My grandson's 125cc atv wouldn't start for nothing last visit. A year ago it did the same thing,my mother in law was in the hospital & I was too busy too work on it so I took it to the shop where we bought it.2 days & $200 later they called & told me it was ready to pickup.I was glad it would be ready for "golden boy" coming back to town.The shop didn't give much detail just claimed it was a bad carburetor & other things ::) .
I wasn't about to let them rip me again so today I hooked up my jump box since the tiny battery was dead from all the attempts to start. This thing wouldn't even start with ether,so I checked the gap on the spark plug & cleaned it.I verified I was getting a spark & it still would not start. So I adjusted the idle screw all the way clockwise & backed it out 1.5 Turns.I gave it a squirt of starting fluid pressed the start button & It fired up with high idle.
I got it idled perfect & ran it enough that the battery charged up. It fires right up every time now.
My theory is with all the bumps & grinds,vibration causes the idle screw to loosen up to a point that the throttle plate isn't allowing enough air in.I know the idle screw was backed out at least 5 turns when I turned it in.
I might put a dab of glyptal 1201 or fingernail polish on the idle screw like we used on trim pots in electronics so it will not keep happening.
Great way to sleuth out a fix! Not sure about your application, but does it have a tension spring on the idle screw? if not, maybe adding one? - john
My thought exactly John. Missing or weak spring. Bill
Update:
The ATV wouldn't start again so I ordered an all-in-one kit that comes with carb & everything from Amazon.I wasn't getting fire so I changed the plug it fired right up.I have been starting it everyday until golden boy arrived.3 days later the starter went out. Amazon delivered the starter in 2 days to get me back where I was ::) Then it would only start with starting fluid :o So I finally replaced the carb,now no spark what-the-hay.I went to Advance Auto & got the NGK version of the spark plug.Got spark again & I tossed the Torch brand plug.We have had the grand kids for 2 weeks until school begins.The ATV has ran better than new actually never missing a beat.Golden boy is 6 yrs old & drives it everyday .