I was having the same problem with my GTO when I first got it running. Took the check valves out, cleaned & inspected to make sure they were working, and they were OK. Then I realized I had the oil tube going to the left cylinder pinched betewwn the carb and the engine case. Took carb back off, adjusted fitting on oil pump to get the oil line to run in a little different direction, and it was OK. It did still take longer to get all the air to bleed out of the left side than it did on the right. And it probably took 7 or 8 minutes of running to get all the air out of the lines. Idled while holding pump cam wide open, revving it every so often.