I'm working on a 2008 Rancher 420 for a customer and can not figure out the issue. I've put a new fuel pump, fuel pump relay, injector, ICAV, petcock, and air filter and still no luck. It will fire but only stay running if you pump the throttle, and when giving it throttle it backfires through the exhaust and air box. Getting frustrated with it after changing so many parts and don't know where to go with it next. Any help would be great, Thanks!

 

I have had 4 or 5 of them to do that and ended up replace coil to fix I know that odd is what I thought but I was butting my head I has another Atv in shop and changed coil out and fixed and I put the question coil on the other Atv and it ran bad also make sure you have new NGK spark plug in atv and have you check valve lash?I would also check charging rate should be around 13-1 to 14-1 also I have had better luck using the factory coils on these 420s.

 

Yes, I would check charging, then go for a coil next but, it could be throttle position sensor, or pickup coil in the engine. Even a faulty temperature sender can fool the ECU into thinking it is hotter or colder than it really is. Welcome to the world of EFI diagnostics. Like papa hog scoot, unless it throws a fault code, I often end up substituting bits from another bike until one of them fixes it. Private owners can't do that, and we only can if we have the same model in the workshop at the time. Had a current model 420 in a couple of years ago with a slight misfire, boss's son had an identical 420 so I swapped every part of the fuel system in turn, nothing fixed it. Finally tried swapping ECUs, instant cure. Tried the "faulty" one on the other bike and it transferred the fault. Kicked myself for not trying that first as an ECU swap is dead easy, but it hadn't thrown a code and we had never had an ECU go bad before, except on an overcharging old model 420.