Tune-Up Tips: The Carburetor
Three things need to happen to gasoline for there to be combustion. It needs to break down into small particles (atomize), mix with air (emulsify), and change phase from a liquid to a gaseous state (vaporize).
The job of the carburetor is to do the first two. The phase change occurs through the latent heat of vaporization. Gasoline needs heat to vaporize. At 60°F., only 50% of the fuel makes this change.
The key to a properly running engine is an efficient phase change of the fuel. This can only occur if the carburetor does a good job atomizing and emulsifying the gasoline. It is your responsibility to keep it in tip-top shape so that happens.
A carburetor works via the pressure differential in the cylinder bore and the atmosphere. The piston creates a vacuum (pressure less than atmospheric pressure) by moving downward. During this time, atmospheric pressure is acting on the fuel in the carburetor float bowl and pushing the gasoline (or pulling it, depending on the way you look at it) through the circuits of the carburetor. It is for this reason an updraft carburetor works. It’s not magic, it’s just a pressure differential.
Airtight carburetor and manifold are key
The low pressure created in the carburetor venturi is called a signal. For the signal to be strong (efficient fuel atomization and emulsification), the carburetor and intake manifold must be tight and not have even the slightest air leak.
So the first step is to make sure all of the fasteners that hold the carburetor together and the intake manifold and cylinder head are tight. Check for deteriorated or soaked gaskets that can cause a slight leak. The best way to check for a slight air leak is by using a propane enrichment tool.
It is extremely important that the carburetor is kept spotlessly clean inside and out. This is easily accomplished with a spray product. The carburetor has tiny orifices that are called air bleeds. These are used to emulsify the gasoline. If they get dirty with varnish, they can be blocked or made smaller in size and be rendered ineffective. If you ever have an engine that suddenly refuses to idle, it probably has a clogged air bleed.
A quick in-the-field remedy is to race the engine and, at the same time, choke the air intake by hand or with the choke plate. Do not let it stall. This often sucks the dirt out of the bleed. It may take a few tries, though.
Float settings and dirty air bleeds
The float level in the carburetor needs to be correct or it will never run properly, let alone at its best. If it is too low, the engine will have an off-idle stumble and be lean from that point to full load. If it’s too high, it can flood easily or be very rich and load up right off idle. The most common issues with a tractor carburetor are the float setting and dirty air bleeds. It will result in a carburetor that is unresponsive to adjustment, or what I call, “having the change lost inside.”
With the engine off, gently seat the mixture screw and count the number of turns. Remove it and then spray carburetor cleaner into the empty passage using the tube on the can.
Inspect the mixture screw for dirt and pitting and the spring for tension. Gently polish the needle of the mixture screw with a fine Scotchbrite. Do not use sandpaper. Clean the needle with carburetor spray when done. Reinstall the mixture screw the same number of turns from seated as before. It will have to be adjusted later, though, which will be discussed in part three of this series.
The tractor carburetor is extremely adjustable, but you need to identify and understand how to work with each circuit. In contrast, a car or truck carburetor is not as tunable.
For example, a Marvel-Schebler model TSX-697 used on Case 350 series tractors has the following external adjustments: idle speed screw, idle mixture jet, power jet, and throttle rod. That’s why it is best to have access to an original service manual that identifies each adjustment point for the carburetor on your tractor(s).
You can have the idle mixture set properly, but if the power jet (which is a load-based fuel-enrichment circuit) is not correct, the engine will idle fine, but it will run poorly as the throttle is opened or the tractor is loaded.