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On my '68 302-2V I have been having this ongoing issue with the engine idle ramping up and dowm. It isn't rough but as it sits there it will climb from about 900 rpm up to about 1400 and then go back down to 900. I rebuilt the carb and set the idle mixture screws at between 1 1/2 to 2 turns out and adjusted the idle speed to 900. I would have set it closer to 650 but with this up and down the engine would sometimes just quite on the down cycle.

Looking this up suggestions led me to look at the vacuum hoses. I decided to replace all of them as they were pretty old. Good thing since they were hooked up wrong to the distributor vacuum valve according to my Chiltons manual. But even then it still has this up and down idle going on. Anyone seen this or have any ideas to share?
 

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It sure does sound like a vacuum leak. Get a can of brake cleaner and spray quick shots around the base of the carb and the carb spacer and see if the idle picks up. If it does then you found your leak.
 

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Try capping off all vacuum realted stuff first - start from there to see if it's something on the motor itself (try spraying around intake runners too). Then start intoducing vac lines/etc until you find which way to look. Divide and conquer!
 
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