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While riding normally, it quit. It will start & idle but any throttle advancement kills it.
 

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my 2005 sportsman 500 HO did the same thing a few weeks back.

Check and make sure your thumb throttle is adjusted correctly. If it is not adjusted correctly, it can cause stalling or hesitation issues.

Your carb might also need to be cleaned / adjusted. If the air mixture screw is not set properly, then you are either getting too much air, or too little air and when you give it throttle, the engine is choking out. Check the throttle adjustment. if that seems like it is adjusted correctly, then check your air / fuel mixture screw and set it to specs. If neither of those help, you might need to clean / rebuild your carb.

It could also be the ECU/ECM. but try the other 2 things first.
 

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Fuel screw

If the air mixture screw is not set properly, then you are either getting too much air, or too little air
The mixture screw on the carb that you have is a fuel screw - older carbs had an air screw. They work the same way, but a fuel screw adjusts the amount of fuel mixed with a set amount of air while an air screw adjustes the amount of air mixed with a set amount of fuel. Both can have the pilot jet changed to change the maximum amount of fuel available to be mixed with the air, but changing a pilot jet in the idle circuit of a carb with a fuel adjustment screw has less effect than adjusting the pilot jet in a carb with an air adjustment screw.

The fuel screw has little effect on performance other than a smooth idle - in some cases being on the rich side of the smoothest idle setting will promote throttle response, but throttle response is dependent on the design and condition of the carb.

There is no spec setting for the fuel screw - the service manual specifies the initial setting of the fuel screw - the initial setting is close enough to allow the engine to be started and warmed to operating temperature - with the engine at normal operating temperature, the fuel screw is adjusted for the smoothest idle. A rule of thumb is; if the fuel screw is turned out more than 4 full turns for a smooth idle, then increase the size of the pilot jet by one increment - i.e., lets say the jets are sized 35, 40, 45, 50 etc - one increment is from 35-40 - if the jets are sized 32, 35, 38, 40, 42, 45 etc, then one increment is 38-40. The other rule of thumb is if the fuel screw is out less than one full turn from lightly seated to attain a smooth idle, then the pilot jet should be downsized one increment. The amount of air passing through the idle circuit may be adjusted also by changing the air jet.

The initial setting of the fuel screw for your 500 is 2 turns out from lightly seated with a 40 pilot jet and a clean 160 air jet
 

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I learn something new every day. I have always heard of the main jet, and the pilot jet. Which jet in these carbs is the air jet?
 

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The air jet is part # 3130619 (key # 17 in the carb schematic) - it is in a passage when looking into the intake bell of the carb

Polaris only offers the one size, but the carb parts suppliers offer various sizes for the serious tuner.
 

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I would like more info. As in, was it running properly and this just started, or has it always been this way? What kind of gas? Has it sat for a long time?

If it was running fine and this just started, I wouldn't mess with jetting or mixture adjustments until I was sure it was getting enough fuel or it wasn't all gummed up with ethanol gas. I run pure gas in mine.
 
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