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Old 09-21-2017, 01:22 PM
Beginner Class
Join Date: Sep 2017
Last Online: 09-28-2017 07:00 AM
Location: Mesa, Arizona
Posts: 12
Unhappy Help with jetting?

can anyone explain to me jetting? I read your guys comments about it and things but im not understanding when you guys say the correct jet or 420-450 jet, I just bought a yz250 2 stroke and I feel like this is important to know.
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Old 09-21-2017, 03:58 PM
wFo250's Avatar
will fall over
Join Date: Jun 2004
Last Online: 09-18-2020 09:04 PM
Location: NTX
Posts: 3,515

You should just google it and study the fundamentals. If there's something specific you don't understand we can help explain it, but the entire concept behind the Venturi carburetor is difficult to convey in a forum post. Simply put, it's a bowl of fuel and a straw. The size of the straw is the main jet. The amount of fuel in the bowl is dictated by tihe float and float needle. The float rises with the fuel level and stabs the float needle tip into the hole stopping flow at a given level, and giving the Venturi effect a supply of fuel from which to draw. If you have a skinny straw it is harder to suck through and you get less fuel. You want less fuel if you live in high elevations and have less air to mix with, or it is very hot and the air is less dense. If you are at sea level and a cold or mild air temperature you can mix more fuel with a larger jet because the air coming in is more dense, that is you can fit more of it into the mixture. The air screw allows for metering of the supply of air for small throttle openings, but not its density, or at full throttle, therefore the jetting is necessary. Too much fuel is considered rich and too little fuel is considered lean. This rich/lean condition is always talking about the fuel not the air. If we say "it's lean" that doesn't mean there's too much air, it means there's too little fuel. The factory jets I believe are richer than optimum, because a rich condition is not potentially harmful, whereas an overly lean condition can damage the motor. It's a cautionary setting from the mfg. I'm no expert, but that's just how I understand it. Someone please correct any misleading information.
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Old 09-21-2017, 09:52 PM
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Join Date: Feb 2012
Last Online: Yesterday 09:37 PM
Location: dallas tx
Posts: 922

Go to you tube they have VIDEOS TO EXPLAIN IT.
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