318 Engine stumble

Ignore that timing lite for a couple of minutes. Just start the car , and advance the timing until it runs decent. Reduce the Curb-idle speed back to 800. Defeat the Vcan. Blip the throttle a few times; Still got backfires? Check that your Accelerator pump is spraying. If yes, and this is a metering rod carb, make sure the rods are staying down with light blips and popping up with hard ones.
Read the timing.
If it's over 20*, reset your Transfer-slot sync. With a smaller than [email protected] cam, close the secondaries up tight but not sticking. Set the mixture screws in the center of their range. Clamp off the brake booster hose, if you have one. Now start her up again. DO NOT touch the curb-idle screw. Instead, STILL ignoring the timing lite, retard the timing to obtain 750rpm. Again blip the throttle. Still got backfires? Leave the timing alone for now, timing is not the problem.
Instead, now rev it up slowly to 3500rpm listening for ignition crap out and backfiring. What you are doing is causing the flyweights to fully advance the rotor, and if the phasing is off, sooner or later, the spark is gonna start jumping to the wrong towers. By 3500 the timing is usually fully advanced, so if it ain't barking, it's fine. Hook the Vcan back up and repeat.
No barking now..... I would be checking split-overlap.
Barking now,
Engine off;I would set the crank to 15*advanced for #1TDC-compression; you will need an indexed balancer or a timing tape, or you can farmer measure it using 1.5 times 10 on the timing tab, accuracy ain't important right now.
Ok so when that is done, pop the cap and make sure the rotor is in the general vicinity of the tower that feeds the wire to #1 cylinder. Then; if it is put it back on.
Next scribe a line on the D on the side of the Distributor body directly in line with the center of the #1 tower. Then scribe shorter lines to the next closer tower in each direction. Again accuracy is not critical; I use a Sharpie.
Next pop the cap off again and set it aside. Observe where the rotor is. It should be closer to that long center scribe-line than to any other.
Grab the rotor and twist it as far as it goes; It should pass the long scribe-line, but stay nearer to it than the next short scribe line. If it gets near to or over the half way point, the spark is gonna take the path of least resistance.

Whenever you blip the throttle, you are increasing the EFFECTIVE compression ratio in every cylinder in it's time. But the chambers adjacent to the one under compression offer less resistance to the spark, so it attacks them. The one behind it in the firing order is on exhaust, so the engine will never notice a fire in that hole,and the rotor is not traveling in that direction, so you can ignore that. But the next one coming up is beginning it's compression stroke, so if the spark jumps to it, it is possible the intake valve is not yet fully closed. Of course if that AF charge gets ignited at this time, the now on-fire gasses are gonna escape up into the intake manifold, and try to lite-off everything in there too. At idle, there is not usually enough oxygen in there to support much so all you get is a lil fizzle. But when you blip the throttle, atmosphere slams down the carb throats, and kaboom up comes the fire. Don't be looking down there and blipping! unless you don't mind looking weird with no eyebrows and smelling singed nose hairs for the next two weeks.

Of course this reverse fire can also happen with an intake valve that is not sealing for some other reason as well. Reasons like weak springs; or bent,burned,sticking,or carboned up valves;or loose-clearance guides. Some of these can be caught by a compression test. But if all cylinders are low, you might not suspect anything except worn out.
And of course, if the cam-timing is out, that would do it too. And that would speak to your odd timing results, and miss-phased rotor as well..