I'm not sure on a 90 but on many Hondas, you loosen two allen head bolts on the rear bearing carrier, then there are slots on the chain side of that carrier next to the sprocket. You put an allen wrench in one of the slots, sideways so as to use it as a lever, and use it to rotate the entire end of the carrier. It is an eccentric so as it rotates, the chain will get tighter or looser, depending on which way you rotate it. Make sure you don't overtighten it because as the bike suspension collapses, as in when you land a jump, the chain will get tighter. If you make it too tight, it will break something. But have a good look at the sprockets and chain before you tighten it. The chain may be worn out. If it is, you will need to replace it AND the sprockets. Never replace a worn out chain without replacing the sprockets too. How do you tell if a chain is worn out? Several ways but the quickest and easiest is to try to pull it away from the back of the rear sprocket. If you can expose more than half of a sprocket tooth, the chain is done.
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