400ex works good when fuel air screw is turned in all the way except a sputter when going slow but when I give it gas it comes out of the sputter and works perfect no stutter at all

 

Help please

 

Brandon, the best tool for carb tuning is a wideband O2. Sounds like a lot of rigamarole, but it's quite easy, once you have a way to monitor A/F ratio. One thing that also helped me tune both of my Arctic Cats was Dynojet stage 1 kits. Check eBay, sometimes you can find em cheap. Typically the PMS screw should be out 1-3 turns. Turning in makes it leaner at part throttle. You may just have some dirt in the starter/slow jet. I like the grooved needles in the Dynojet kits, really let you zoom in on correct A/F.

 

baseline for air/fuel mixture screw is 2.5 turns out from lightly seated...Turning it out richens, in leans. Need a lot more info:
1) is the motor stock
2) are the jets stock
3) is the airbox stock
4) is the needle seated properly


Have you read your plug? is it running rich or lean at cruising (sputtering) speeds?


When was the last time it was thoroughly cleaned?


The air/fuel screw is generally the fine tuning area...