Marvel Carburetor Troubleshooting

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hotrodmac
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Marvel Carburetor Troubleshooting

Post by hotrodmac »

Hi all,

I'm getting ready to go into annual and have an interesting annoyance that I'm looking for some advice on. I have a C85-12 with a Marvel MA-3SPA carb that was installed as a factory re-manufactured unit when my engine was overhauled by the previous owner A&P/IA last year. When I bought the airplane, he told me it runs rich and that it needs to always be leaned out. True to his comment, if I don't lean it out at least a finger-width on the ground, by the time the engine is warmed up and I'm to the run-up area, the bottom plugs are fouled. In cooler weather when the air temp is below about 60F, I have to fly with partial or full carb heat to get it to run smooth. In the winter, I cannot find a mixture setting that will make it run smooth.

My A&P/IA said to contact Marvel for some advice. I did that and didn't really get much help since I didn't have the exact model and serial number at my fingertips when I made the call. The only advice I got was to cap off the primer line and "see if that helps." My mechanic thinks that the idle mixture is not set correctly, however I do get the appropriate RPM rise at idle cut-off, so it's a head-scratcher.

I'm looking for some advice here on what my be the culprit. We have not tried capping the primer yet, will probably do that when we annual the plane here in a couple weeks.

Thanks much!

Lance
'46 C140 - Flying
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V529
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Re: Marvel Carburetor Troubleshooting

Post by V529 »

I suspect you may have two problems.

1. leaning out to avoid fouling spark plugs at run up.
1A. Yes this happens and if you lean it out sometime that helps........but..........it can also be oil leaking past the rings that has fouled out the spark plugs. To check this, after the engine has been sitting(at least overnight), pull all 4 top spark plugs and look down into the cylinder. Do you have a puddle of oil? is the lower spark plug(should be viewable) bathed in oil? Then that is your true fouling issue.
1B. If you get to the run up area, and have a rough mag check, power up the engine to 2000 RPM's, lean for about 30-40 seconds (almost to the point of RPM's falling off) if the mag check clears up, great. I would still inspect the lower spark plugs for either oil or fuel deposits. They look different. Fuel will look like molten lead, (obviously.......lead in the fuel.....) and oil will be the same as the build up on top of your pistons, usually brownish crud.

2. Your rough running and requiring the carb heat to be pulled partially out, especially in cold weather.
This is a classic Marvel 3 SPA issue. Check and see if you have A. the one piece venturi in the carb B. the "pepperbox" nozzle. (as opposed to the straight nozzle that just has a hole in the end.......a pepperbox will be perforated close to the top) Either the one piece venturi or the Pepperbox nozzle will cause the engine to run lean, especially in the mid RPM ranges. Does this at all sound like your issue? If so, you may need to either install a straight nozzle and/or the old 2 piece venturi(which requires the 100 hour inspection) I had to do this to my carb as have many others. While it's a pain? to check the venturi each annual (or 100 hour) it's better than having a rough running engine.


The "classic" lean RPM problem manifests as: very rough RPMs around 1500 to 2000. Sometimes the only way to get the engine to accelerate through this zone is to pull the carb heat. If any of this "sounds familiar"........that will probably be your issue.

Do check the primer for leakage by blocking it off....I don't think this is your problem. If a primer is leaking the engine will barely run at idle and belches lots of black smoke. We run into this about once a year with our trainers where the students will complain of a "bad running engine"........we "seat" the primer for them and the problem magically dis-appears.
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Re: Marvel Carburetor Troubleshooting

Post by 6643 »

And people complain about Strombergs! ;)
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Re: Marvel Carburetor Troubleshooting

Post by V529 »

6643 wrote: Mon Jul 13, 2020 6:31 pm And people complain about Strombergs! ;)
yer a funny guy.......... 8-)
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Re: Marvel Carburetor Troubleshooting

Post by phil123 »

Send the carb in to Marvel Schabler in Gibsonvile NC for testing (after verifying correct p/n), they can also tell you if the wrong parts were installed ...jets..etc. The carb is nothing to mess with and an unknown from previous owners unless you have receipts and tags. If it test fine then you have other issues. That's what I would do :)
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Re: Marvel Carburetor Troubleshooting

Post by 4004 »

Before you continue on a "witch hunt" and I'm surprised your IA hasn't verified that you have the correct MS carb for your 85. Can do this for yourself by checking the appropriate TCDSs. Reason I say this, I remember listening to our David Lowe give a presentation many years ago when the MS one piece venturi conversion had major issues - David had flown his 120/140 only a short distance and consumed almost all the fuel. Also check with the engine OH people re the carb, etc re documentation etc. Also, I remember David talking about obtaining an STC to put the C150 O-200 carb (10-4894-1 IIRC) on the 85 said it just used a bit more fuel!! Rich like your situation. If you had had your carb number the MS folks may have "nipped the problem in the bud". Major problem with getting the one piece venturi resolved was the nozzle inside the MS - in addition, I remember, "centuries ago" the Borg Warner (MS) guru preaching about carb set ups and that a lot of "black magic" went into it and that a "flow bench" was critical for a proper set up.! 2c FWIW

PS: phil123, just adding a bit of history - your advice is "spot on" in the modern vernacular!
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Re: Marvel Carburetor Troubleshooting

Post by V529 »

Assuming you have the correct p/n for your engine. Even if you have the "right parts" for your carb and your engine, it can still perform poorly. Many, many small continental engine powered aircraft owners have had to revert back to old style nozzles and 2 Piece venturis.

The one piece venturi and the Pepper box nozzle just seem to run on the lean side.

It is possible if you can find a shop that will "work with you" to install different nozzle/venturi combinations that will flow more towards the rich side.

Just realize that the fuel scheduling for many carburetors, not just the Marvel's is:
Rich at idle
leaner towards mid range
Then rich at full power.

The issue is "too lean" at mid range. (see my first paragraph about "many, many") this is no joke, when the "new parts came out", many aircraft couldn't even make it to 2000 rpm.
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Re: Marvel Carburetor Troubleshooting

Post by 4390 »

There are some service bulletins that might be helpful concerning you mixture problems: https://msacarbs.com/technical-data/service-bulletins/
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Re: Marvel Carburetor Troubleshooting

Post by atypicalguy »

OK related issue.

1946 140, metalized wings. c85 stroker, done in 2008 by Don's. In 2012, prior to my ownership, they swapped out the Stromberg for a Marvel. But they used the 0-200 Marvel, which is 4894 I think, instead of the correct 4020 for a c85 - unclear why. (Of course all the motor guts and the case are all 0-200, but Don says it is still a c85 and should be treated as such). Then they applied an STC to it for auto fuel jets. No notation of why this was done either, but STC is from Lowe Aviation in KY.

Ran fine for a couple hundred hours, including the flight from TX to CA when I bought it earlier this year. I put about 40 hours on it learning to fly, pretty much all pattern work, without any issue flying at or near gross with two 200lb guys in it.

Then last week my CFI flew it 20min to meet me without issue, shut it down briefly while I hopped in, and it started fine and taxi out was non-eventful. We skipped the run up because he had just landed, which turned out to be a mistake as it started to miss on the takeoff roll. Pulled the plugs, all very black. #3 is running cold. Compression good. Problem persists across mags, so seems unlikely ignition related. The motor only has a few hundred hours on it and it is a Don's rebuild with Slick mags. Valves look fine. If you lean it all the way out, it gets better, but still does not run right. Intake path is clear.

So two questions:
1) does this carb seem OK for a c85? Don in Alabama says it is the wrong carb for the motor.
2) what on Earth is going on with the carb to make the motor run so fat so suddenly? I don't know if it has been running rich all along, but I suspect perhaps it has. It uses maybe 6gal/hr in cruise. But there was an acute change from CFI landing to takeoff 20 min later which appears to have worsened the problem. Did it eat a venturi? I read about some debacle changing to the one piece, but consensus seems to be to keep the 2 piece and do the 100 hour inspection on it.
3) Should I bother fixing this carb given that it is not the correct one for the motor? Or just replace it with a new one? Would prefer to just repair I guess as it ran fine for a long time, but maybe that is not the correct way to proceed.

Thanks,
Karl in LA
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Re: Marvel Carburetor Troubleshooting

Post by 6643 »

Have someone open the carb up and take a look.

I believe David Lowe has an STC to modify the O-200 carb for use on a C85, and I suspect that is what you have. I am not aware of any carb modifications for MoGas.
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