victor Bravo]

Borrow another carburetor and bolt it on, changing nothing else, and at least isolate the problem to that, or it could verify that the issue is not the carb.

If it is the carb, then the jets, air passages, idle circuit, etc. etc. or some little speck of dirt in one of the passages could cause these problems.

 

[dwalker]

Borrow another carburetor and bolt it on, changing nothing else, and at least isolate the problem to that, or it could verify that the issue is not the carb.

Possibly the best advice so far- test with a KNOWN GOOD part, and note results.

 

[Magisterol]

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.

Well this is what I think it is. I should ask my AME to gauge the jet. Last December, one AME performed the AD and changed the float, Venturi and jet. The airplane was running very lean. The EGT was around 1500F and I couldn’t lean the engine in cruise. The moment I move the mixture, engine started to go rough and rpm dropped. So I went back and he changed the jet to the original one (47-773). Now I am thinking about moving it back to the new Venturi 47-862 and play with the idle mixture. Right now the idle mixture is 1.5 turns out but on shutdown I still don’t get any rise in the rpm before engine shuts down. Don’t know what else to do. I am going to the hangar tomorrow and ask the AME to do this again and see what it is. At least I will be able to fly it. Right now the pucker factor is very high when I hear it. The valve and compression test was done in may during my annual at one of the AME on the field.

 

[Magisterol]

Borrow another carburetor and bolt it on, changing nothing else, and at least isolate the problem to that, or it could verify that the issue is not the carb.

If it is the carb, then the jets, air passages, idle circuit, etc. etc. or some little speck of dirt in one of the passages could cause these problems.

I asked around at the airport. Either nobody has it or nobody wants to go through the trouble. And I don’t blame them.

 

[Victor Bravo]

Your explanation in Post #25 gives you the answer. Take the carburetor off, send it out to a DIFFERENT shop, a place that specializes in aircraft carburetors, and ask that shop to diagnose the cause of the issue. You may be able to explain to them that you don't need a carburetor overhaul (if it had already been OH'ed), you just need to find whatever was accidentally mis-adjusted during the previous OH.

 

[dwalker]

If the float is not adjust properly this could be your issue. I am with Victor- send the carb out to a specialist, or find the manual and work through it yourself.

 

[Magisterol]

Your explanation in Post #25 gives you the answer. Take the carburetor off, send it out to a DIFFERENT shop, a place that specializes in aircraft carburetors, and ask that shop to diagnose the cause of the issue. You may be able to explain to them that you don't need a carburetor overhaul (if it had already been OH'ed), you just need to find whatever was accidentally mis-adjusted during the previous OH.

Well. I have to ask the guys at the airport where is the closest specialized shop. In Canada we don’t have them everywhere.

 

[Dan Thomas]

Are you sure someone didn't put Jet-A in your tanks?

 

[Magisterol]

Are you sure someone didn't put Jet-A in your tanks?

No way. I don’t even know if they have it...

 

[Victor Bravo]

Well. I have to ask the guys at the airport where is the closest specialized shop. In Canada we don’t have them everywhere.

IMHO ask a larger group than just the guys at the local airport. Your data pool is possibly a lot smaller than it needs to be.

Ask the internet, ask our HBA forum, ask COPA or EAA. Ask Cessna Pilots Association or Piper Owners... even if your airplane is not a C or P the fact is that the O-320 you have is used on tens of thousands of C and P airplanes.

 

[Twodeaddogs]

Is it sucking in air from a dodgy inlet manifold joint?

 

[Dan Thomas]

Is it sucking in air from a dodgy inlet manifold joint?

That would make it lean, not rich.

 

[Kiwi303]

Borrow another carburetor and bolt it on, changing nothing else, and at least isolate the problem to that, or it could verify that the issue is not the carb.

If it is the carb, then the jets, air passages, idle circuit, etc. etc. or some little speck of dirt in one of the passages could cause these problems.

VB beat me to it. THIS is what I would be doing given spark appears to check out fine and it's not eating oil. That leaves fuelling, replacing a carb with a known good item and seeing if the problem goes away isolates if it's a carb problem.

If it's all still going wrong with the new carb I would check the fuel pump, is it electric or mechanical? if electric, can it have shorted or something to push more pressure at a higher flow and forcing fuel through instead of just putting it there for the carb to suck? A fouled carb bowl stop needle would imitate the same problems, not cutting off the flow when the bowl is full/float is up, forcing excess fuel through the jets at pressure, bit if that was the case the carb swap would eliminate it.

 

[Magisterol]

IMHO ask a larger group than just the guys at the local airport. Your data pool is possibly a lot smaller than it needs to be.

Ask the internet, ask our HBA forum, ask COPA or EAA. Ask Cessna Pilots Association or Piper Owners... even if your airplane is not a C or P the fact is that the O-320 you have is used on tens of thousands of C and P airplanes.

I am heading tomorrow at the field and start asking.

 

[Magisterol]

Is it sucking in air from a dodgy inlet manifold joint?

That would make it a lean mixture, not rich.

 

[Magisterol]

VB beat me to it. THIS is what I would be doing given spark appears to check out fine and it's not eating oil. That leaves fuelling, replacing a carb with a known good item and seeing if the problem goes away isolates if it's a carb problem.

If it's all still going wrong with the new carb I would check the fuel pump, is it electric or mechanical? if electric, can it have shorted or something to push more pressure at a higher flow and forcing fuel through instead of just putting it there for the carb to suck? A fouled carb bowl stop needle would imitate the same problems, not cutting off the flow when the bowl is full/float is up, forcing excess fuel through the jets at pressure, bit if that was the case the carb swap would eliminate it.

I will try to find a carb to swap, but slim chances.....

 

[Victor Bravo]

What kind of airplane and fuel system? Pump driven, gravity, gravity with a boost pump????

 

[Pops]

Your explanation in Post #25 gives you the answer. Take the carburetor off, send it out to a DIFFERENT shop, a place that specializes in aircraft carburetors, and ask that shop to diagnose the cause of the issue. You may be able to explain to them that you don't need a carburetor overhaul (if it had already been OH'ed), you just need to find whatever was accidentally mis-adjusted during the previous OH.

What I would do. A shop that specializes in aircraft carburetors.

Well this is the AD that Lycoming says you have to do. This is why the AMEs are scrat their heads. They say it is a simple O-320 with nothing fancy on it and can’t figure out what is wrong.

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.