Marvel Schebler Carb

B

Bob Turner

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Apr 4, 2018
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I use Strombergs, and am completely set up for cleaning, calibration, etc., but have no in depth experience with the M-S.

Are there any “gotchas” when doing something as simple as a fuel strainer check? I would just disconnect the fuel line and back out the nut, expecting a cylindrical screen. I have heard that some applications have an in-line fuel filter as well, but the Citabria Maintenance Manual does not mention any additional filters. I will clean the gascolator, of course.

I did have an M-S come apart on me during an epic cross-country, but all that took was a screwdriver, after three hours of messing around in the hot desert sun.
 
Thank you. Perfect. Only unexpected thing is that gasket with bendable lock.

Well, I expected a more complex carburetor, but this one seems to have more parts than a Ford four barrel! I have successfully overhauled one of those.

No wonder the shop just ordered a replacement for the local M-S equipped Cub.
 
You are right. Easier than the Stromberg!

Gascolator and carb strainer both pristine clean! Looks brand new in there.

Maybe you can aim me more precisely - brought the airplane from Denver - 160 Lyc in a 7ECA. Flawless trip. Starts right up, doesn’t burn oil, 6.3 gph. Climbs at full power to 11,000 feet to cross Angel Fire.

Sea level, half tanks - first takeoff - at 200’ I lost 200 rpm just long enough to consider landing, then it went right back to full power. I did another circuit - same thing.

Since I have forced landing contingencies completely mapped out, and actually practice them, I decided to continue. Six circuits - always the same. At 200’ I get a 200 rpm drop lasting less than three seconds.

Remember, this was a first attempt at field elevations of less than 3500’.

Suggestions?
 
Checked both screens, isolated the primer, test-flew. Same deal. But if I restrict the RPM to 2400 it doesn't do it. Peculiar, because it happens at 300', lasts less than two seconds, and then it runs at full power all the way up to 1000' agl.

Only thoughts I can come up with are trash in the on/off valve, a flap in the flex line to the carb, or a carb set up for high altitude airports. I rather doubt the last one - you set these things up standard, then pull the mixture for best power at Denver. I am going to fill the tanks tomorrow, for more head pressure, and try it.
 
It's a fuel flow over time issue? 🤔 thinking here....

Have you tried throttling back at say 1000 ft for a minute, then go into a full power climb again? See if the problem comes back? Continue until you have a low enough DA that it stops doing it.....

Being a 7ECA, it came with a O200 or O235, I don't remember, are the fuel line size to the gascolator the same as a 160hp?

Is the carb jet inside the right size for a 160 HP? Being from Denver. They may have stuck a 150 HP jet in and it never showed its face due to lower DA.

When I bought my 160hp GCBC, I had fuel starvation issues and high CHTs in the cold winter, turned out it had a 150hp jet in there.
 
That might well be - I have questions about the carb. But why would the 160 jet be different - it burns less fuel than the 150 by a bunch! I was thinking improper float level, and not by much.
 
There's a formula for fuel flow per horsepower. Not sure what it is off the top of my head. I do remember I'm looking for 14.7-14.8 gph at redline rpm full throttle for the 160hp.

I'm not a full on carburator guy, but there are different jets.

How I figured out mine was set wrong was rpm would RISE and the engine would run smoother when carb heat was applied at cruise thru full throttle settings. This was before I had fuel flow.
 
May I take a shot at this? This is only a guess but what if the needle valve is sticking just a little, on the ground the angle of the carb float I think might be lowere then level flight. So as you’re rolling the tail comes up, now the float might be a little higher causing the needle valve to closed slightly tighter than normal causing it to stick ever so slightly. So in the initial climb fuel flow is reduced, as the fuel is used the level in carb gets low but just till the valve opens again. Which might be the stumble you experience. If you’re into the carb I think I would change the float(unless it’s metal) and the needle valve when you drop the bowel you can see if there is any trash. Best of luck in your quest!
 
Thanks. Yes, the carb is suspect. Since it is not my aircraft, the solution, once the flow tests are done, is a rebuilt carb from Spruce.
We did that on a Marvel-equipped J3 - cost a grand, I think - solved a whole bunch of problems.
 
Nah-hah! My spies tell me that symptoms just like this have been traced to earlier O-320 sumps. More as I get more info. See my thread on fuel flow at supercubdotorg.
 
Found your thread on SC dot org. Never heard of the sump being a problem, but I wouldn't rule it out.

On another project, I had a customer with a super cub that the engine would stumble on go arounds. Long story, but I found that the mechanics that installed the engine 1.5 yrs prior, had sqeezed the intake tube clamps so tight as to crush the tubes on the sump..... all of them.

Looks like lycoming is looking for a min fuel flow of around 13.25 GPH according to their chart
 
Thanks! We decided to have a real pro look at it. If he can’t get to it this month, I will do the flow test. Appreciate your #s.

Turns out the Robinson uses the engine in a horizontal mount, so the sump may not be our problem. That is, the sump probably came from the factory with the engine. But check out the photos in the thread - dramatically different.

I think our “pro” has a way of checking the float level without disassembling the carb. I have no idea how to do that.
 
Bob, if you scroll down to page 13 in the file in the link I send you (post #2) you can see how, you could do that yourself as well...
 
How neat is that? Thank you. I guess it should have been obvious, but it wasn’t.

I guess I could review the math and set up head pressure for .4 and 6 psi. Hopefully the mechanic has all that stuff in a kit.
 
Turns out the wrong carburetor was installed. Put the correct carburetor on, adjusted it, and now it is like a brand new aircraft.
 
Happy you found it! Makes you wonder however such a wrong type carb managed to climb onboard somewhere in history, doesn't it?
Probably with a complete engine change and nobody noticed the carb type...?
 
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