We have two 400s From the beginning, mine has intermittently sputtered. It is more pronounced when climbing hills. In fact, it gets so bad the machine just powers out and won't climb. Not much fun and dangerous.
My wifes never did it till recently, now it acts just like mine. Hers was involved in the recall for the ECM problem, mine wasn't
Loosening the fuel cap sometimes helps a little but doesn't solve the problem.
Both are equipped with EBS.

What we have tried:
Installed K&N air filters, filters are clean
Re-jetted my quad, shop went up one size
Replaced fuel filters with Polaris filters
Changed plugs to the NGK iridium
Blew out fuel tank vents, they were open (what's up with the tiny hose?)

This has been very frustrating as the shops can't replicate the problem as they have no hills to climb and a google search turns up nothing.
Anyone else experience this?

 

WE had the same problem on a 2004.5 Sportsman 400....The atv was jetted wrong from factory...The dealer rejetted it and has ran fine since....Iwill try and find what jetting was installed..The small plastic hose from the gas tank is the vent hose..If this is pinched it will do the same thing...(sputter)It should go down into your steering stem...check to be sure it is not pinched..What altitude are you riding?? our problem seemed to get worse above 3000 ft..

 

Thanks. The vent tube isn't pinched or blocked, I was wondering why the diameter is so small?
We have experienced the same thing above 3000'.
We live at 2800' and ride up to about 6000'. Polaris says the machines are jetted to 6000' but the did re-jet mine but it didn't help.

 

The 400 SPortsman were jetted ridiculously rich. More than likely you came with a 172 Main, and will need to drop to a 167.5. Your Dealer jetted up because of the filter, which is often the norm, but in the case of your machine, I can gurantee you it was jetted rich enough from the factory to handle a k&N and STILL be dropped. Pull your plug and look at it, Ill bet money its nice and dark (rich).

 

I agree with ktmguy70 it's jetted too rich and if the dealer went up one it's way rich now....