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Just finished rebuilding my 2007 400ex and got a couple hours riding time on it and it runs great but when i let it sit and idle after 30-60 sec it will slow down like its gettting too much fuel or not enough fuel and when you go to give it gas it will just kill. Carb is brand new, I have a 42 idle jet and a 175 main. Bike has 440 big bore, k&n filter no air box lid, wrapped stock header, lexx exhaust, hotcam stage 2. Fuel screw is at factory setting is this what I need to adjust?
 

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Yeah, sounds like it's loading up when idling, but when the engine is hot, it should not be let to sit and idle - it will over heat in less than 3 minutes - in stock configuration it is only supposed to be let idling for 5 minutes from cold.

Why a #42 pilot jet? Yes, adjust the fuel screw for a better idle.

Falling on its face when giving throttle is usually caused by a lean condition. The accelerator pump need to be checked and adjusted and the idle speed probably needs to be increased.

When you open the throttle, you are giving the engine more air - not gas. It is the job of the carb to mix the proper amount of fuel with the air to promote combustion. Too rich it will run ragged until it leans out. Too lean, the fire goes out until it richens up enough to light again. When you open the throttle from a low speed, there is a vacuum drop and the velocity of air entering the engine is slowed. It's the vacuum that pulls fuel up out of the float bowl, so when the vacuum drops, the fuel flow stops. The accelerator pump shoots a small jet of fuel into the air stream to replace the fuel lost during the vacuum drop. The amount of fuel sprayed into the throat of the carb is set by the carb manufacturer, but the initiation of the jet can be adjusted by the mechanic.

The pilot jet controls the amount of fuel available for idle and the fuel screw sets the amount of fuel mixed with the air to support clean combustion. The main jet controls the amount of fuel mixed with the air at full throttle and the engine at maximum RPM. The slide needle and slide needle jet controls the fuel mixed with the air at all throttle settings between idle and WFO (Wound Flat Out)
 
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