I replaced the original 735 CFM Holley as one of the many up-grades to my Pantera. I selected a Speed Demon 650 CFM with vacuum secondaries similar to the Holly. I found the secondaries are not opening or not opening fully. I changed the return spring in the diaphragm that opens the secondaries to the minimum tension. I still only get a partial opening. I installed a vacuum gauge and I read about 3� of vacuum at full throttle through 1st and 2nd gear, which tells me the engine is flowing near maximum CFM. This would indicates to me that there isn�t any more �displacement� to cause a vacuum. I think this is a self-regulating effect of the drop in vacuum and movement of the secondaries. Does this level of vacuum seem correct for this size carb. and engine displacement?
(idling vacuum about 20�� and steady, 25K miles on engine, all engine parts original except intake manifold)
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