Vacc leaks on an L-jet car will often result in a car that idles very poorly, rather than one that idles too high. A malfunctioning Decel Valve might cause the symptoms you're describing.
The ignition timing could also give you these kinds of symptoms as well. Make sure the hoses are hooked up correctly (i.e., if you only have one vacuum fitting on the throttle body it goes to the retard fitting pointing back toward the distributor body, and the advance fitting does not get connected to anything). Make sure the diaphragm in the vacuum dashpot is not broken. Make sure that the points plate moves smoothly when you apply a vacuum to the retard fitting. And make sure that the points plate moves reasonably smoothly by hand as well.
--DD
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Pelican Parts 914 Tech Support
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