IO-360 rough idle

[dell30rb]

Got a little troubleshooting to do here.

The airplane is a 1946 Globe Swift with a 180HP lycoming and CS prop. Has GAMI injectors and an engine monitor (MVP 50). Idles poorly below about 1100 RPM on the ground. When you pull the throttle back in the pattern for a power off landing, you can hear it pop and fart a little bit.

Right now my best guess is the idle circuit is set improperly. But I don't know enough about the injection system on these, to know if this would be a possibility.

The injectors seem to be relatively clog free and it will run okay (smooth with maybe a hint of roughness) at 20 degrees LOP. Passes a mag check with minimal drop. Passes the GAMI lean test with about .3 GPH from the first cylinder peak to last cylinder peak.

 

[dell30rb]

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[Dan Thomas]

Got a little troubleshooting to do here.

The airplane is a 1946 Globe Swift with a 180HP lycoming and CS prop. Has GAMI injectors and an engine monitor (MVP 50). Idles poorly below about 1100 RPM on the ground. When you pull the throttle back in the pattern for a power off landing, you can hear it pop and fart a little bit.

Right now my best guess is the idle circuit is set improperly. But I don't know enough about the injection system on these, to know if this would be a possibility.

The injectors seem to be relatively clog free and it will run okay (smooth with maybe a hint of roughness) at 20 degrees LOP. Passes a mag check with minimal drop. Passes the GAMI lean test with about .3 GPH from the first cylinder peak to last cylinder peak.

How much of an RPM rise do you get when you go to idle cutoff when it's idling at about 800 RPM?

Dan

 

[dell30rb]

How much of an RPM rise do you get when you go to idle cutoff when it's idling at about 800 RPM?

Dan

Usually I kill it at 1000 RPM. Don't recall much of an RPM rise. Just shuts off.

Glad you raised this question. Possibly too lean on the idle circuit?

 

[Piloto]

Sounds like an induction leak. When there is an induction leak in one of the cylinders the engine will pop as you describe. It will not pop at high power settings because the leak is too small in comparison to the airflow through the intake pipe.

One way to find the leaky intake pipe is by running the boost pump.
With the engine off
1. Set mixture and throttle full forward.
2. Run the boost pump for about 15 seconds
3. Check for fuel stains around the intake pipe flange that mates into the cylinder.
4. Replace the flange gasket with a new one. Use a gasket sealer to improve sealing.
5. Recheck leak next day.

José

 

[dell30rb]

Sounds like an induction leak. When there is an induction leak in one of the cylinders the engine will pop as you describe. It will not pop at high power settings because the leak is too small in comparison to the airflow through the intake pipe.

One way to find the leaky intake pipe is by running the boost pump.
With the engine off
1. Set mixture and throttle full forward.
2. Run the boost pump for about 15 seconds
3. Check for fuel stains around the intake pipe flange that mates into the cylinder.
4. Replace the flange gasket with a new one. Use a gasket sealer to improve sealing.
5. Recheck leak next day.

José

I might rather find the leak with a little air pressure and soapy water, but this is a good explanation for the problem

 

[Henning]

Usually I kill it at 1000 RPM. Don't recall much of an RPM rise. Just shuts off.

Glad you raised this question. Possibly too lean on the idle circuit?

We don't know anything about the idle circuit yet. At idle, 650-750 rpm, shut off the engine with ICO mixture. You should see a 75 rpm rise just as it's shutting down. I would also check the timing on both mags.

 

[dell30rb]

We don't know anything about the idle circuit yet. At idle, 650-750 rpm, shut off the engine with ICO mixture. You should see a 75 rpm rise just as it's shutting down. I would also check the timing on both mags.

True, but how about the popping and farting when I pull the power to idle abeam the numbers? The reason I suspect the idle circuit is from my experience with motorcycles. These symptoms would indicate a too-lean idle circuit or induction leak.

 

[Piloto]

I might rather find the leak with a little air pressure and soapy water, but this is a good explanation for the problem

I have used the air pressure test but found the fuel leak test quicker and easier to do. But it only works for fuel injected engines.


José

 

[Henning]

True, but how about the popping and farting when I pull the power to idle abeam the numbers? The reason I suspect the idle circuit is from my experience with motorcycles. These symptoms would indicate a too-lean idle circuit or induction leak.

Are you slamming the throttle shut abeam the numbers? I can make them bang, fart, and pop being too rich as well, same for incorrect timing or a bad condenser.

 

[dell30rb]

Are you slamming the throttle shut abeam the numbers? I can make them bang, fart, and pop being too rich as well, same for incorrect timing or a bad condenser.

Full rich, from about 2300 rpm (prop full forward) to idle throttle in 2 seconds. Typical power off 180 drill...

 

[Henning]

Full rich, from about 2300 rpm (prop full forward) to idle throttle in 2 seconds

Probably have more fuel loaded in it vs being lean.

Get your basic mixture set as I said above, and your timing checked and verified. That's step one, it will either solve the problem or indicate what step 2 is.

 

[flyingmoose]

-Did this problem just develop?
-Did you change anything before this started?
-Have you checked the Mag timing?
-Did you do a Mag check at power?
-Is it rough at idle on the ground or when flying?

 

[dell30rb]

-Did this problem just develop?
-Did you change anything before this started?
-Have you checked the Mag timing?
-Did you do a Mag check at power?
-Is it rough at idle on the ground or when flying?

No idea how long the problem has been there. It is a new (to us) airplane. 700 SMOH. Apparently it didn't show up in the prebuy/annual.

Haven't checked the mag timing. The plugs were gapped and cleaned at pre-buy / annual - about 30 hours ago.

Have only checked the mags during a ground run up. Does fine there. I'll try it in flight.

As I mentioned earlier, it runs rough on the ground and when you pull the throttle back to idle when flying, it pops and farts a little bit. You don't feel it, but you hear it.

One note on the airplane, it has done a lot of sitting for the last 1-2 yrs or so. I know 100LL does not gum up like automotive fuel, but this still makes me suspect the fuel system. It was hangared with an engine saver (dehumidifier) hooked up to the crankcase.

 

[whifferdill]

Ren- with these engines, minor backfiring or popping when pulling power off is not uncommon. It's residual unburned gas igniting in the exhaust. Mine does it for a few seconds after pulling power to idle on downwind, and then goes away. Is the idle RPM setting very low? Some have found that the popping goes away after increasing the idle a little, since that lets more air in when the throttle is closed. A low idle seems to contribute to it. Lots of airplanes pop a little for a bit when pulling power off - P-51s, radials, Pitts'. But mine idles smooth enough on the ground, though I avoid letting it idle continuously for plug fouling reasons. And it's leaned way out for all ground ops. Rough idle and a little popping after pulling power could possibly be unrelated issues. Idle mixture may be a little on the rich side.

I sat in the Swift over the weekend. I'm pretty jammed in. Sweet little airplane though. Looks good in there with the Pitts.

 

[dell30rb]

It does need a little help. I have flown many airplanes w/ IO-360's and this one is worse than the others at idle. I can't say i've had much time to mess around with troubleshooting. I would guess it idles around 750 rpm. It does not spend much time there, usually If I am warming it up, i'll do it at 1000rpm with the mixture leaned way out. Runs much better there.

Not really a big issue but something to get checked out. It runs very smoothly and has good power otherwise. It will do close to 140kts true at just over 8gph

 

[dell30rb]

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[whifferdill]

I would guess it idles around 750 rpm.

That's on the high side of a reasonable idle RPM setting, so I'd imagine that's not contributing to popping. And others may pop a little, but the sound may be masked by the muffler. More noticeable on planes with no muffler. I saw the pipes on the Swift. I assume no muffler? The Pitts sure doesn't have one.

 

[dell30rb]

That's on the high side of a reasonable idle RPM setting, so I'd imagine that's not contributing to popping. And others may pop a little, but the sound may be masked by the muffler. More noticeable on planes with no muffler. I saw the pipes on the Swift. I assume no muffler? The Pitts sure doesn't have one.

I honestly don't know if it has a muffler or not! The cowling is pretty tight. You have a good point there

 

[FastEddieB]

Mine had a rough idle right before it froze an exhaust valve, leading to an emergency situation, with details and photo in the "Emergency" thread.

Lycoming I-360, carbureted.

There was a "wobble test" to be performed on that engine.

Just a thought...

 

[Henning]

No idea how long the problem has been there. It is a new (to us) airplane. 700 SMOH. Apparently it didn't show up in the prebuy/annual.

Haven't checked the mag timing. The plugs were gapped and cleaned at pre-buy / annual - about 30 hours ago.

Have only checked the mags during a ground run up. Does fine there. I'll try it in flight.

As I mentioned earlier, it runs rough on the ground and when you pull the throttle back to idle when flying, it pops and farts a little bit. You don't feel it, but you hear it.

One note on the airplane, it has done a lot of sitting for the last 1-2 yrs or so. I know 100LL does not gum up like automotive fuel, but this still makes me suspect the fuel system. It was hangared with an engine saver (dehumidifier) hooked up to the crankcase.

Uh oh, you lost a cam lobe.

 

[KSMooniac]

If it has a sniffle valve, you might check/replace that as well.

 

[RoscoeT]

Uh oh, you lost a cam lobe.

Amazing remote diagnostic powers you have.

 

[Henning]

Amazing remote diagnostic powers you have.

Yep... when he threw the last piece of the puzzle in there....

 

[RoscoeT]

Yep... when he threw the last piece of the puzzle in there....

I hardly think so Monsieur Poirot. Do you know this history of the airplane, and how well this particular engine dehumidifier worked? Would your diagnosis change if this aiplane had lived in Arizona...which was not mentioned?

 

[Henning]

I hardly think so Monsieur Poirot. Do you know this history of the airplane, and how well this particular engine dehumidifier worked? Would your diagnosis change if this aiplane had lived in Arizona...which was not mentioned?

Not particularly. All it takes is for a spot of surface rust to start, and the cam will be shot inside 100hrs.

 

[dell30rb]

Hopefully a bad cam lobe would show up in the prebuy/oil analysis. Sure, could be a sticky valve. But its not displaying the classic morning sickness.. Fast Eddie I believe you mentioned the first and only indication of the valve problem was a rough idle after a cold start.

I believe stuck valves are frequently caused by lead buildup and this motor is relatively low time.. plus the owner ran it LOP. I've had a stuck valve on a few motors (boat and a car) in the past.. definitely does get your attention.

This is a pretty minor problem. The guy who did my checkout in the plane is an A&P and commented on the rough idle. He told the owner he might want to get it looked at sometime, probably dirty injectors or something. If he suspected a more serious issue i'm sure he would have spoken up.

 

[Henning]

Hopefully a bad cam lobe would show up in the prebuy/oil analysis. Sure, could be a sticky valve. But its not displaying the classic morning sickness.. Fast Eddie I believe you mentioned the first and only indication of the valve problem was a rough idle after a cold start.

I believe stuck valves are usually caused by lead buildup and this motor is relatively low time.. plus the owner ran it LOP. I've had a stuck valve on a few motors (boat and a car) in the past.. definitely does get your attention.

This is a pretty minor problem. The guy who did my checkout in the plane is an A&P and commented on the rough idle. He told the owner he might want to get it looked at sometime, probably dirty injectors or something. If he suspected a more serious issue i'm sure he would have spoken up.

If the plane had minimal running time in the couple years prior to the inspection oil change/analysis, there is no particular reason it would have shown anything. It takes a few hours to get through the hardened surface, and then it goes pretty quick. What I would do on your next oil change is to filter the oil through a coffee filter and cut open the oil filter and see what's to be seen. Go ahead and check the timing in the mean time and see what you can find on an induction leak (my method is to get a can of starting fluid and with the engine running start spraying at all the junction points and listening for a change) but I wouldn't spend a bunch of money chasing the problem without going to the camshaft.

 

[FastEddieB]

For those who missed the prior thread, what I discovered post-emergency:

And once the exhaust valve is frozen closed, I can tell you a four cylinder engine runs VERY rough and with a LOT of backfiring (OK, after-firing to be precise).

 

[Piloto]

For those who missed the prior thread, what I discovered post-emergency:

And once the exhaust valve is frozen closed, I can tell you a four cylinder engine runs VERY rough and with a LOT of backfiring (OK, after-firing to be precise).

What kind of oil you were using?

José