Look closely where the handle attaches. Should be a couple nuts and I believe a nylon bushing. It should be a matter of adjusting the two nuts. One controls the tension, the other is a jam nut
The 2 nuts (80,90) compress a couple of washers (60,70) to give the lever tension, also check the feed back rod on the other side to make sure it's connected.
I had to tighten mine a while back. Doesn't have enough room for two wrenchs. Happened to luck out, and the skinny wrenches that came with my die grinder happened to be the right size.