Basically, "filling" the crank helps increase flow velocity, packs the case, and helps get rid of the dead flow area of the crank that's immediately under the carb...

I have used JB weld, but was always wondering if it would hold. So far it has, but recently I started using flowable silicone...the runny stuff that's used to seep under windshield trim and seal the water leaks around a cars windshield/rear window.