Sounds like the Holley carb needs some tweaking... Changing the vacuum spring will change the rate and time at which the secondaries kick in. Get a timing light and see where the timing is set at, and make sure the vacuum and mechanical advance is coming in correctly. Could be it's set with too low of intial timing and/or mechanical advance is coming in too soon.... To me, I think the engine should be producing a lot more vacuum, which could indicate the timing being off or perhaps a valve out of adjustment....