They stop fresh air from being injected into the engine during deceleration which is done to burn off any unburnt fuel vapor. They also stop most if not all of the popping and backfiring you hear when you get off the throttle. You can accomplish the same thing with the marble mod which uses marbles to plug the hoses that carry the fresh air. If you have stock exhaust, reed valve covers (Coasters) are unnecessary.
ok well i guess i should have looked before asking. i bought the bike used, and i just walked outside to cover her and took a flash light with me. and the reed valve system is gone already. i wondered why is smelled like it was burning a little rich.
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