How Small Engine Carburetors Works?

From Go-karts, mini bikes to lawnmowers engines, carburetors are used in pretty much all small engines such as the predator 212. A carburetor is used to prepare the air-fuel mixture for combustion that takes place inside the engine.

Even though their function might seem complicated, their design is very simple and easy to understand, in this article we will go through the parts, working mechanisms of the stock carburetors that come along with the go-kart engines such as the predator 212 or the honda Gx200 or clones.

The Mikuni Carburetors used as a performance upgrade Carburetors work a little differently in comparison, you can check this post to understand their workings.

Parts Diagram

Apart from the parts shown in this diagram, your carburetor will also have a pilot jet.

The bowl is not labeled, but it is the outer-lower surface below the float, that holds the fuel. We will go through the functions of all the parts one by one.

How it works

As you might already know, there are small combustions taking place internally in the engine to convert the fuel into mechanical work. But no combustion can take place without the presence of oxygen. So, we need a mixture of both oxygen and fuel to be supplied to the engine for the combustion to take place.

The carburetor is the device through which the fuel gets mixed with the oxygen and the mixture is then passed onto the combustion chamber where the mixture is burned and the energy is released.

So the next question is, how do we create this mixture? We need to get the fuel to come in close contact with the oxygen in a place from which the mixture can be then pushed towards the combustion chamber due to some pressure difference.

So the place at which they come in contact is the Venturi (in the section diagram, the venturi is in between the choke shutter and the throttle shutter)

The air enters the carburetor from the Air filter into the upper half of the carburetor and the fuel comes in from the fuel tank through the gap between the inlet needle and seat and settles in the bowl. (If you’ve ever taken apart your go-kart carb, then the lower outer surface that is screwed into the carburetor is the bowl).

When the engine is running, the air enters from the filter, and the fuel is sucked in from the bowl into the venturi, where they both combine and the mixture is then passed to the combustion chamber (If you have some time in your hand, check this video of the carburetor working from a transparent view)

Choke and throttle valve

The 2 valves in the upper part are called the choke valve, and the throttle valve. The function of the choke is to block the entrance of air when the engine is cold so that the air-fuel mixture is rich and therefore easy to start.

The throttle valve will control the amount of mixture that leaves the carb and is directly related to the rpm of the engine. When the throttle valve is fully open, the rpm is high and when the throttle valve is closed, the Engine rpm is low.

In idle conditions, the throttle valve will be semi or fully closed, at fully closed, the mixtures are still able to form due to the presence of other small passages for the air to flow through,

There is usually an idle/pilot screw and an idle/pilot jet in the carburetor, the idle screw will determine how much the throttle valve is open in the idle condition and the idle jet will control the amount of fuel for the mixture in the idle condition.

In many situations, you’ll need to adjust the idle screw, for example when your go-kart clutch or torque converter takes off at idle condition, then it’s possible that the idle speed is set too high. For such situations, you’ll have to adjust the idle screw (it’s located between the 2 valves).

If you move the screw further inside, the idle rpm will increase, if you loosen that screw, the idle rpm will decrease.

The place between the 2 valves has a dip in cross-section called the venturi, this is where the fuel and air combine. Due to the smaller cross-section, there is low pressure in this region. If you want to learn more about their working then check this article.

Bowl and Float and inlet needle

The bowl is the part that holds the fuel on the carburetor and when you remove the bowl you’ll be able to see the float, which is a hollow cylinder usually made of plastic, metal, or fuel-resistant foam.

The float is connected with the inlet needle and the seat, as you can see from the lower half diagram:

The fuel comes in from the fuel tank through the space between the inlet needle and the seat and starts filling up the bowl.

As the bowl is filling up with the fuel, the floating part starts to float on the fuel, and after a certain amount of oil is filled in the bowl, the float will be pushed upwards due to the fuel along with the inlet needle which will fit in the seat, blocking any further incoming fuel.

The system is build so that the carburetor only takes the amount of fuel necessary for operation.

Main Jet and the emulsion tube

The fuel needs a passage to go from the bowl to the venturi and this passage is provided by the main jet and the distribution tube.

When you open up the carburetor from the bottom end, you’ll find a bolt that is holding the bowl to a cylindrical pipe inside, when you look inside that pipe, you’ll find the main jet with a hole and the distribution tube, which is a long cylindrical pipe with several holes on its side.

So the fuel goes in from the main jet into the emulsion tube and then into the venturi where it mixes with the air.

But the fuel has to get from the bowl to the main jet, for this purpose, some carburetors have 2 fixed holes in the cylindrical pipe (the pipe that holds the bolt and has the main jet inside of it), some carburetors have holes in the bolts (the bolts that hold to bowl to the tube) for the fuel to pass.

When the throttle opens, the main jet activates and the fuel starts getting sucked upwards. As it is passing through the emulsion tube, the fuel mixes with the air from the holes in the emulsion tube. Yes, the fuel is emulsified with some air even before it enters the venturi. In the venturi, this emulsifiers fuel is atomized.

Summary

Fuel enters the bowl from the fuel tank through the fuel inlet
The bowl starts filling up and the float starts floating
After sufficient fuel the inlet closes
The fuel gets sucked in through the main jet and distribution tube into the venturi
Fuel and air combines
The mixture is sent to the combustion chamber for combustion.