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SBC 350 Surging at Low Speed Cruise - Vacuum Gauge Reading Interpretation?

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SBC 350 crate engine with mild cam,  Holley Street Avenger carburetor (575 cfm), MSD Street Fire HEI distributor and wires, mild cam, new plugs gapped at .45. No vacuum leaks found. Engine installed in a 72 240z in 2009. Engine ran perfect for over ten years, but started surging at low speed cruise recently. Blocked all vacuum ports and connected vac. gauge to manifold vacuum on carburetor. Reading taken at warm idle.


In the video the engine can be heard speeding up and slowing down (surging) as needle moves on the vac. gauge.


Other: adjusting idle screws has no effect. Replaced needle valve/seat. Float level adjusted per specs. Went up two sizes on primary jets, but no change. Power valve OK. No vacuum leaks.




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