SER FAQ: LMFAQ: Basic operation of a float type carburetor

S.E.R. FAQ

NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Basic operation of a float type carburet..
Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

<< Crankshaft anatomy | Index | The four strokes of a fou.. >>

Basic operation of a float type carburetor

The diagram below shows a schematic of a typical float type carburetor with the engine running at high speed. The choke plate is fully open and the throttle plate is opened the proper amount be feedback from the governor to maintain the speed set by the user throttle control.

Air is sucked through the venturi by the intact stroke of the piston. The lower pressure created by the faster moving air draws gasoline up through the main nozzle and into the air stream. This aerosol of gasoline vaporized almost instantly.
The air bleed allows the pressure inside the float chamber to equalize as fuel it drawn off
The float maintains a more or less constant level of liquid gasoline in the float chamber by opening the inlet needle valve when the level drops. The fuel supply is either above the level of the carburetor or there is a separate fuel pump (usually operated by crankcase pressure pulsations or a direct mechanical linkage).
The main nozzle either has precisely machined orifices to set the ratio of air to gasoline in the mixture or has an adjustable needle valve for this purpose (not shown).
Closing the choke forces more gasoline into the mixture and makes it richer.
The throttle plate is used to vary the amount of the air-fuel mixture to the engine but maintains nearly the same ratio of air to fuel over a wide range of engine speeds.
The user throttle (speed) control adjusts the spring force against which the governor must act to close the throttle plate. Increasing the spring force increases the engine speed.
Operation at idle speed depends on a separate system of an idle nozzle and idle speed adjustment (not shown) and does not depend on governor feedback to control the throttle - which is kept open just a small amount.


            Air filter       (Air: -->, Air+fuel: ==>)
          _____________
         /             \  Choke plate               Throttle plate
          | ||||||||| |  (Fully open) Venturi      (Partially open)  
          | ||||||||| |______________    v   _______________________________
          | |||||||||                \______/
          | ||||||||| -> -> -> -> ->           ==> ==>      /  ==>  Intake
          | ||||||||| ->  ----O----  ---> ===> ==> ==>    O    ==>  pipe to 
          |           -> -> -> -> ->  __||__   ==> ==>  / |    ==>  cylinder
           \____________________  ___/  ||  \_____________|__________________
                                ||      ||                |
    Fuel __________ Air bleed ->||      ||<- Main nozzle  o---------+ 
   Inlet ______    |____________||______||__                        |--> Pull
            |  |_ _|                    ||  \       Speed <--/\/\/--+    from
   Needle ->|  __A_______________       ||   |     control   Spring    governor
   Valve    | |o__/              \      ||   |                          closes
            |-----|     Float     |-----||---|                         throttle
            | . . |               | Gas || . |<- Float bowl              plate
            | . . .\_____________/. . . || . |
             \______________________________/

<<Crankshaft anatomy | ToC | The four strokes of a fou..>>