I'm going to guess, in addition to what Willy has said, that you have some water in the spark plug boot and/or spark plug bore. After starting this water heats up and starts to evaporate, but condenses on the inside of the boot, shorting spark to case. Unplug and dry the boot and then the bore thorougly (a hair blow-dryer applied to the areas for several minutes can help) and keep trying.
'01 660 Raptor, Burgard +2+1 arms, YFZ Shocks, LSR anti-vibe, CTR pipe, jetted, K&N
'01 Polaris Scrambler 400, K&N
'02 Honda 400ex, Big Gun pipe, jetted, K&N, Lonestar +2 A-arms, Lonestar anti-vibe
'04 Suzuki LTZ400
'16 RZR XP4 1000 Turbo