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Discussion Starter · #1 ·
I've got a 300 fourtrax. I'm mechanically capable of doing about anything, but as the title says me and carbs don't know each other very well.

I have taken the carb apart and cleaned it. Not very well I guess, as there is some varnish but all the passage ways are clear and I got a carb rebuild kit. Everything seems to install one way with the exception of two pieces. The idle mixture screw (I understand this) and the needle. What does this needle do, and how does positioning it differently change the power?

I'm also thinking of changing the exhaust and modifying the intake for better flow. I've read that this should be rejetted, but not one thread I've found has given an exact answer. I'm not even sure how to tell if I'm running lean or rich. I'm used to plugging in a "programmer" and knowing so please excuse my ignorance.. I'll take a shot in the dark and say I'd need a 128 and keep the stock 40?

While I'm here another question.. The atv seems to stumble or sputter during partial throttle. What could cause this?

Yes I've searched. I've lurked around a lot recently. ?
 

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The needle you are referring to... are you talking about the Needle Jet needle? The one that has an e-clip near the top, with 5 clip slots to choose from? If so, the e-clip probably belongs in the center (3rd) slot if your altitude is less than 1000 feet above sea level. Altering that e-clip position will cause the motor to run either slightly lean or slightly rich at part throttle. Look in the service manual for your machine for the factory e-clip slot location.
 

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Discussion Starter · #4 ·
Thanks. Yes that needle. I replaced it when I got the rebuild kit and it was / still is in the middle. So if I raise or lower it maybe my part throttle will be fixed? I'm right at 1k elevation. Where I'm taking it at the end of the month will be up to or higher than 2k elevation. Should I raise or lower it? Thanks for the help.
 

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Under 2000 should not require rejetting. My suggestion is to clean the carb again using extra time and careful thinking. Use bright lights and magnifiers if needed. Pay careful attention to the emulsion tube if it has one. Also the jet sizes can't be trusted sometimes unless it is OEM or just a few after market brands. Someone else here can tell you the names of "trusted" aftermarket brands. As far as the understanding of the needle position I will give you a quick answer. This is probably the most important adjustment for fine tuning because the needle meters the throttle response from roughly one fourth throttle to three fourths. You see it is tapered right? It lifts from the emulsion tube/ needle jet/ main jet as you throttle up. The taper allows more fuel thru the set circuits mentioned above. If you lower the needle (raise the clip) this will restrict fuel and vise versa.The prime area for the needle is half throttle. It blends with the pilot/slow jet in a progressive manner as you throttle up from idle. It begins to use the size of the main jet more and more above half throttle to about three quarters throttle. Remember always that the needle gets it fuel from the main jet first so if main jet way off then the needle setting will be impossible to achieve until the main is very close or spot on.
 
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