welcome to the boards Bruce!
this site is fairly new, and so are everyone's RZR's.
I haven't came across the issue of melted plastic yet.. but the air box dust you are expirencing might be from a warped plastic engine cover that sits behind the seats.
the air intake that goes into the air box and filter is located between the seats on the panel that is just in front of the bed. the intake is suposed to pull in air from the middle of the cockpit. sometimes the engine cover can get warped, and block off this opening, causing all the intake air to get pulled up from the bottom of the car, and the engine area, where there is going to be a lot more heat and dust than what is in the cockpit.
we came up with a pretty cool little solution that lets you open up that air intake to get more fresh air in, and help the engine and clutch system breath better. basically, you cut open the panel using a supplied template, and install a stainless steel grill that goes on top of the opening. It gives a cool look, and helps keep larger particles from entering. If you are running in super dusty and muddy conditions, where you are going to be coated with dust and mud as the driver and passenger, i would suggest a small amount of foam in the area, so direct shots of mud can't enter into the 1st stage of the collection area.
But under normal riding conditions, this is a quick and simple fix that also has a cool look.
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