LA small block timing is suddenly way off

Update based on your replies so far: I think the reason I could not see the timing mark earlier this summer was just because the harmonic damper is painted orange, the timing mark is orange with no contrast, and the lower radiator hose blocks most of my view of the timing mark so it's hard to see. Yesterday I marked it with a black marker to fix that. I did confirm back in spring that the timing mark was at 0°C TDC for cyl 1 on the compression stroke. By ear I would guess I set the timing 8° advanced or so, but can't be sure. Since the missing problem began I have verified the distributor rotor points to cyl 1 wire at TDC, checked spark plug wiring, pulled distributor cap checked rotor and weights and springs, verified strong a consistent spark with inline spark checker between coil and distributor even when idling badly, installed new spark plugs, and checked for carburetion problems but did not find anything carb-related yet. Get a lot of smoke and fumes out the exhaust, probably just a lot of unburned gas coming out.