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Thread: How a carb works...

  1. #1
    Join Date
    Aug 2008
    Brownstown, MI

    Post How a carb works...

    The carburetor is probably the most misunderstood mechanicalpart on a motorcycle. This article was written by John Glimmerveen and explainsthe internal workings of the carburetor to set the correct ratio of fuel andair for optimum engine performance.
    All engines require a proper mixture of air and fuel forcombustion. The carburetor controls the ratio of the fuel/air mixture enteringthe engine. This sounds simple enough, but there are many carburetor partsthat, if not set properly, will at best cause the bike to run badly, or atworst keep it from running at all.

    How It Works. Air enters the carburetor from the air intakeand speeds up drastically due to the narrowing of the interior walls of thecarburetor. This air is blowing perpendicular to the throttle slide, a valvecontrolled from the throttle cable. When the throttle is opened, the cableraises the throttle slide located inside the carburetor’s main body. As theslide rises, the fast moving air pulls the fuel up the main jet from the floatchamber.

    This works automatically because the fuel wants to travelfrom an area of high pressure (the float chamber) to an area of low pressure(the carburetor main body). The fuel mixes with air and heads into the engine.The amount of fuel that flows is dependent on the position and size of theneedle valve (explained below), the size of the main jet, and the height levelof the fuel in the float chamber. The fuel height in the float chamber iscontrolled by the floats. Adjusting these floats is explained at the followingpage.
    Inside the Float Chamber. Looking more specifically at thefloat chamber, you will find the float. This simple device controls the amountof fuel that enters the float chamber, restricting or cutting off the supply byits settings. The Float Chamber Components are: Needle valve assembly, floats,float pivot rod, Chamber and gasket, Drain plug.

    The float chamber is a reservoir for fuel, and contains allof the working components of the float. Most float chambers are fixed to thebase of the carburetor, but some early machines used a remote system where thechamber was located some distance away from the main carburetor body. Mostfloat chambers have a drain fitted for maintenance and, in some cases, tomeasure the actual fuel height.

    The Float and Needle. The float chamber actually workssimilarly to a toilet tank. The floats essentially “float” on the fuel in thefloat chamber. The floats pivot on a rod and, via the tang, open or close theneedle valve, causing fuel to enter or not enter the chamber. When fuel isdrawn up the main jet, the fuel level in the chamber drops, thus the float alsodrops. This opens the needle valve allowing more fuel to enter the chamber.When the float again rises with the fuel level, the needle valve will close offthe fuel supply.

    Needle Valve. With a spring-loaded clip at one end and atapered rubber tip at the other, the needle valve works in unison with thefloat. The needle valve also works with the seat assembly, which is simply ascrew-in brass bolt – drilled to take the shape of the rubber tipped needle.The seat assembly is the end of the fuel line, where fuel is waiting to enterthe chamber. When the chamber is full, the rubber-tipped needle is pressed intothe seat, preventing fuel from overflowing the chamber. (Note: Different sizedneedles and seat assemblies are available for specialty requirements likeracing and altitude corrections, the fuel requirement is lower at higheraltitudes due to a lower atmospheric pressure.)

    Setting the Float Height. You want to set the float heightso the bike runs on the exact amount of fuel it needs – no more, no less. Toset the float height, it is first necessary to determine the current settings.This can be done in two ways: by using an external measuring pipe, or bymeasuring the physical height of the float above the gasket face.

    External Measuring Pipe Method. This method of determiningthe actual fuel level (against the float height level) uses an adapter thatfits into the drain hole of the float chamber. A clear fuel line withgraduations is fitted to the adapter. Once the fuel is turned on, fuel flows upthe pipe until the needle valve assembly closes the supply; the fuel height inthe tube corresponds to the fuel height inside the float chamber.

    (Note: Some carburetorshave a drilled protrusion on the base of the float chamber to which the clearfuel pipe can be attached for fuel height measurements.)

    Physical Height Measurement Method. When using this methodto check float height, first remove the float chamber bowl from the maincarburetor body. With the chamber removed, tip the carb onto its side, in thesame position it takes while fitted to the bike. Now blow into the fuel supplypipe, while simultaneously lifting the floats slowly until the air flow stops.This is the measuring point for the specified float height. The distance tomeasure is available from the manufacturer and can also be found in all goodshop manuals (double-check any distances quoted in forums on the internet). Thedistance is typically measured from the float chamber face – or gasket surface– to the highest point of the floats. If the floats are not at the rightheight, the tang on the floats should be bent toward the needle valve for lessfuel to be allowed in (the bike runs leaner) or, conversely, away from thevalve to increase the fuel level (bike runs richer).

    Symptoms of Incorrect Float Heights. In an extreme case, ifthe floats are set too high, fuel will overflow via drillings inside the carbbody. In addition, fuel may flow into the engine unrestricted, which, if theengine is not running, can cause hydraulic lock – that is, as the piston riseson the compression stroke it cannot compress the fuel. If fuel is leaking fromthe carb, it can potentially cause a serious problem – fire. If the fuel heightis too high but the bike is running, the engine will have a tendency to displaya rich running condition, which will make the throttle response slow and theengine note muffled. This condition is generally accompanied by a strong smellof unburnt fuel from the muffler. If the fuel height is too low, the enginewill display a lean running condition, where the engine typically hesitatesbefore accelerating or surges as the throttle is opened. The bike may alsomisfire when the throttle is closed.
    Last edited by Gas Man; 01-08-2015 at 10:05 AM.
    ^^My $0.02 not yours^^
    aka Gas Man

    "Why pay somebody else to fuck up your bike?"
    "Custom don't bolt on!"

  2. #2
    Join Date
    Dec 2009


    Nice write up!

  3. #3


    Be right back , I need to ask google what a "carburetor" is

  4. #4
    Join Date
    Oct 2009
    Marine City, MI


    what is google?

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