Mixing it up: Carburetor service

Three things need to happen to gasoline to allow for combustion in an engine. It needs to be broken down into small particles (atomize), mixed with air (emulsify), and change phase from a liquid to a gaseous (rarefied) 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 degrees 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 an excellent job atomizing and emulsifying the gasoline. It is your responsibility to keep it in tip-top shape, so that happens.

How it works

A carburetor works via the pressure differential in the cylinder bore and the atmosphere.

The piston creates a vacuum (pressure less than atmospheric) by moving downward. During this time, atmospheric pressure is acting on the fuel in the carburetor float bowl. It pushes the gasoline (or pulls it depending on the way you look at it) through the carburetor’s circuits.

For this reason, an updraft carburetor works —- it is not magic; it is just a pressure differential.

The low pressure created in the carburetor venturi is called a signal. For the signal to be strong (efficient atomization and emulsification), the carburetor and intake manifold must be tight and have not even the slightest air leak.

So, the first step is to make sure all the fasteners that hold the carburetor and the intake manifold to the cylinder head are tight.

Check for deteriorated or soaked gaskets that can cause a slight leak.

The best way to check for an air leak is with a propane enrichment tool.

The carburetor must be 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 will suck the dirt out of the bleed. It may take a few tries, though.

The float level in the carburetor needs to be correct, or it will never run properly or 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 too high, it can flood quickly or be 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 we call “having the change lost inside.”

With the engine off, gently seat the mixture screw and count the number of turns. Then remove it and 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. I like to gently polish the needle of the mixture screw with fine Scotchbrite. Do NOT use sandpaper. Clean with carburetor spray when done. Reinstall the same number of turns from seated as before.

For further information listen to the Hot Rod Farmer’s podcast: Let’s keep it carbureted!

Know your carburetor

The farm tractor carburetor was extremely adjustable, but you need to identify and understand how to work with each circuit.

In contrast, a car or truck carburetor was not as tuneable.

For example, a Marvel-Schebler Model TSX-697 used on the Case “350” Series Tractors had the following external adjustments: idle speed screw, idle mixture jet, power jet, and throttle rod. Thus, it is best to have access to an original service manual that identifies each adjustment point for the carburetor on your tractor.

You may have the idle mixture set correctly, but the power jet may be misadjusted. The engine will then idle fine but run poorly as the throttle is opened or the tractor is loaded.

Part 3 will explore how to bring the tune together.