I concur. I can pull a plug wire totally off my PV's B20 and it will start just about as easily on 3 cylinders as it does on 4.

Just do a couple of basic diagnostics. Cars need 3 things to run (although there's a wide range between 'running' and running well).

1) Fuel
2) Spark
3) Compression

1) Fuel - There's a whole set of mechanical bits related to delivering fuel, even on a simple carb car. But as a quick check to rule out the whole fuel system as a culprit, just spray a some starting fluid in both carbs and try to start it. If it then starts up and runs for a few seconds, then it means something is wrong with the fuel system, because the starting fluid allowed it to run for a little bit.

2) Spark - needs a good spark, and it needs to be at the right time. First basic check - is it making any sparks at all? Pull a plug wire off and place it about 1/8th " from a ground (like the engine block) and watch while the engine is cranked. You should see a nice healthy spark. If not, issues exist with the ignition system. And there are 2 sides to the ignition system, a low tension (i.e. 12 volts) side and a high tension (~30,000 volts, the sparky side) side. The coil needs to get 12V+ to one side for it to work. On an old Volvo this is done via an armored cable under the dash leading from the ignition switch to the base of the coil, protruding through the firewall. Unless some irate mechanic has chopped into it in the past. The other teminal of the coil (the only one visible on an OEM old Volvo coil) goes to the distributor, where a connection to ground is alternately made and broke as the distributor turns. This on-off ground is quite often the root of issues, related to points or malfunctioning electronic points-replacing gizmos.

3) Compression - last and least likely. but various mechanical problesm can befall a motor that can prevent it from cycling the air through the engine and compressing it properly for a proper combustion cycle. First suspect on an old pushrod Volvo motor is the cam gear, once it falls apart the valves no longer open and close (incidentally, this would also knock the ignition system out of comission since the distributor would no longer be turning). Pistons can also get holed, HG's can leak, all of which could be checked by a simple compression check.
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'63 PV544 rat rod, '93 Classic #1141 245 +t