I would recommend taking the airbox cover off....but I guess if you have deep water holes that's gonna be tricky. I would try your best to get as much air as possible into that airbox. Like maybe multiple snorkels like they do on utility quads setup for mud bogging?? I would use the largest diameter material you can, and do more than one. Maybe have two 3-4" pipes connecting to the airbox?? Cut holes in the sides and run them backwards and up at the back of the quad. It's best to have them go forward, as it will pick up less dust, but I'm just thinking in terms of being able to fit big air pipes, going forward could require a lot of work. Then get them to stick up as high as possible. Put a U at the end so that the opening is pointing down. You could even put a low spot in the snorkel so that if it did suck in some water, it would fall to the bottom and not get sucked into the intake. A well oiled filter will keep a little water out. Put panty hose over the intake as a pre-filter for large particles.
Also, in terms of jetting, was it running good with the 280?? Is that what you had for stock? Before changing the jet maybe try putting the needle clip a little lower. The clip can make a HUGE difference!!
