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Hello Everyone,
well i have picked up a book for basic mechanics of motorcycle . currently i am studying the basic motorcycle carburetor operation.
as i have no background of physics understanding i have difficult to learn simple things truly.

well the part that i do not understand is the choke . as i understand , the venturi of the carburator with the float chamber works on the principle of Bernoulli question . so as more air flows and reach the venturi closely and closely the velocity is increasing and the pressure is decreasing resulting in pumping more and more fuel . here coming the but thing :

statement : for cold startup of engine , choke is used , the choke is closing nearly completely the air instake resulting in vaccum with pumping more reach mixture of fuel ( correct me if i am wrong ) .

well , how come ? the velocity is clearly low as the volume of the air is low resulting in nearly no increase at venturi ( as we are talking about situation were we want to reach a vacuum state ) it should also resulting in no significant air pressure drop as the parameter's not changing dramatically between the states before the venturi and at the venturi . so how come more fuel is drawn .

would be glad for a explanation, hop it was not a boring explanation of my issue of understnading , thanks.
 

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When the choke is closed, it blocks most of the air flow, so the piston, on the intake stroke, pulls on the intake and creates a vacuum at the carburetor, which forces the fuel to rise through the main jet, and also through the pilot jet. There is air coming in through the intake air jets, which normally just add bubbles to the passages above the jets, to get a better mixture; this air is not blocked by the choke, so there is a rich fuel/air mixture.

But, in most, if not all, carburetors made since around 1980, there is no real choke plate, but an enrichment valve which, when open, directly feeds a rich fuel/air mixture around the throttle plate. On this type carburetor, the throttle should not be opened for starting, as opening it dilutes the mixture from the enrichment valve, and can prevent starting.
 
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