re: thick old dried up grease.
Lay the bike on the left side and drip a little motor oil and silicone lubricant in there. Pick the bike up and spin the freewheel a bunch. It will loosen up. A lot of freewheels can be cleaned and re-packed, but there are about 50 bb's about 1mm dia. in there, and if 1 is missing it can jam later. Also the pawls and springs may make re-assembly difficult. Canned spray grease also works ok if you can get it in there with the straw.
NOTE: if you are using a cassette hub, you will need a cassette remover tool, a proper wrench, vise grip, and cone wrenches, to disassemble and repack your hub shell/bearings and freehub body.
Old dead freewheels make good gifts if you vat them out really really well and oven dry them. Use them for nifty paperweights that make a loud ZIZZZZ sound.
Originally Posted by donutguy
A couple years ago....I had a freewheel slip on one of my regular (non-motorized) bikes.
It was doing exactly the same thing....I would be pedaling and it would slip and the pedals would advance about 45 degrees (out of a full 360 degree pedal stroke)
The problem turned out to be old congealed grease in the freehub body as it was catching and not releasing the internal sprags from time to time.