The triangle holes in the air cleaner housing are so it's gets fed air that passed through the rotary screen on the flywheel. This does 2 things: Makes sure the air has very little grass or large debris in it, and 2.) should be feeding it cooler air, as opposed to air from the engine compartment which would be very warm/hot.
I think most guys cut the back off. There is another style of cleaner with a "snap strap" cover. You may be able to remove that style without mods.
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