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I bought my son an 2002 scrambler 90 and I can't get it to run right. It wouldn't accelerate at all at first. So I turned both these screws in all the way and backed them both out 1.5 turns. Quad started up and was idling at like 2k. I took turns and tried to adjust each screw individually to see if either one would make it idle down to acceptable speed but neither screw seemed to do anything. I took it for a ride and it was fast but backfired every time I let off the gas.
 

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Yeah, idle speed screw is run into the carb body and stops the slide inside to set the idle speed. You need to make that adjustment with the throttle open a bit so the screw doesn't get driven into the side of the slide. If you can get it to idle down, then run the mixture screw in until it misfires, then all the way out until it misfires, counting the turns. Split the difference and run it in that many turns. Should get yo in the ballpark. Adjust the idle again and repeat if needed.
 

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I had a 2003 Predator 90. Same 2 stroke they used on all the 90cc machines until 2007 or 2008 when they changed to the 4 strokes. Another thing to look at is the CDI box. Longish electronic box under the front hood. You can probably see it from the left side under the front fender. On my Predator, they came with a jumper on there that limited the RPM for the new user. I made a wire since mine didn't have it while the grandkids were learning. You can check and see if there is something on the end opposite the wiring harness
 
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