Recently bought a 2000 Xplorer 400 new from the dealer a few months ago. I had my dealer rejet it before I took it off the showroom floor, since I do all my riding at elevations between 5300-9000 ft. He put in a 185 jet, taking out the factory 195 jet. I though he should change the clutch weights also, but he advised against it. To date I have 300 miles on this quad, and have had to change out the plug 5 times now because it gets fouled. It doesnt seem to matter whether the air temp is cold or warm (have ridden it in 65 degrees weather and as low as -4 degrees Farenheit), I still foul plugs. It just isnt running smoothly, sputters alot. My question: Could it be the jetting? or is there a carburator adjustment I should look at?

 

Had the same problem with my 95 explorer.Check alignment marks on oil pump may be running to rich.This was not my problem though.I put in 1 size smaller pilot jet and it cured my problem.Running too rich on bottom end or idle end.

 

It sounds to me like your oil injection unit is pumping in to much oil and fouling out the plugs.Have the dealership check the alignment/adjustment of the cable,
Bill

 

Another thing to check would be the setting for the choke. A partially engaged choke caused by not enough slack in the choke cable line could also foul plugs.