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I agree with Roger. the "jumping" AFR ratio is what we were discussing. My other thought was the gauge or sensor had a malfunction.

Speaking of vacuum.. have you measured it? Have you adjusted the carb using a vacuum gauge? We don't know what cam you have. If it's race only, It may be a dog under 2 or 3 k rpms.

Interesting that it appears to be present then not.. the no load revving and loaded stumble isn't really revealing but. What are the 2 vacuum lines running? Is 1 of them to a vacuum advance on the...