O-470L fuel on carb body

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Narwhal

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I have attached 2 pictures of the carburetor from an O-470L on a '59 C182B. It has an autogas STC. It is run on ethanol-free 91 octane autogas about 20% of the time. There has been about 500 hours and 10 years since overhaul.

There are fuel stains on the carb, as you can see. There hasn't ever been fuel on the ground during preflight. I have read that the O-470, especially with autogas, can leak some fuel at shutdown.

The only thing wrong with the operation of the engine -

1. Sometimes the primer will not work, it just pushes air. This seems random, 90% of the time the primer works fine. I have read that the primer can leak fuel into carb. Could this be the cause? I lack the mechanical knowledge to understand if these two things could be related.

2. There is sometimes an audible "pop" when pulling the engine to idle for landing with the carb heat on.

3. For a time, there was random roughness that I laid out in this thread: https://www.pilotsofamerica.com/com...gine-roughness-at-cruise.130588/#post-3042022 ; I had the magnetos inspected/refurbished and that problem seems to have gone away (they were at about 480 hours). I have only flown it once since the magnetos were re-installed, and this is when the fuel stain was noticed.

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The A&P I was working on this with tightened the bolts and just recommended we keep an eye on it.

Similar problem from BCP: https://backcountrypilot.org/forum/carb-leak-cessna-180-182-o-470-23098
Similar problem on an O-320 at SCorg: https://www.supercub.org/forum/showthread.php?52873-fuel-leak-through-carburettor
 
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Narwhal said:
I have attached 2 pictures of the carburetor from an O-470L on a '59 C182B. It has an autogas STC. It is run on ethanol-free 91 octane autogas about 20% of the time. There has been about 500 hours and 10 years since overhaul.

There are fuel stains on the carb, as you can see. There hasn't ever been fuel on the ground during preflight. I have read that the O-470, especially with autogas, can leak some fuel at shutdown.

The only thing wrong with the operation of the engine -

1. Sometimes the primer will not work, it just pushes air. This seems random, 90% of the time the primer works fine. I have read that the primer can leak fuel into carb. Could this be the cause? I lack the mechanical knowledge to understand if these two things could be related.

2. There is sometimes an audible "pop" when pulling the engine to idle for landing with the carb heat on.

3. For a time, there was random roughness that I laid out in this thread: https://www.pilotsofamerica.com/com...gine-roughness-at-cruise.130588/#post-3042022 ; I had the magnetos inspected/refurbished and that problem seems to have gone away (they were at about 480 hours). I have only flown it once since the magnetos were re-installed, and this is when the fuel stain was noticed.

View attachment 94615 View attachment 94614

The A&P I was working on this with tightened the bolts and just recommended we keep an eye on it.

Similar problem from BCP: https://backcountrypilot.org/forum/carb-leak-cessna-180-182-o-470-23098
Similar problem on an O-320 at SCorg: https://www.supercub.org/forum/showthread.php?52873-fuel-leak-through-carburettor
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It's that filter cavity plug that's leaking. The thing with the 95-500 number on it. Needs a new gasket.

Primers with worn O-rings will suck air. And sometimes they get a bit of crud in them that interferes with the tiny check valves in them. You are pulling the primer out and waiting for it to fill with fuel, right?
 
The primer thing is common. Especially if it hasn’t been exercised lately. I agree with Dan about the leak source. I’m surprised your mechanic didn’t fix it. Re: the pop? Also common. Try pushing carb heat back in on short final. If you had ice it’ll be gone by now. It’s a good habit in case you need to go-around.
 
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I had a similar problem on my old Cherokee 140. The bolts holding the float bowl to the throttle body would loosen after a few hours of flying. They were a pain to get to, so the second time it happened, after initially snugging all the bolts, I pulled them one at a time and used locktite on the threads. I never had another problem after that.

Based on your pictures the leak appears to be under a plug bolt head. It could be a straight thread plug that seals on a copper or aluminum washer. These are like the hollow bolts commonly used for banjo fittings, but the plugs are shorter because there is no fitting under them. That's what it looks like from your picture. YMMV after you look more closely
 
Dan Thomas said:
It's that filter cavity plug that's leaking. The thing with the 95-500 number on it. Needs a new gasket.

Primers with worn O-rings will suck air. And sometimes they get a bit of crud in them that interferes with the tiny check valves in them. You are pulling the primer out and waiting for it to fill with fuel, right?
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Thanks, I'll recommend to my mechanic that we change the gasket on the filter cavity plug!

Yes, I wait for the primer to fill with fuel. Changing the O rings was the first thing we tried but it still does it, have had 3 different A&P's, 2 with IA's, look at it. The 10% of the time when it doesn't work, there is no resistance on the aft stroke and nothing happens no matter how long you wait (no squeee sound, no resistance), and sometimes no matter how many times you re-prime nothing will happen. This usually happens when the plane has been sitting a week or more without flying.
 
William Pete Hodges said:
Based on your pictures the leak appears to be under a plug bolt head. It could be a straight thread plug that seals on a copper or aluminum washer. These are like the hollow bolts commonly used for banjo fittings, but the plugs are shorter because there is no fitting under them. That's what it looks like from your picture. YMMV after you look more closely
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That gasket is visible. It has the two long tabs that fit over a small lug on the body, and several smaller tabs that are to be bent over the hex on the plug to prevent its loosening. Part #49 in this picture:

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#48 is the last-chance fuel screen. The leak is at that end. The fuel comes in at the other end of that bore, at fitting #46.
 
Latest problem. Engine still shuts down at idle after replacing gasket. It’s idling at about 520 rpm and dies if it idles for more than 10 or 15 seconds.


it doesn’t die at idle with carb heat on. It will die a few seconds after I turn carb heat off.

No RPM rise when pulling mixture from rich toward idle.

Obviously I’m not flying it like this, but just doing some preliminary troubleshooting for the benefit of the mechanic.
 
More likely the idle needle is set to max lean, and the engine just starves for fuel after the throttle is closed. Carb heat richens the mixture, remember? No RPM rise on ICO confirms a lean idle mix.

Carb ice is a possibility, but if the carb heat has been on for a minute or two at idle, there's no ice in there to kill the engine after 15 or 20 seconds.

One other more remote possibility is weak spark. Weak spark ignites rich mixtures more easily than lean mixtures. If those mags haven't been off in the 500 hours since engine overhaul, it's time to do them.
 
Ok thank you very much. My A&P and I are going to tackle this on Friday. I’m sure he probably doesn’t want/need my help aside from getting the cowling on and off and maybe doing an engine run to warm it up, but it’s good to be armed with some knowledge considering he works on many different types of airplanes.
 
Had to pull the carb and send it off. The mixture adjustment wasn't able to generate an RPM rise at shutdown to the mechanic's satisfaction, even at the extreme end of the adjustment range. Also discovered that fuel was still dribbling out of the fuel line with the fuel selector off, even though the fuel selector was replaced less than 2 years ago. Maintenance whack a mole continues.....to be expected on a 61 year old machine I guess.
 
Got the carb back from overhaul and reinstalled. Unfortunately the plane was down for 6 weeks because this happened right before the annual. It seems to run great now. I still hear some popping sounds at idle sometimes in-flght but it's running great aside from that. It's idling at about 600 rpm and I get about 40 RPM rise when I pull the mixture to shutdown at 1000 rpm.
 
Narwhal said:
Got the carb back from overhaul and reinstalled. Unfortunately the plane was down for 6 weeks because this happened right before the annual. It seems to run great now. I still hear some popping sounds at idle sometimes in-flght but it's running great aside from that. It's idling at about 600 rpm and I get about 40 RPM rise when I pull the mixture to shutdown at 1000 rpm.
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40 RPM rise indicates a rich idle mixture, which can give you that popping sound (afterfiring) at idle in the glide. Mechanics really should consult the manuals.

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25 RPM maximum at 1000 RPM. That will change as the air temp and altitude change, as well. Leaner in cold weather, richer in hot. Got to sort of shoot for the middle.
 

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Dan Thomas said:
40 RPM rise indicates a rich idle mixture, which can give you that popping sound (afterfiring) at idle in the glide. Mechanics really should consult the manuals.

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25 RPM maximum at 1000 RPM. That will change as the air temp and altitude change, as well. Leaner in cold weather, richer in hot. Got to sort of shoot for the middle.
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Thanks, I'll mention that to him!
 
Narwhal said:
Thanks, I'll mention that to him!
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Fuel injection systems often require a 25-50 RPM rise on ICO, and mechanics can forget that carbs are different. The '70s 182 manuals called for a 25 max RPM rise, and the oldest (up to 1962 or so) called for a 20 RPM rise. In any case, 40 is too much. And checking right at idle (around 600) can get different numbers. Try it as that excerpt I posted says, at 1000 RPM after clearing the engine, and see what you get.

25 is pretty hard to spot on the tach, especially if the needle is waving around some. Old tach cables, with dried-out lube, can do that. Electronic tachs are a real improvement. I would never want to replace a mechanical tach with another mechanical tach unless the airplane had no electrical system or was maintained as an original classic or if there was no STC for an electronic tach. Mechanical tachs are a real pain. In Canada we have to check magnetic-drag tachs (which is most of them) annually to see that they're within 4% of actual RPM. AS they age the magnet in them loses its strength and the tach begins to under-read. Sometimes they'll really jump around or read really high because the oil in the tiny bearing (that the far end of the needle shaft rests in, in the input shaft) begins to harden and the needle shaft gets dragged all over the place.
 
Dan Thomas said:
Fuel injection systems often require a 25-50 RPM rise on ICO, and mechanics can forget that carbs are different. The '70s 182 manuals called for a 25 max RPM rise, and the oldest (up to 1962 or so) called for a 20 RPM rise. In any case, 40 is too much. And checking right at idle (around 600) can get different numbers. Try it as that excerpt I posted says, at 1000 RPM after clearing the engine, and see what you get.

25 is pretty hard to spot on the tach, especially if the needle is waving around some. Old tach cables, with dried-out lube, can do that. Electronic tachs are a real improvement. I would never want to replace a mechanical tach with another mechanical tach unless the airplane had no electrical system or was maintained as an original classic or if there was no STC for an electronic tach. Mechanical tachs are a real pain. In Canada we have to check magnetic-drag tachs (which is most of them) annually to see that they're within 4% of actual RPM. AS they age the magnet in them loses its strength and the tach begins to under-read. Sometimes they'll really jump around or read really high because the oil in the tiny bearing (that the far end of the needle shaft rests in, in the input shaft) begins to harden and the needle shaft gets dragged all over the place.
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Yeah, luckily I have an electronic tach. I'm planning to fly again today so I'll be sure to check the RPM rise very closely again.
 
Tested again. Didn't seem to be any popping and it looked like a 23 RPM rise (1005 to 1028). I'm not sure what might've changed other than about 5 degrees Celsius warmer on the outside temp.
 
Narwhal said:
Tested again. Didn't seem to be any popping and it looked like a 23 RPM rise (1005 to 1028). I'm not sure what might've changed other than about 5 degrees Celsius warmer on the outside temp.
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Perfect. Doing it at 1000 instead of idle might have changed it.
 
Bad news.

It's still dying at idle, but only after the engine is hot. After running for about an hour, the engine will start dying on the landing rollout. During the initial runup at the beginning of the day and landings during the first hour, there is no problem. I can only speculate now that perhaps the mixture adjustment screw on the carburetor was set too rich (Just like Dan T said earlier), but it is worse now than it has ever been even after 12 new spark plugs, overhauled carb, and "refreshed" bendix magnetos. Kinda frustratiing to shell out $5k USD on maintenance and have very little to show for it. Keeping the carb heat does not stop it from dying any longer, and anything below about 1000 rpm on the throttle is enough for it to sputter and die when warm.

Obviously going to have the adjustments on the carb looked at again. It is about 10-20 degree C warmer on the outside temps now than when the carb was put back on. With that said, it seems crazy that the motor wouldn't be able to handle those kin of temperature swings.
 
Narwhal said:
Bad news.

It's still dying at idle, but only after the engine is hot. After running for about an hour, the engine will start dying on the landing rollout. During the initial runup at the beginning of the day and landings during the first hour, there is no problem. I can only speculate now that perhaps the mixture adjustment screw on the carburetor was set too rich (Just like Dan T said earlier), but it is worse now than it has ever been even after 12 new spark plugs, overhauled carb, and "refreshed" bendix magnetos. Kinda frustratiing to shell out $5k USD on maintenance and have very little to show for it. Keeping the carb heat does not stop it from dying any longer, and anything below about 1000 rpm on the throttle is enough for it to sputter and die when warm.

Obviously going to have the adjustments on the carb looked at again. It is about 10-20 degree C warmer on the outside temps now than when the carb was put back on. With that said, it seems crazy that the motor wouldn't be able to handle those kin of temperature swings.
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Something definitely wrong there. What was involved in "refreshing" the mags? Were they done by someone who knows what they're doing? Just replacing the points isn't enough; the E-gap has to be set, and in some Bendixes the cam can be set wrong on the shaft and then everything is haywire.

Try checking the mags at 1000 or 1100. You might find a seriously weak or dead mag there.
 
Serious question: Does a hot engine run leaner than a cold engine?

Are you rolling out full rich?
 
Domenick said:
Serious question: Does a hot engine run leaner than a cold engine?

Are you rolling out full rich?
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Should run leaner cold and richer hot IMHO. The fuel distribution is so poor on o470 they run like crap cold, as they warm up fuel is vaporized better and distributed better and run better.
 
Narwhal said:
Bad news.

It's still dying at idle, but only after the engine is hot. After running for about an hour, the engine will start dying on the landing rollout. During the initial runup at the beginning of the day and landings during the first hour, there is no problem. I can only speculate now that perhaps the mixture adjustment screw on the carburetor was set too rich (Just like Dan T said earlier), but it is worse now than it has ever been even after 12 new spark plugs, overhauled carb, and "refreshed" bendix magnetos. Kinda frustratiing to shell out $5k USD on maintenance and have very little to show for it. Keeping the carb heat does not stop it from dying any longer, and anything below about 1000 rpm on the throttle is enough for it to sputter and die when warm.

Obviously going to have the adjustments on the carb looked at again. It is about 10-20 degree C warmer on the outside temps now than when the carb was put back on. With that said, it seems crazy that the motor wouldn't be able to handle those kin of temperature swings.
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Did it ever not do this? IOW has it been doing this the whole time you have owned it?

Fly it, if it dies again at idle, quickly open a fuel cap and listen for a whooshing sound?
 
Lean or rich is a factor of ambient temp, not engine temp. If the intake system is healthy it should run fine cold or warm. In warmer temps it may need to be leaned more aggressively for less than full power ops but that assumes it isn’t running too lean to begin with, and lean is VERY common. Every big bore Continental owner needs to know their leaning authority and how it differs between winter and summer. Quitting on rollout isn’t indicative of a typical induction leak but I’d always check the rubbers first and closely inspect the balance tube for cracks after that.

I’ve learned to appreciate 6-point engine monitoring and a fuel flow instrument.
 
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bnt83 said:
Did it ever not do this? IOW has it been doing this the whole time you have owned it?

Fly it, if it dies again at idle, quickly open a fuel cap and listen for a whooshing sound?
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If it was a fuel flow issue, the thing shouldn't even be able to fly. Those old 182s are gravity-feed anyhow. Any fuel system restriction would end in disaster on takeoff.

Weak spark at idle. Or induction leakage. Or a bunch of "new" old stock Champions, though even those shouldn't be giving trouble this early. Or the idle mix is too rich, something we discussed earlier. A cold engine would appreciate a bit of extra fuel at idle, a hot engine wouldn't. The OP could try restarting after it quits, then lean it out right close to ICO, and see if it will idle there OK. If it does, the idle mix is too rich.
 
Ok, it was a loose mixture adjustment screw on the carb. Apparently the screw had backed out to the point that the engine was dying at idle. Rescured the screw and the engine ran fine at idle indefinitely. I don't know what would cause it to run fine for the first half of the day and the screw to suddenly back out and cause the engine to die at idle. Maybe vibrations? Either way at least it was a quick fix. It was flown for 2 hours today and never had an issue.

No, it hasn't always done this. The initial dying at idle problem started about 5 months ago before the carb was overhauled. Then it worked fine for about 3 hours after the carb/mag overhaul, then the screw apparently backed out causing the most recent issue. Yikes!
 
Narwhal said:
Ok, it was a loose mixture adjustment screw on the carb. Apparently the screw had backed out to the point that the engine was dying at idle. Rescured the screw and the engine ran fine at idle indefinitely. I don't know what would cause it to run fine for the first half of the day and the screw to suddenly back out and cause the engine to die at idle. Maybe vibrations? Either way at least it was a quick fix. It was flown for 2 hours today and never had an issue.

No, it hasn't always done this. The initial dying at idle problem started about 5 months ago before the carb was overhauled. Then it worked fine for about 3 hours after the carb/mag overhaul, then the screw apparently backed out causing the most recent issue. Yikes!
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There is supposed to be a stiff little spring on that mixture screw that keeps tension on it and discourages creep.

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Maybe that spring is shot or missing or the wrong spring. If there are washers under it it can slip.
 
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