brand new 2012 850 ho eps.. first ride 100' then backfire.. the tech where i bought it said it was part of the breakin period.. break in period is over.. had oil changed and top of the line spark plugs put in still backfiring. It does it on the lower end of deceleration.. almost close to idle...the thing smells rich, a few guys i ride with say the same thing. I havent checked the exahst yet for leaks..

 

mine did it also even with 300 miles on it- was told it's normal from polaris.
shouldn't have to worry about rich / lean running with efi-

 

Same thing when you install a new battery on a car or truck especially one with a lot of miles on it. It takes a little time for the sensors to send info back to the computer on fuel air mixture,temperature,air flow,etc. That's why they run crappy for a few minutes until all the systems are showing current engine status. Same on the 550 and 850s. They would pop and backfire for some time until the engine breaks in. First thing I noticed on the Xps when they first came out. OPT