LA small block timing is suddenly way off

Thanks for the efforts guys. I made progress tonight and found an error in my earlier timing analysis. Tonight I removed valve covers, removed distributor cap, removed spark plugs. Turned engine over slowly by hand and confirmed that harmonic balancer 0° mark perfectly aligns with TDC of compression stroke on cyl 1 (i.e, both cyl1 valves closed, thumb on spark plug hole to feel compression building, a wooden dowel on top of piston until crankshaft is just barely past the point where piston stops moving). At this point the distributor rotor points about 30° past the cyl 1 post on the distributor cap, with the distributor set where I previously had it. This means my previous ear tuning (fast smooth idle) must have been 30° advance timing, but the engine did seemingly run well and I never detected any knock or ping. Aha, but I then found that the harmonic damper has at least 3 other strike marks around the perimeter. So when I reported 30° retarded timing (using a 12V inductive pickup timing light on cyl 1 wire) at the start of this thread, the real 0° mark is not yet visible and I was instead seeing one of those other strike marks that comes around about 1/3 rev before the real timing mark.

So other than the fact that I have been using about 30° advanced timing at idle which is probably not good, I no longer think I have a timing problem causing the bad missing. I can turn the distributor in the retard direction to properly set timing, and I did try this a couple days ago but it was making the miss even worse (daughter in car feathering the throttle and keeping the rpm's up to keep the engine limping along). But I still have not found the real problem, so maybe I had a backfire that did something to one of my Edelbrock 1404 carbs. The name of this thread is now wrong, but before I started thinking I had a timing problem I did remove all 4 fuel/air idle screws and shoot a blast of compressed air in each hole to remove any debris from the idle circuit, this tip came from an Edelbrock youtube video I saw earlier from an Edelbrock guy. I also pulled out all 4 metering rods and wiped them off but they were clean.