How Motorcycle Carburetors Work

How Motorcycle Carburetors Work

If you’re new to motorcycle repair and maintenance, carburetors can sometimes be the most frustrating part. Cleaning carbs is tedious enough, and tuning them only gets more complex the more carbs you have and the further you move away from a completely stock setup.

Motorcycle carburetors work through a venturi effect. Air enters the engine through the large hole in the back of the carb and creates a low pressure system or vacuum as airflow increases.

The vacuum moves around the carb and gas is pulled through different jets and is vaporized as it is pulled into the combustion chamber.

Motorcycle Carbs Exploded View

Different jets and air passages in the carb are controlled by different throttle positions. At certain throttle positions, some of the systems will overlap. The breakdowns below serve as a rough guide of what each component controls:

  • The idle jet system controls about 0 to 25% throttle opening.
  • The throttle valves controls about 0 to 35% of throttle opening.
  • The needle jet and jet needle system control about 15 to 80% of throttle opening.
  • The main jet controls from about 60 to 100% throttle opening.

Components of a Motorcycle Carburetor

Each part and system of your motorcycle’s carburetor has a specific responsibility. After awhile, you’ll be able to diagnose symptoms of your carbs and know exactly where to look first to fix the problem.

Fuel Levels & Carb Floats

The fuel levels in your motorcycle’s carbs are controlled by the floats and the float valve/needle seat.

Carb floats are made of material that floats in gas. Floats are typically made out of plastic, cork, or hollow brass. Hollow floats can form pinholes and fail – these can usually be replaced with plastic floats from later models.

The floats are connected to a tab that presses against the float needle, which rests in the needle seat. The tab is typically bendable so the fuel level can be adjusted up or down.

Carb Floats

Motorcycle Carb Floats

As the gas level changes, the tab connected to the floats will either lift or depress the needle in its seat and either stop or allow fuel to flow into the float bowls.

If the fuel level is set too high, fuel can either leak out of the carbs or into your crankcases.

Incorrect fuel levels can also cause rich or lean running conditions. Too low a fuel level can cause a lean mixture. Too high a fuel level can cause a rich mixture.

Float Needle & Needle Seat

The needle seat and float valve control the fuel entering the float bowl. The needle seat typically screws into the carb body with a thin washer, and the float needle rests in the needle seat between the carb float tangs.

The float needle can wear out over time and particles can get between it and the seat. Both situations can cause fuel to overflow the carbs. If there is a particle lodged in the seat, you can sometimes fix an overflow by gently tapping on the carburetor with a plastic handle of a screwdriver.

Float Needle & Needle Seat

Carburetor Float Valves

Pilot/Idle Jet System

The idle jet system is responsible for about 0 to 25% throttle positions. The idle circuit consists of an idle jet, a screw that adjust the air mixture, and a screw that adjust the idle position.

Channels in the carb run from the idle jets and air screws and exit just in front of the throttle plate. As the vacuum increases, the control moves to the needle jet.

Throttle Slide/Throttle Valve

The throttle slide typically has a flat side and an angled side. The throttle slides control the fuel/air mixture from idle to about 35% open throttle in conjunction with the idle circuit.

Most throttle slides or throttle valves have a flat side and a cut out/angled side. The cut side faces the air cleaner inlet and affects the fuel mixture in the carb.

You probably do not need to worry about changing your throttle slides unless they are severely damaged.

Needle Jet

The needle jet sits in the center of the carb above the main jet, controlling about 15 to 60% throttle opening. The jet needle enters the top of the needle jet and the main jet typically screws into the bottom.

Jet Needle

The jet needle is found in the center of the throttle slide. Different carbs have needles with different tapers.

Some have grooves near the top which will allow you to change the clip position. Moving the clip up effectively moves the needle down and will lean out the mixture from about 20 to 80% open throttle. Moving the clip down will raise where the needle sits and make the fuel/air mix richer.

If you have a jet needle with only one clip position, you can usually replace it with a jet needle with multiple positions to allow for adjustment.

Jet Needle & Clip

Carburetor Needles

Main Jet

The main jet is one of the largest jets in the carb and controls the fuel mixture from about 60% to wide open throttle.

Main jets screw into the bottom of the needle jet and control from about 65% to wide open throttle.

Main jets are easy to swap out and test.

Choke/Enrichment Systems

When your motorcycle engine is cold, it cannot vaporize fuel well. To compensate for this, carburetors are built with systems that work to enrichen the mixture at start up, effectively adding more gas so that it can vaporize and ignite in the combustion chamber.

A guillotine or plate style choke mechanism are a common application on many old motorcycle carbs. Other carbs include enrichment circuits that work with float bowl plunger levers.