My atv often loses idle after 10-20 minutes of slow' low-range mountain riding. (Which is mostly what I use the atv for) It will keep running great as long as you are giving any throttle at all, but if you try to idle, it just dies. If you try the idle adjustment, engine speed will surge a few times, then dies anyway. I tried to cure the problem with repeated carb cleanings, new fuel lines and filter, sea foam, non-ethanol fuel, valve clearance adjustment,new plug, and finally a full carb rebuild but the problem keeps happening. After a cooling-off period, it idles great again. The fan works, coolant stays put, so I don't think it is overheating, but engine temp is definitely a factor. Could the carb or fuel lines be getting hot enough to cause this? Is there a cure? Thanks

 

If you've don't have the correct thumb throttle slack,the etc contacts in the throttle housing will touch and die no matter how much the adjust the idle at the carb.Has to have only 1/8" slack maximum,more than that it will die.Adjustment is on the cable down from the throttle housing. Plus check that the tank vent line along with the carb atmospheric vent are clear and open.Also some cable idle adjusters would vibrate and back off,mostly on ones that the wire carb holder would break and adjuster ended up laying on the starter. Float height is also critical on the BST40 carb,here's a carb similar to yours,but used on a Ktm. Shows how to set the float correctly.The BST-40 Bible - ADVrider Also check that the tank heat shield is properly in place. Some have added extra heat shielding on atvs that travel slow quite a bit. Fuel can start heating up to the point of bubbling that can cause problems.