Idle (as in- as slow as it will run with the throttle lever pushed forward) is affected by the idle mixture screw. Any increase in engine speed above maybe 750rpm will be un-affected by the idle mixture screw. The carb runs on the main jet passages and nozzles when in mid rpm ranges. That mixture is adjustable with the load screw (bottom front). A perfect mixture of fuel and air is between 14 and 15 to one ratio. The best a carburetor can do is to attempt to make that happen based on temperature, external air pressure (varies with altitude) and applied vacuum at the discharge ports in the carb. The highest vacuum above the throttle plate is applied at low speed idle. (the throttle plate is closed to its stop, and restricting the air flow. This restricted flow is so small that the main jet has no flow of air past it and no vacuume is applied to the main nozzle. (all the vacuum is on top of the throttle plate, pulling fuel through the idle circuit.) At higher speeds the air flow past the venturi (where the main nozzle is located) causes vacuume there pulling proportioned fuel into the airstream. At full throttle and full load, the throttle plate is wide open, and the fule is pulled from the main jet, and load screw, and discharged out the high speed nozzles in the venturi. Your updraft carb is similar to the ones shown below, but upside down.(makes no difference) yours will not have an accellerator pump, nor a hot idle compensator, nor a power circuit. it is more basic. Your idle richness adjustment bleeds in more air as it is screwed out, and less fuel. Many adjust the opposite way more fuel out and less fuel in. I hope this helps. Jim
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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