I am pleased to be able to offer the following resource on Marvel Schebler Carburetors. This page would not be possible without the contributions of GB_MT and JMOR frequent Internet Forum contributors.
Use the quick links here to navigate the entire carburetor section.
Carburetor Teardown and Cleaning
Carburetor Assembly and "The Carb Shack"
The carburetor operates as follows:
The purpose of the carburetor is to atomize the fuel in the air supplied to the motor at the right ratio. Fuel enters the carburetor through the elbow strainer. From there it arrives at the float valve. The float controls the amount of fuel that's allowed to be stored in the float bowl. When the float drops the needle valve opens allowing fuel to enter the bowl. Fuel in the bowl passes through the main/power jet and fills the chamber where the fuel nozzle resides. The main jet meters the rate at which the fuel flows into the main jet cavity. The nozzle with holes in the sides allows extra fuel to be stored in the area so that if the throttle is opened quickly the extra fuel stored is available to quickly enrich the increased air volume for quick acceleration while the main jet meters what can continue to enter. More modern carburetors used accelerator pumps for this purpose.
The mainjet passageway is also the source of fuel for the idle circuit. Fuel at idle is drawn out of the mainjet area through the idle jet. The idle adjustment screw determines how much air is mixed with the fuel. Screwing the adjustment in reduces air creating a richer mix.
The economizer jet plays a role in regulating the pressure/vacuum in the fuel bowl. The vacuum applied to the bowl and resulting air movement changes with relation to the throttle position and also as the throttle position is changed. The pressure/vacuum in the throttle bowl are a factor that affects fuel movement from the bowl through the main jet.
The choke is used to create a richer mixture of fuel sometimes needed on cold starts. This additional fuel mix is required due to the way fuel atomizes in cold temps. The choke plate is used to restrict the amount of air entering the carburetor and increase the suction on the fuel nozzle.The increased suction results in a richer mix. A few degrees in movement in the choke plate makes a big change in the fuel mixture ratio. The spring loaded relief valve is provided to reduce the amount of manipulation that would be required by the operator during use. Due to how much more fuel the choke is capable of delivering, its usually easy to flood an N by using the choke too soon during cranking or using it too long. Many operators find it best to try to start the tractor first with 1/3-1/2 throttle. If it doesn't start try again with light operation of choke as the motor turns over followed by a quick release of the the choke.
In order to service these carburetors you must understand the passageways in the carburetor. These passageways make up specific circuits like the idle circuit, power circuit, and choke circuit.
The following images are color coded so that you can follow them through the carb and see where they connect to one another. The pictures are presented as identification of the passageways and how to get to them for purposes of cleaning. Some of the passageways are as small as ~.030 or #20 - #220 gauge telephone wire. Copper wire can be used to insert in the passageways to verify that they are clear and copper being soft won't damage the carb.
Bowl and upper body passages
Idle and power passages
This picture shows the passages that make hard turns inside the carburetor (blind alley ~110 deg turns) Note the color of the wires so that you can see which ports are connected.
Click here to download "GB in MT and JMOR carb cleaning instruction" pdf
These Carburetors are nearly 70 years old and Marvel Schebler was a carburetor provider to numerous tractor manufacturers. Many of the parts will interchange between different models. In addition, due to the upgrades to the carburetors over the years and different sizes being furnished by providers unless your carb runs correctly you should not assume it has the correct jets in it. The following information is provided as a resource to id the jets used for the TSX 33 and 241.
Here are some existing jets along side of new jets. Note the difference in orifice sizes. There are too many jets that will fit to assume you have the correct ones.
M/S carb jet sizes for 241B overhaul spec's
Used by (GB in MT)
Idle jet..... part#..9N 9596......0.032 orifice
Econo jet.....part#..9N 9914......0.046 orifice
Main jet.....part#..9N 9533......0.043 orifice
Above listed jets are the only jets I use, and I have found them be the smoothest running sizes, for all "N" series tractors, and all M/S TSX carbs.
Image used with permission from Just 8N's
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