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hey... awesome video..nicely explained !
just one question. Please do tel me wts AIR NEEDLE and LOW JET used for again. ? i mean how and what do they work?
The air needle is like a transitional restriction to increase vacuum pressure and tapered to prime the main jet flow gradually to ramp up fuel flow from the idle/low circuit to the main circuit. Think of the circuits like gears or stairs. The needle covers transition between 3rd gear to about 5th gear on a six speed. It overlaps basically. Fuel is getting by it from the idle/low- on through to where it's completely out of the way of the main jet and the main jet fuel flow is roaring like the straw/compressor example.
Without the needle the carb would work, but not as smoothly through middle throttle (where most people ride and have the most fine input or throttle roll back and forth.
The low jet is like second gear on a transmission. You can shift from 1st to 3rd, but again you risk stalling and it won't be as smooth. Also there isn't really the mechanical inertia to carry the process on like you have with a rolling vehicle with a flywheel on the engine and the wheels turning. The pistons still create vacuum every other time they go down, but air just doesn't have the punch that steel does when in motion.
The circuits in a carburetor have to be there or you would have a limit on RPM range. An idle jet can't deliver enough fuel to really scream, and the main jet couldn't go slow and would be VERY hard to start because the engine would have to be spun up really fast. So you have usually four or five circuits (I want to say jets, but their physically different) that get air flow like stairs as the throttle opens. They are typically 1. Starting circuit (choke on) 2. Idle circuit 3. low jet 4. Jet Needle 5. Main jet.
How Motorcycle Carburetors work and how to tune and clean them Check out my other channel here www.youtube.com/watch