Carburetor basics

By Dave Mowitz

The most important thing happening in your carburetor is the Venturi Principle. It says, "air or liquid (or atomized liquid, like the fuel/air mix) that travels through a constriction (that's the smooth, often middle, part of the carb) increases in speed and decreases in pressure."

That's basically how a carburetor gets fuel into the cylinder. The carburetor sits between the air filter and the intake side of the engine. It reacts to pressure changes in the cylinder, and to input from the throttle. The pistons' downstroke during intake causes a negative pressure that starts airflow from the filter, through the carburetor and into the cylinder's intake. This airflow through the carb causes a pressure drop. With a higher pressure now present in the float bowl (where the fuel sits at the base of carburetor), fuel is pushed through a jet and into a venturi. How much fuel is pushed? That depends on the size of the jet and the pressure difference.

A carburetor consists of lots of little parts, with little passages, sitting in volatile chemicals, often working in dirty conditions. But its not so much dirt as it is old fuel that causes carbs the most troubles. Gas sitting in the float bowl for long periods will change composition often referred to as goo and varnish. Actually, goo isn't the scientific name, but is a great description of gas that has started to solidify. Varnish, on the other hand, is an almost scientific term describing the way old gas coats parts. No matter what you call it, old gas is bad. Really, really old gas . . . the kind of stuff that has been sitting in a tractor for years . . . is really, really, really bad stuff.

If you discover old fuel in a tractor, just drain it. The stuff doesn't even make a good solvent. And be sure to drain the entire fuel system - tank, lines and carburetor. And if you are planning to leave fuel in a tank, say, over the winter make sure it is treated with a stablizing additive. This stuff is cheap insurance again varnish build up. For really gummed up carburetor, and fuel systems for that matter, you are going to have to use a carburetor and fuel system cleaner to scrub down the tractor's fuel system.

Now if you want to learn about how a carburetor works, take one apart sometime. Oh, and get that particular carburetor's service guide to assist in this learning processing. I'll guarantee you that by the time you have taken the carb entirely apart, the whole time referring to the service manual, you will have a far better understanding on how all carburetors work. But be prepared to encounter an army of tiny parts.

Carbs are a minature world of parts. There are tiny tubes called jets. And there are springs, metal tubes, needles and clips, slides and/or butterfly valves connected to the throttle cable. All these parts combine to inject fuel into the air flow going into the top of the engine.

First fuel flows into the bowl at the base of the carburetor. A "float" in this "bowl" controls the amount of fuel used. In operation when the pressure drops in the engine's cylinder, a higher pressure in the float bowl pushes fuel out.

At idle and low-engine rpm, the fuel first moves through the "pilot jet" which is sometime also referred to as the slow or idle jet.

When fuel is pushed through the pilot jet and into the carb's venturi (passage way through which air flows through the carburetor) fuel begins to mix with the flow of air. This air-fuel mixture is carried to each cylinder entering when the intake valve opens.

When you give the engine some throttle, you're opening or closing down the choke plate inside the carb's venturi.

Opening the choke plate allows allows more air to travel through the venturi. This in turn causes more fuel to be pushed out of the bowl as the pressure difference between the venturi and the bowl increases with higher engine rpm.

At higher operating speeds the engine needs more fuel and certainly more fuel than the pilot jet can deliver into the venturi.

This is where the needle jet comes in to supply more fuel. This jet features a needle which moves in and out as the slide moves down and up with the movement of the throttle. This needle is tapered so it gradually lets more fuel through as it moves out of the jet.

The depth of the needle can be adjusted for different fuel delivery rates in this engine operating speed range. When you are reaching three-quarters to full throttle the carburetor employs the main jet which resembles a short and fat brass tube that is often connected to the needle jet. This is the fuel hog of the system capable of keeping up with an engine's needs.

Of course not all carburetors are the same as the system described above. Still this gives you a basic understanding of carburetors.

As I said before, go find an old junked carburetor and take it apart to really get a feel for this mechanism's operation. This makes for a great winter job which, if performed on the kitchen table, will create in a pile of small parts scattered across the table and often on the floor. True, this is sure to cause a reaction from your wife. Try NOT to anger your wife. Instead, the desired affect is to frustrate your wife to the point that she asks why you are not taking that "thingamabob" apart in your shop.

"My shop is not heated, sweetness, otherwise I would be in there this very moment and not messing up your table or floor," you respond. Thus, a heating system is justified.

Now if I could find a similar way to justify a new set of Snap-On tools.