In addition to my 67 Chevelle SS I also play around with a 65 Impala SS- 327-300hp -auto with powerglide trans. The motor is stock. I had the Carter AFB all rebuilt and I put in a new Pertronix Distributor which has the vac advance. I believe it gets its vacum from the carb port. The car never had a snappy throttle response even before these upgrades and still hesitates a bit when you step on the gas from a dead start. Its annoying. I have had more than one mechanic and a friend who is really talented at these things redo the timing and adjust the carb a few times. We have gotten it somewhat better by our constant tweaking but still that little flat spot remains. Any ideas would be helpful--John
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Make sure the accelerator pump is working fully (not sure how those are set up on a Carter) and make sure you ignition's mechanical advance is curved so that it's all in by 3000 RPMs. I can almost guarantee that the advance springs in the distributor are ridiculously stiff and would benefit from having softer springs in there.