Engine running rough

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Pops

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Yes, the mixture metering valve can be put in backwards. Makes the fuel shut off with the mixture knob in instead of out.
Daughter and her husband bought a cessna 150 that was seating in a hanger and the owner couldn't get it to run except on the primer fuel. Gave up and wanted to sell it. They bought it and I took a carb off another 150 and put it on and flew it home.
 
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Dan Thomas

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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.
They might be making them that way now. I thought you figured the jet was just machined into the carb casting.
 

Dan Thomas

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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?
The mixture control can't be assembled backwards. The female part of it screws into the carb body, and if it comes loose it leans out the mixture and can cut it right off.

And yes, I once had a new engine running rough because the accelerator pump was letting fuel though it during high power ops. One of the tiny check valve springs in the pump assembly was missing. It needs to be there to prevent fuel delivery unless the throttle is opening. But it's a high-power thing, not low power. It only happens when the airflow through the venturi is high.
 

Dan Thomas

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Yes, the mixture metering valve can be put in backwards. Makes the fuel shut off with the mixture knob in instead of out.
Daughter and her husband bought a cessna 150 that was seating in a hanger and the owner couldn't get it to run except on the primer fuel. Gave up and wanted to sell it. They bought it and I took a carb off another 150 and put it on and flew it home.
It must have had the lever installed 180° out? There's one flat on the shaft to prevent that.
 

Magisterol

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They might be making them that way now. I thought you figured the jet was just machined into the carb casting.
Never saw the jet/nozzle thing. i only saw the manual and that was my conclusion albeit wrong apparently.
 

Pops

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It must have had the lever installed 180° out? There's one flat on the shaft to prevent that.
Sent the carb out to be overhauled and they said the metering valve was backwards, maybe they meant the lever ??
Got the overhauled carb back and replaced the loaner carb and all was good.
 

Talon38

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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...
Full rated power is not developed until the aircraft is in motion / flying. The De Havilland Dash 8 requires prop balancing to be accomplished in the air. Most aircraft props I have balanced were satisfactorily accomplished on the ground e.g. Mitsubishi MU2, Beechcraft 90, 100, 200, 300.
 

Magisterol

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Full rated power is not developed until the aircraft is in motion / flying. The De Havilland Dash 8 requires prop balancing to be accomplished in the air. Most aircraft props I have balanced were satisfactorily accomplished on the ground e.g. Mitsubishi MU2, Beechcraft 90, 100, 200, 300.
The vibration is not from an unbalanced prop. It is from the engine.
 

Magisterol

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Well, knock on wood, looks like the roughness is gone, with the replacement of the fuel nozzle. Still don’t understand why is started to happen all of a sudden. My only explanation would be that when the mechanic put the original nozzle back on somehow the hole or holes got messed up (unintentionally) and since then the roughness. Don’t know what to say.... Anyway, thanks everybody for the help. Cheers.
 

Magisterol

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Well, looks like the roughness is back. And at rpm above 2500, the engine feels like is ”coughing “. A periodic short”cough” or “sneeze “ or “stall. Don’t even know what to call it. The worst part is that you can feel the whole airplane jolting, like you are pulled back for a fraction of a second and then released. It is disconcertin, especially flying over water... The opinions are divided between carb and mags, but nobody has an explanation without a rebuttal... I was coming back from a trip at at 7500ft I decided to bring the rpm up to 2700 and check the TAS as I got an OAT indicator. As I got over 2500rpm, it started. It drops about 100rpm or so. Well at least it is not doing below, as before. I wander if the engine should have been run as if you are braking it in. It was sitting a lot before I bought it. In the last few years before I bought it, it only flew about 1 hr or so after each annual.
 

Dan Thomas

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Is there a mechanical fuel pump in the system? It might be shot. Has the fuel strainer been apart and the screen cleaned? Has the inlet strainer on the carb been out and cleaned? Hesitation and coughing at high power levels implies insufficient fuel flow, and the engine leans out and starves intermittently. It causes an RPM drop to a point where the fuel flow is sufficient and then catches again.

If you have an electric boost pump, turn it on and see if it helps. Push the mixture in and see if it gets worse. But check those screens first.
 

Magisterol

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No fuel pump on the airplane. I played with the mixture, but no difference. And looks like the compass is spinning too, when this thing is happening. all the fuel filters and screens were checked and were clean.
 

Dan Thomas

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And looks like the compass is spinning too, when this thing is happening.
Vertical card compass? They are really vibration-sensitive. If not.....something electrical is goofing up. Does the ammeter spike at the time? A big charge current can generate a field strong enough to bother a compass if the cable runs too close to it. Or is something shorting power to the mag P-leads? At the mag switch? Loose hot terminal (for the start function) contacting a P-lead? The mag wouldn't like that. It's a stretch, I know.

Maybe it's a UFO following you around.
 

Magisterol

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Vertical card compass? They are really vibration-sensitive. If not.....something electrical is goofing up. Does the ammeter spike at the time? A big charge current can generate a field strong enough to bother a compass if the cable runs too close to it. Or is something shorting power to the mag P-leads? At the mag switch? Loose hot terminal (for the start function) contacting a P-lead? The mag wouldn't like that. It's a stretch, I know.

Maybe it's a UFO following you around.
It is a vertical card. I was thinking about intermittent grounding of the mags, but I did a mag check and there is no difference with one or two mags. Did not see any spike or movement on the ammeter. I have a rear view mirror, but I didn’t check to see. Next time ...😊 If the weather is good tomorrow, I will head to the field and fly it again, maybe I will find new clues.
 

TFF

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I don’t remember, does it have an EGT gauge? If it does, what does it do? If it doesn’t, even a single cylinder will give you good information. Low wing airplane with no fuel pump is not normal. Is the fuel in the wings or under the cowl? Also is this fixed pitch? Because you can get 2700 rpm it might start running out in the float bowl. At 2500 the consumption might be low enough that the float bowl stays full. Ambient temperature has changed, cold needs more fuel.
 

Magisterol

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I don’t remember, does it have an EGT gauge? If it does, what does it do? If it doesn’t, even a single cylinder will give you good information. Low wing airplane with no fuel pump is not normal. Is the fuel in the wings or under the cowl? Also is this fixed pitch? Because you can get 2700 rpm it might start running out in the float bowl. At 2500 the consumption might be low enough that the float bowl stays full. Ambient temperature has changed, cold needs more fuel.
It does have an EGT gauge, but no change on it. It is getting info from cylinder #3. The fuel tank is in the fuselage. Fix pitch prop. If it runs out of the fuel in the bowl, would mean that the diameter of the needle valve seat is too small. It used to do this “cough” above 2300-2400rpm but now it does it above 2500. Don’t know what changed. Maybe I should run the engine at higher rpm for a few month instead of 2400. 2400 rpm is my comfortable rpm for xcountry... See if it changes.
 

Dan Thomas

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It does have an EGT gauge, but no change on it. It is getting info from cylinder #3. The fuel tank is in the fuselage. Fix pitch prop. If it runs out of the fuel in the bowl, would mean that the diameter of the needle valve seat is too small. It used to do this “cough” above 2300-2400rpm but now it does it above 2500. Don’t know what changed. Maybe I should run the engine at higher rpm for a few month instead of 2400. 2400 rpm is my comfortable rpm for xcountry... See if it changes.
I wonder if the carb you have is intended for a pumped fuel system. You might check the TCDS for your particular engine model and suffix and see what the carb number is supposed to be, and see if a fuel pump is also specified for that engine. Whatever fuel head you have may be insufficient for full power, although one would expect serious problems in the climb or when the tank's fuel level is low. Leaning the engine would reduce the fuel demand and maybe stop the coughing.

Sticking valves are a possibility. A multi-point EGT would be handy.

A crossfiring mag can cause all sort of problems, firing at the wrong time and setting off the mixture while an intake is still open, burning all the mix in the intake manifold. But you've tried checking the mags at the time and nothing changed. Hmph.

Do you have an ACS ignition switch? Not a Bendix? There are ADs on both switches, and in the ACS it addresses burned contacts that can cause running issues. I think the starter contact plate also acts as the mag contact for the left mag, and it burns due to the voltage spike spark generated by the starter contactor coil. The AD demands a shunt diode on the start contactor coil to stop that spike, and a 2000-hour internal inspection of the switch. I could imagine burned junk in there rattling around and getting into places it shouldn't. Just another idea.
 

Magisterol

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I wonder if the carb you have is intended for a pumped fuel system. You might check the TCDS for your particular engine model and suffix and see what the carb number is supposed to be, and see if a fuel pump is also specified for that engine. Whatever fuel head you have may be insufficient for full power, although one would expect serious problems in the climb or when the tank's fuel level is low. Leaning the engine would reduce the fuel demand and maybe stop the coughing.

Sticking valves are a possibility. A multi-point EGT would be handy.

A crossfiring mag can cause all sort of problems, firing at the wrong time and setting off the mixture while an intake is still open, burning all the mix in the intake manifold. But you've tried checking the mags at the time and nothing changed. Hmph.

Do you have an ACS ignition switch? Not a Bendix? There are ADs on both switches, and in the ACS it addresses burned contacts that can cause running issues. I think the starter contact plate also acts as the mag contact for the left mag, and it burns due to the voltage spike spark generated by the starter contactor coil. The AD demands a shunt diode on the start contactor coil to stop that spike, and a 2000-hour internal inspection of the switch. I could imagine burned junk in there rattling around and getting into places it shouldn't. Just another idea.
The carb I have on is MA-4SPA 10-3678-32V serial # A 26 16009 and the engine is an O-320E2A serial # L-26926-27A. Is was modified to 8.5:1 compression. In climb I tried to climb at a higher speed to get about 2600rpm and it doesn’t cough. Now it is true that this time I put MMO in it compared to last time. I checked the ACS ignition switch and it looks like out of the box inside. No corrosion or dark spots on the contacts.
I wander about that fuel head pressure.yesterday I had full tank, but last time I checked, I had less than half. I should have tried yesterday after that 2 hrs flight. Maybe I should try again with half tank, see what it does. I should get one of those Facet fuel pumps. Easy to install and no moving parts to brake down.
i forgot say, but I have the SB 22 that ask you to replace the old nozzle, with the new one #47-862.
 
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