while the bike is running you are supposed to loosen the big bolt (leaving the smaller one on top tight) while the engine is warm. now that i think about it hot would be even better...
the function of the big bolt is to tighten two collars (like ones on bar end mirrors) that when squeezed clamp a rod attached to the tensioner (to lock it in place). while the bike is running, you are in effect letting the tensioner move freely, so the spring will push it (up, as if it was going to pop out of the top of the motor), bending a long plastic slide that takes up the slack in the cam chain.
on MY bike, the spring was so old that it couldn't apply enough pressure to actually bend the slide, so when i first tried nothing changed. the spring was cheap at the local stealership, like 3 bucks. BUT you have to take off that side of the case and the flywheel, which doesn't exactly slide off. if you are crafty you can get it off using a bolt that threads into it, but you need to be careful not to wreck the crankshaft (you can put a bit of copper or some soft metal inbetween, AT YOUR OWN RISK or just buy the tool).
i hope i did a OK job explaining the situtation. i wouldn't be suprised if it was the same spring as on my xl185 and the xr200's. i bought a spring for a 2002 xr200, it was a slightly different design and had a bit more pressure to it. after putting it in all the chain noise in my bike went away, it was like it was a sewing machine. it is also possible that the slides (one on each side of the chain running up and down in the motor) are very warn out... mine were not the greatest, i used the best from the two motors i had apart.