Topic: Newbie engine question

[Burnboy]

What exactly makes our bikes idle? I can understand the idea of it on a Fuel injected vehicle but not a carb'd one. I assume its some sort of loop system, i.e. engine fires, rotates, more fuel moves in and gets spark from the movement of the motor and so on. But Id like to get a more complete picture like is there a vacuum created to pull more fuel in without opening the throttle? What determines idle speed and so on. Thanks in advance

[robe0531]

i belive when the pistons go down and the intake valves open it causes a vacum and sucks gas in then the piston goes up cause the explosion from the spark of the plugs and so on and so on
but not 100% sure on this
just how i think it works

[Really?]

I think he was trying to say suck squeeze bang blow.  Just how I like my evenings.  It's the suck thing you are referring to.  The answer to your question is basically yes.  Fuel is pulled in 'cause the slides are partially open.  Open the throttle, more vacuum and it pulls more fuel.  It goes a bit deeper than that (how much fuel and when, other circuits that feed fuel up to a certain RPM where the idle jets no longer really function, main jets kicking in, bla bla bla) but that is the jist.

[punch455]

When the butterfly valve (or in our case, the throttle slide) is closed, air and fuel are bypassed into a seperate circuit, called the idle circuit.  The slow/idle jet, using engine vacuum created by the piston moving downwards in the cylinder, sucks up fuel from the float bowl. The main jet at this time is completely plugged by the needle, and therefor sucking no fuel.  Through the slow jet, the fuel is then mixed with a certain amount of air that is metered by the air mixture screw, and released into the carburetor venturi.  YAY, we now have a precise and constant amount of fuel and air entering the cylinder! (assuming everything is working as it should, which I haven't had the pleasure of experiencing quite yet...