Hard to start, found problem. Now Why?

Had a problem after new rebuild on 72 340 engine. Block 0.030" over, 9.5 CR, hughes hyd cam, stock thermoquad, stock electronic distr. Got engine started quickly for the cam breakin and everything went well. Later when cold, tried to start and it would crank but wouldn't even pop. Went thru everything, had fuel and spark but not strong fire. Compression test showed all 8 at 175+ psi. Finally found volts from ignition switch was only 10v. Replaced ignition switch now 12v, but would only start with shot of starting fluid. Tried a proven ecu & distr too & check for vacuum leaks. Vacuum at idle 15 in-hg and timing 18*initial. Finally thought why did start for breakin? Only thing different was that I had a plug on the carb port for pcv during startup. Sure enough when I plugged that port, it will start on gasoline. So know the problem source.
But WHY won't start with pcv hooked up. Pcv seems good. Sure glad I had that plugged for first start. Would appreciate help to know why.