Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 18 Times in 13 Posts
Is it very short on power too? Your 304 engine is notorious for jumping the timing chain a tooth, which is a common cause for your malady. When that happens the ignition timing will be off, so a timing light will confirm the diagnosis. There's no other likely cause for the timing to be off.
As for a guide to the process, the only things normally done in a tuneup on that engine are spark plug replacement, and checking the ignition components and filters. If the spark plug wires are cracked or the cap or rotor is burned they need to be replaced. A good check is to look for arcing around the cap and wires with the engine running in total darkness.
The carburetor might need a rebuild too, but most rebuilds done are unnecessary, IMO.