Vacuum leak ? 20 degrees initial timing should be close give or take a couple degrees.
Factory Holley 750 settings should be close enough for a better generic tune than you have.
I believe you have a vacuum leak that is altering the tune.
I'll double check that. I've done a full engine rebuild, so all gaskets are new. Vacuum to brake booster and pcv. Everything else is capped. Manifold plugs are sealed. I'll spray around the carb.
I bought the carb used from someone that didn't need to be messing with anything mechanical. 750, vacuum secondaries. Rebuilt it, set it up to factory specs, added a secondary metering block. Haven't looked at the transfer slot since that first rebuild so that's definitely a place I'll start at.
