Simply put, when the throttle is open and airflow passes through the carburetor, it passes through a constriction in the throat of the carburetor; we refer to this restriction as the venturi. The venturi sees a speed increase in the airflow and a pressure drop. A fuel port, or jet, is located in this low pressure area. The faster the airflow, the greater the pressure drop, the greater fuel flow that is drawn into the airflow.
So far, so good.
Close the throttle. Very little air flowing through the venturi. The engine is a suction pump, and the throttle is like your hand over the hose of a vacum cleaner. What does the vacum cleaner do when you cap your hand over the hose? The pressure drops in the hose even more (like manifold pressure does when you close the throttle, right??), and the engine strains to get more air.
Same for your engine. The engine still gets fuel, or it wouldn't run, but it's not getting the fuel through the main fuel jet. There's no air flowing past that jet to draw the fuel out. Your mixture controls that jet...your mixture isn't doing anything because that jet isn't doing anything. Your mixture isn't controlling or adjusting fuel flow, because the fuel source that the mixture adjusts isn't in use.
Where the throttle is closed, it doesn't seal perfectly. Otherwise the engine would simply die. Along the edge of the throttle plate a little air gets by. It rushes in due to the low pressure on the engine side of the throttle plate..."suction," if you will. Suction, caused by the suction machine, the engine.
Adjacent to this spot where the air leaks by is the idle jet. This jet also has it's own mixture control, but it's a fixed control set by a mechanic when the meachanic adjusts the idle mixture. The only control you have over this is the idle cutoff function of the mixture control. This is a very rough control; you're actually cutting off the fuel rather than adjusting it's flow, and it's a very, very minute range in which this works.
If your idle mixture is set properly you should be able to set the throttle to idle, and oh-so-slowly retard the mixture toward the cutoff position. At some point the engine rpm should rise about 25 rpm, and the engine should then die. If it doesn't rise at least that, then you're operating with too lean an idle mixure. If it rises much more than that, you're operating with too rich an idle mixture. Either condition needs maintenance.
That little bump, that little 25 rpm rise, is all the mixture adjustment you get in idle. Otherwise, your mixture isn't doing a darn thing no matter how much you twiddle with it. That little 25 rpm jump isn't mixture control, either, but shutting off the fuel, instead. Something entirely different. Far less reliable, far less repeatable, harder to locate, and not consistant.
If you're working an injected engine, the semantics are somewhat different.
So far, so good.
Close the throttle. Very little air flowing through the venturi. The engine is a suction pump, and the throttle is like your hand over the hose of a vacum cleaner. What does the vacum cleaner do when you cap your hand over the hose? The pressure drops in the hose even more (like manifold pressure does when you close the throttle, right??), and the engine strains to get more air.
Same for your engine. The engine still gets fuel, or it wouldn't run, but it's not getting the fuel through the main fuel jet. There's no air flowing past that jet to draw the fuel out. Your mixture controls that jet...your mixture isn't doing anything because that jet isn't doing anything. Your mixture isn't controlling or adjusting fuel flow, because the fuel source that the mixture adjusts isn't in use.
Where the throttle is closed, it doesn't seal perfectly. Otherwise the engine would simply die. Along the edge of the throttle plate a little air gets by. It rushes in due to the low pressure on the engine side of the throttle plate..."suction," if you will. Suction, caused by the suction machine, the engine.
Adjacent to this spot where the air leaks by is the idle jet. This jet also has it's own mixture control, but it's a fixed control set by a mechanic when the meachanic adjusts the idle mixture. The only control you have over this is the idle cutoff function of the mixture control. This is a very rough control; you're actually cutting off the fuel rather than adjusting it's flow, and it's a very, very minute range in which this works.
If your idle mixture is set properly you should be able to set the throttle to idle, and oh-so-slowly retard the mixture toward the cutoff position. At some point the engine rpm should rise about 25 rpm, and the engine should then die. If it doesn't rise at least that, then you're operating with too lean an idle mixure. If it rises much more than that, you're operating with too rich an idle mixture. Either condition needs maintenance.
That little bump, that little 25 rpm rise, is all the mixture adjustment you get in idle. Otherwise, your mixture isn't doing a darn thing no matter how much you twiddle with it. That little 25 rpm jump isn't mixture control, either, but shutting off the fuel, instead. Something entirely different. Far less reliable, far less repeatable, harder to locate, and not consistant.
If you're working an injected engine, the semantics are somewhat different.
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