Simple drawing of a slide type carb
This is a very, very simple drawing of a ‘slide’ type carb. Yep, there is stuff missing from the drawing like the float and you can’t see where your accelerator is attached, but the stuff in a carb that mixes gas with air and allows you to make the scoot go faster is shown. This drawing shows what things look like when you are at idle. You can see that the slide is blocking about 1/2 of tunnel where the air goes into the motor.
That is why a pilot jet (idle jet) exists on every carb in the world. Yes there is a hole into the ‘throat’ of your carb right where it’s shown and gas gets sucked up so it will idle. It is a pointy needle shaped thing, and you can adjust it in or out and get your scoot to idle at the correct speed.
That thingy sticking out the bottom of the slide is a needle about 3″ long and it is also tapered, and it sticks all the way thru the ‘jet holder’ and into the jet. In this position there is no gas going thru the main jet so your scoot would not start. Your accelerator cable is attached to the top of the slide so that when you turn the throttle the slide is lifted UP and out of the way so more air can get sucked into the engine and the tapered needle goes up too, so more gas gets in and zoom you go.
Now, some of you might be thinkin’ that gas is always coming out thru the idle jet! You are right, that why it is important to set the idle mixture correctly. If you set it too lean or too rich at idle, the air/fuel mix will be wrong all the time. Bummer
The ‘jet holder’ looks like it has 3 holes in it, and it has even more than that. They are very small holes, and when the carb sucks up gas it mixes air with the gas in itty-bitty droplets for a good burn.
The PB20 Keihin carb on your scoot is a very simple device, almost never breaks, and would take years to wear out. It is brain-dead simple, easy to clean, and is seldom the issue when your scoot doesn’t run properly.
Awesome! Thanks for the clear explanation! Much appreciated.