typically, to tighten that type of lever, you have to loosen the tension on the brake cable by disconnecting it from the brakes, press the lever to the bar to open the guts of the lever, pull out the top of the cable. This will allow you to see the top of the allen bolt that tightens the band holding the lever to the bars. Stick your allen wrench in there and tighten that puppy up. The you have to reset your brake cable and attach it to your brakes. A bit of a pain, but doable.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"