Hopefully someone can help! I have a 2000 425 expedition 4x4 with standard shift,It Idles beautiful but as soon as I hit the throttle it backfires through the carb & will not rev up & doesn't matter if it is in any gear or neutral, I have put a Brand New Carb on it because it was acting lean I thought but still acted the same,,I also pulled plug out of muffler to see if that was plugged & still same, it also has new head gaskets & was bought not running & the guy had past away that owned it so have no idea why it has new gaskets? Exhaust Loeb on Cam? Any suggestions of What it may be or where to start please?

 

Look at the lobes on the cam first as this machine was in the year range that did have problems. When worn down enough most would crank and idle good,but back fire out the carb once throttle was applied. It may look like the one this member had problems with. 425/500 models were affected. https://atvconnection.com/forums/pol...ml#post3165857 Plus doen't over look the air box and filter also. It has to be sealed or the engine won't run right either.