Sluggish Motor
Short answer: Missing or surging at a steady cruise (but fine under acceleration) usually comes down to ignition advance, a lean cruise mixture, or the distributor.
- Test the advance: disconnect and plug the vacuum-advance hose and drive. If the surge clears, check the initial, centrifugal, and vacuum advance — a broken internal stop in the vacuum advance can let the rod travel far too much.
- Lean cruise can surge too; a crude check is to lightly restrict the air horn at 2000–2500 RPM (warm, choke off) — if RPM picks up sharply and smooths out, it’s running lean.
- On a Quadrajet, a sticking throttle-slide (the brass metering-rod cylinder) can cause it — buff it with a Scotch-Brite pad.
- In this case the cause was a dangling wire on the distributor’s vacuum-advance module; swapping in an HEI distributor (a cheap junkyard find that fits without modifying the firewall) cured it.
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Content last modified: June 13, 2026 at 10:06 am
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