I just picked up a 99 scrambler 400 for $200. Overall in decent shape. Guy couldnt get it running and didnt want to deal with it. Anyway, I swapped my old wheels/tires (from my 500 scrambler) onto it. Messed with the choke (which he didnt realize wasnt hooked up) and got her running. She runs good as long as your into it. Will NOT idle at all. Throttle cable is sticking badly and is all chewed up. Bushings are all shot. I guess my questions are regarding the throttle cable and idling. The cable for the throttle splits and appears to connect to the oil injection pump and at the carb. I can replace the cable but is there a trick to "tunning or setting" it once installed. Also it wont idle at all. Not sure if this is due to the cable or not. Also has an FMF header on it and a stock can. Not sure of the sense in that but could that be an issue? Im not familar with 2 strokes at all. Any hints or starting point would be great. compression is good also.

 

You might try to remove the washer in front of the idle adjust spring as over the years the idle screw tip can wear down and not touch the carb slide enough. Just an old trick,dont know if will help in your condition. Plus if doesnt work you might try to adjust the cable adjuster on top of the carb. OPT

 

For what you have described, sounds like this ATV has been really abused. If you are going for a new cable, one part does run down to the pump, run the cable of the wheel, and line up the marks inside the box.
As for it not idling at all, Old Polaris tech has a great idea, the other is to rebuild the carb, making sure you clean up the pilot jet for sure.

 

It is sucking air from somewhere is my thought. Doc