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Joe, thank you for your response. My L75 300hp is a 1965 so it has an AFB carburetor (3721). It does not have a Holley, the 1966 300hp engine had the Holley. The specs on the crane cam are: intake lift @ cam.3114 @ valve .467. Exhaust lift @ cam .3294 @valve .494. This is with a rocker arm ratio of 1.5. The cam timing @.004 tappet lift is 278 degrees intake & 290 degrees exhaust. The intake duration is 222 degrees (measured at 109 degrees ATDC) and the exhaust duration is 234 degrees (measured 119 degrees BTDC). I think you are correct about the vacuum gage rapid needle flutter associated with an intake valve. My idle setting is 700-800 rpm, but this setting appears to be affected by my pulsing vacuum.
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Re: L75 Vacuum Reading
If the OE VAC (vaccum advance control) is still installed, it's not suitable to that amount of valve overlap. The VAC signal line on all Corvette AFBs is full manifold vacuum, not "ported". What is the data stamped on the VAC mounting bracket?
Idle vacuum typically fluctuates on high overlap cams, and if the VAC is not pulled to the limit at idle it will fluctuate worse due to spark timing variation and the idle will destabilize.
Also, the vacuum reading at the VAC signal line should be the same as at the PB booster.
Any forty year old carb. with no maintenance/repair history can have all kinds of problems. Chances are it needs a good cleaning. There are lots of small passages, which can get deposit/dirt buildup over time that will foul up fuel metering and cause driveability problems.
Duke
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