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Marvel Schebler Troubleshooting

Flooding

Seems it is flooding (getting too much fuel).
Check the pressure of the fuel pump assuming you have one. New pumps are especially suspect. Some have gravity fed carburetors, but they usually have a tendency to run lean, not rich.
Fuel could be leaking around the seat. Check carefully for any cracks. Make sure you installed the gasket. Also check to make sure the old gasket wasn’t left.
Make sure he float isn’t hanging up somehow and not closing the needle.
Be sure not to put pressure on the needle when adjusting the float. The viton tip can easily be damaged.
Clean the viton tip with mineral spirits by wiping off with a paper towel until you don’t get any black on the towel.

Message from customer: It,s a John Deere  1020 tractor.  Looking at your carburetor commentary the common problem with gas running out of it , is often dirt from the gas.
I blew out the fuel intake line with air and it started.
Thanks for a good site

Marvel Schebler Parts


Runs out of power at high RPM1, a week coil will fail under heavy load, but would continue to fire if the mixture is richer (choke out)2, he should clean it again, and run a wire or small drill bit through ALL the passages and jets. sometimes that hard build up ofdeposits will stick in the small passages and wont come out with compressed air. Of coarse you know this would cause a lean mixture and a stall or backfire3, some times when we rebuild a carb, dirt can get in the fuel inlet or break loose from in the fuel line and plug up the newly rebuilt carb.I have had this happen a few times.also have him check point gap and distributor shaft for play. at 1300 rpm the advance is starting to rotate and may pull the points away from the cam, or pull the cam away from the points if the bushings are shot.

Can’t get the jets to come out. Seem frozen

Use a screw driver that fills the jet slot as full as possible. Tap the screw driver a few times. This may knock the jets loose.
Heat the outside around the jet.
Using a small drill, drill out the the jet increasing the drill size until it is just shy of the overall diameter. Most of the time the jet will wrap around the bit and come out. Keep in mind that not all Marvel Schebler carburetors use a jet.


My Ferguson tractor starts really easy; but only runs about 3 mins. I have replaced all the electrical parts, cleaned the carburetor, checked the lines and cleaned the tank. It still starts easily; but only runs about 3 mins.


My thoughts on this….the 2 electrical things that can heat up an short out are the coil and the condenser. Even new ones. I would run it until it dies then check for spark, if that’s ok then remove carb drain plug and see if there is fuel in the carb. (may be a restriction in the shut off or fuel bowl assy. only allowing a trickle of fuel
Is there a jet that the main adjustment rod fits into? Referring to the 3″ or so adjustment needle that is used on the B type.

There is a brass jet that is pressed into the bottom of the carburetor. These are not removed normally and are not available.


Doesn’t Run Without the Choke Being On

Either there is a vacuum leak, or it isn’t getting enough fuel.

For the air leak spray carburetor cleaner around the base of the carburetor and the intake manifold. If the RPM picks up, then you found the problem.

Check for a worn throttle shaft. You can spray some carb cleaner around the shaft ends while running. If the RPM changes, then you found the problem.

Marvel Schebler carburetors that use a gravity fed fuel system are having problems with the fuel of today. The needle sticks on the float valve and there isn’t enough pressure from the gas to open it. If you have the type of needle that includes a clip to connect the needle to the float then be sure to use it. When the float drops it will force the needle out of the seat.

Wipe the needle viton tip with mineral spirits. That might help it from sticking.

Be sure the power adjustment needle isn’t turned all the way in.

Any filter between the tank and carburetor should be changed. 

Make sure the venturi is in the right side up. The rounded lip would be up.

That is the weep hole for the throat — gasoline dripping into the throat after shut-down (or when the carb floods) can drip out of it. If it leaks a tiny bit upon shut-down, that is normal for an updraft-carbureted engine; if it leaks a lot all of he time, that is an overfilling bowl. 

Usually, there was a sintered metal “plug” pressed into the large diameter of that weep hole which would allow gas to drip out but prevent most debris from being sucked in; it is missing on this carburetor. You can stuff a piece of felt in there and it will do the same thing, but you’ll have to figure out how to keep it there. It isn’t uncommon for a Marvel Schebler to have gas drip out of the air horn while sitting around. This is due to the weak Marvel Schebler design. To eliminate this problem many installations have a shut off valve installed on the inlet fuel line. This allows the fuel to be shut off when not in use. 

Seems To Run Out Of Gas

Here is a problem we hear a lot about the Marvel Schebler carburetor.

I have a marvel schleber carburetor. It would take throttle then start to run out of gas. I rebuilt it, but now it will run for 3-5 minutes and cut out. Take it back to idle 30 seconds and it will recover and go back to full throttle.

I checked the float it is at 1/4 in from the gasket the bowl was 1/2 full where do I go from here.

If you have a gravity fed fuel system, then the needle may not be opening as it should. It’s been a problem with the Marvel Schebler and the lousy gas we now have.

Wipe the viton tip on the needle with mineral spirits.

Check the viton tip for any scoring. This happens when the float is adjusted and too much pressure is put on the needle. Scoring will cause the needle to stick.

Make sure there are no kinks or restrictions in the fuel line leading to the carburetor.

Is there a clip that attaches the needle to the float? That helps pull the needle out when the float drops.First of all, I would suggest checking the gas tank vent and be certain that it is fully open.
I would also suggest that, if the vehicle in question (tractor?) has a fuel pump, he ensures that it is pumping consistently.
If this carb is on a piece of equipment with the fuel tank located above the engine, I suggest that he check to see if the tank is getting hot (I once saw gasoline literally boiling in the tank of a tractor which was running hot — and this was long before the advent of ethanol!)
Also, he needs to be certain that his fuel filter is clean and that the pipe coming out of the fuel tank to the filter or pump is not corroded inside or otherwise restricted. I had this corrosion situation on my own tractor some years back and it just about drove me nuts before I found it; his symptoms sound almost identical.

Other than that, just poke a wire through all of the various jets and orifices in the carburetor, and follow with some compressed air if available . . . but the problem really does sound more like a function of fuel delivery to me.


Flooding

Flooding simply means you are getting more gas in the carburetor than needed. One of the most common problems is the needle & seat. A bad needle (viton tip) can cause gas to leak into the bowl even when closed. New needle – wipe the viton tip with mineral spirits. For gravity flow type of fuel systems you may need to give the needle & seat time to seat. This can take 10 or more starts of the engine. Check the float to make sure there isn’t any gas inside. If so, then replace the float. Check the float carefully to be sure it is centered and not hanging up. I do this by pulling the float up and down manually and feel for any bumps. A worn float pin could cause this.

I rebuilt the carb because of fuel leaking from air horn after sitting several days, still have the same problem.

Did you replace the float? If not then test the brass float by heating up water just prior to boiling. Immerse the float and watch for any bubbles. As the float heats up inside it will expand and force air out of any hole.

If the fuel is running out when turned off, then you may need the fuel shutoff found on many of the tractors that use the Marvel Schebler.

It is possible the fuel pump is putting out too much pressure assuming you have a fuel pump and it is not gravity flow. You can test this by rigging up a gravity flow tank which would eliminate the pump. New fuel pumps are especially suspect. The import pumps have not been very good.

When Parked on a Hill Carburetor Stumbles
Is the level of the fuel tank about the same when this happens? If gravity fed then look for fuel filters. I once had a customer find that the screen at the bottom of the fuel tank was plugged. Test this by getting on the hill which causes the problem, then disconnect the fuel line to see fuel comes out strong.
It’s possible the fuel is slopping over the float bowl which could be due to the wrong fuel bowl gasket. To test this remove the carb and fill with fuel then tip it the same as if it is on the hill. Does gas leak out of the carburetor?

Updated on 10/01/2021

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