The engine will always increase speed slightly if you pull a small manifold vacuum hose. If you don't have nozzle drip and control with the mixture screws it doesn't need more bypass air.
The only time I'll add more bypass air if no nozzle drip is when there is too much transfer slot exposed and I don't have control with the mixture screws.
10" vacuum sounds about right for that engine combo as it's got a pretty "hefty" cam in it on a tight LSA.....
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73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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