Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Pontiac 400 Idles to fast
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Old 04-02-2010, 03:52 PM
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Most likley cause is a manifold/carb/vacuum hose vacuum leak.

Pinch off the vacuum hose to the booster with vise grips to see if its leaking.

Stuck distributor advance weights. Broken/jambed vacuum advance.

On some Q jets there is a hot idle AFR conpensation air valve (bi-metal flat spring covering a air port) at the back of the carb body that opens up to allow more idle airflow to increase the engine idle speed and lean the idle AFR when the carb body gets too hot and over rich. (prevents hot engine over rich stalling) If this spring valve is stuck or broke off the port will be uncovered and the idle will be high even with closed throttles.

On Qjet carbs the choke and fast idle system is controled by the secondary vacuum brake pull off diaphram (it also controls the sec air valve tip in delay)
If the diaphram is shot the choke/fast idle pull off will be inconsistent.
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