Re: 1970 Chevelle 307 rebuild BTDC question(s)
for break-in at 2000 to 3000 RPM you need to have timing at 45 or 50 degrees to avoid over heating. ignore the initial, when the engine starts immediately raise the RPM to 2200-2500 and check the timing. it needs to be advanced, like 45-50 degrees. the advance mechanism built into the distributor will be in play. hook up the vacuum advance to manifold vacuum, that will also help.
if the engine spins over freely but won't quite start, the timing is probably retarded. advance it. pull the vacuum advance toward the front of the engine.
Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife.