wow, I step out for a while and this thread gets *very* long.

Okay... I've read down and not seen an explanation of how I understand this.

Early volvo engines B18A and maybe the early B18D did not have a PCV system and they ran just fine. The PCV is a good system, but you should be able to run without it even - if just to diagnose your problem. Early engine configurations don't even have the center port on the intake manifold. The flame trap is just an open breathing pipe called a "Road Pipe" It is a little different on the B20, but I think you can get away with what I am going to suggest.

Eliminate the PCV system (temporarily) and narrow down the problem. You have too many uncontrolled variables in your "experiment"

Plug your vacuum port on the manifold and you should be running BETTER if not PERFECTLY. Plug the PCV fitting at the manifold (put a 1/4" NPT plug into the hole for the fitting or put a rubber plug on the fitting). Adjust your carbs so that your mixture is correct and your idle is good.

If that brings it to life. Unplug the port on the intake manifold and attach your PCV hose. You'll need to adjust it slightly to account for the extra air coming in through PCV valve, but otherwise you should be good.

As I understand it, your carbs mix the air and fuel and deliver that balanced mixture to the valves/pistons through the intake manifold. When extra air enters through a faulty pcv port *after* the carbs, but *before* it gets to the valves/pistons the air/fuel mixture is leaned out by the extra air. This causes poor performance, "lean-pop" and other nastiness. You probably adjust the carbs richer to compensate for the extra air, but that results in an over-rich condition and other performance problems.

When I initially suggested pulling the PCV hose, I suggested that because it would lean out your mixture and should have stalled your engine. If the engine idle increases and stays high, you had way too much extra fuel in the mix, and allowing all that extra air in brought the mixture closer to "correct" If you run better with the intake sucking in un-metered air through the PCV fitting, then your carbs are still way too rich.

You should not be able to run at all with that hose off and the intake unplugged.