Mixture problems at low engine speed are caused by the pilot jets or maybe a clogged air bleed that feeds through the idle and transition ports. The 150 mains might be a little big but, since it seems to be OK after you get going, they probably aren't the problem.
When your engine is running slow, below around 3000 - 3400, it is getting gas from the idle jet and a couple of transition ports located just outside of the throttle plates, all fed by the pilot jets. There is an air bleed port running from the inlet end of the carburettors all the way in to the idle port that syphon gas from the pilot jets. Once the engine picks up speed, the slides start to open and fuel feed moves to the needles and main jets. At high speeds, enough air is flowing through the carburettors that the idle and transition ports begin to feed fuel again, acting like the power enrichment valve on car carburettors.
