My flying site is very dusty. Where can I find some air filters for the carb. How do I make some home made ones.

My other question is with regards to the wire from the ignition box to the spark plug. The wire that is on the outside, braided wire. What is it for? Mine has been partially cut by rubbing against the cowl. The engine now is not that smooth. Is it due to this wire being partially cut. I though this wire is only a sheild against RF interfearance.