My wife and I each have 2005, 400's. From the start, mine has never run well at any elevation over 4000 feet. I installed K&N filters and had mine re-jetted, it didn't help. Her's always ran fine.
This weekend we were riding with a group and both our quads were acting up. One of the other riders suggested that we loosen the gas caps, we did and they ran great.
But, later on the started running rough again even with the cap removed.
Anything else to look for?

 

make sure the vent hoses are clear and not pinched.

 

Pulling the cap wouldn't eliminate that as the problem?

 

pulling the cap is to make sure its not air locked. i know if the vent tubes from the carbs are pinched it will not run right.

 

Thanks for the clarification