150 Running Rough- Carb Overhaul?

[VictorMike]

Good people, I am hoping I can find some assistance with my probable carb issue. Engine runs like garbage even still after replacing both mags. Short story is I was ferrying the plane and about 5 minutes into the flight the engine started running a little rough. Turned around and had the shop tear into it. They found the mags were unable to be timed and since they were slick 4201's no one would take the cores... 2000$ gone.. cool. New mags put on, runs better but the engine is still running rough and will die off a couple hundred RPM while running up not yet doing a mag check. Shop wants to put on a new overhauled carb since this one is a 2 piece and even though compliance with the AD was noted they said there was no way it has been touched in 30 years based on how it looks. I would love to make my life easy and get one from spruce but those carbs are probably worse garbage than the one on the plane already. Anyone have thoughts or comments?

 

[Dan Thomas]

Good people, I am hoping I can find some assistance with my probable carb issue. Engine runs like garbage even still after replacing both mags. Short story is I was ferrying the plane and about 5 minutes into the flight the engine started running a little rough. Turned around and had the shop tear into it. They found the mags were unable to be timed and since they were slick 4201's no one would take the cores... 2000$ gone.. cool. New mags put on, runs better but the engine is still running rough and will die off a couple hundred RPM while running up not yet doing a mag check. Shop wants to put on a new overhauled carb since this one is a 2 piece and even though compliance with the AD was noted they said there was no way it has been touched in 30 years based on how it looks. I would love to make my life easy and get one from spruce but those carbs are probably worse garbage than the one on the plane already. Anyone have thoughts or comments?

Go here and see my comments on the carb mounting. This is an old subject that comes up periodically. https://www.pilotsofamerica.com/community/threads/continental-engine-stumble.130180/#post-3029901

Have the sparkplugs been spark-tested under pressure?

 

[VictorMike]

Yes, the spark plugs were tested under pressure, changed, harness changed, etc. No fix. It was bone dry out and after I checked the carb ice chart I actually became concerned it was the engine.. I still wish it was ice. The shop doesn't feel 100% comfortable taking it apart to clean it either after the plane sat for a few years in its hangar. I suspect it does need to come off the engine and get some TLC but I need someone who actually knows about these 2 piece carbs. Recommendations are welcome.

 

[Dan Thomas]

So see if the carb spider is rigidly mounted. You should be able to grab the airbox and swing it side to side a little. I bet it's torqued right up tight.

 

[VictorMike]

So see if the carb spider is rigidly mounted. You should be able to grab the airbox and swing it side to side a little. I bet it's torqued right up tight.

They said its not all that tight. Here is the going theory right now: Plane sat with some mogas for some time. It varnished up the carb and something is sticking. How do we know this? Because there's some varnish in the fuel tanks.

 

[Howard Wilson]

I'm sure that you guys looked... Gascolater screen? Varnish in the fuel tanks could be flaking into it.

 

[MBDiagMan]

Are you sure that the valves aren’t carboned up? Does it make any fire out of the exhaust when this rough running is occurring? When the valves get sticky, they cause unburned fuel to mix with the incoming charge causing crossfiring that cannot be fixed with carb and ignition work.

By removing the valve springs you can move the valves back and forth to determine if they are sticky with carbon build up. If they are, a savvy mechanic can use hooks and magnets through the spark plug holes and guide to remove the valves, swab the guides, push the valve stem through the spark plug hole for cleaning, then with a hook through the spark plug hole and a magnet through the guide, pull it back in place. It is quite an operation to observe.

 

[Jdm]

And why do you think spruce carbs are junk? I honestly didn’t know Spruce made carbs.
Me, I’d check the simple stuff first, like intake gaskets.

 

[MBDiagMan]

Yes a vacuum leak will cause no or rough idle and poor running when advancing the throttle from low speed. A vacuum leak at an in individual intake runner like Jim talks about can cause permanent damage by leaning out one cylinder.

 

[Dan Thomas]

Yes a vacuum leak will cause no or rough idle and poor running when advancing the throttle from low speed. A vacuum leak at an in individual intake runner like Jim talks about can cause permanent damage by leaning out one cylinder.

It started running rough in flight. Now it loses 200 RPM approaching runup RPM. If it was a leaking intake gasket it would be most noticeable at idle and not much anywhere else. At idle the manifold pressure is the lowest and sucks the most air past a bad gasket.

The 200 RPM loss in RPM is typical of the problem I pointed out in the first place. There's a resonance that starts shaking the engine at certain RPMs, shaking that carb and sloshing fuel out of the bowl vent into the carb throat, causing rich misfire. The O-200 is a bad one this way. It doesn't show up on other engines.

 

[MBDiagMan]

I wasn’t clear.

If there is a leak on an individual runner between carb and intake valve, it will lean only that cylinder and do it at all RPM. You would probably feel a mild vibration because of decreased torque delivery by that cylinder, but it would indeed show up at high RPM and would indeed lower your max RPM.

 

[mondtster]

I wasn’t clear.

If there is a leak on an individual runner between carb and intake valve, it will lean only that cylinder and do it at all RPM. You would probably feel a mild vibration because of decreased torque delivery by that cylinder, but it would indeed show up at high RPM and would indeed lower your max RPM.

An induction leak will not cause much trouble during flight because the delta pressure between ambient and manifold is typically prett small. In other words, the air leak will be small which will cause minimal mixture change.

But you are correct, this is not directly engine speed related.

 

[Dan Thomas]