Im not aware of any adjustment on the sieve. I could be wrong, but its just attatched to the shaker arms and bolted in place. Makes me think its out of time, not an adjustment. Hopefully someone else will chime in with better info than I can give. But theres my 2cents.
Make sure the holes in the shoe aren't becomeing egg shaped. Any play in the chaffer will cause it to blow apart if it already weak. Like stated above, check front rod for tightness, weld bar on back of chaffer etc.. I fixed mine multiple times then bought a new one before bean harvest.