I recently began troubleshooting the same syptoms on a Yamaha. The start button actually completes it's circuit by grounding the low-current side of the solenoid (relay). With all the wires that rub the frame in numerous areas, it's almost a matter a time before something gets shorted to ground. Water seems to help the shorting conditon as well.
Next time your ATV demonstrates the problem, wiggle any wires that you can reach to see if it affects the problem. This will help you get started with finding out where the problem is.
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2004 Yamaha YFZ450
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