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I'm working on a 66 GTO for a guy. It has a fresh rebuild of 428 Pontiac engine Edelbrock performer intake, Holley 800 spread bore double pumper carb. MSD mechanical advance distributor. It has Comp XE274H Hydraulic flat tapper cam.
It speeds up and slows down by itself. Here is a video of it running. What can cause this condition? I tried blocking the PCV off and it still does it.
Thanks
Danford1

https://vimeo.com/199460492
 

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It is possible that the carb is actually running lean on that engine. Just for fun, after checking all the vacuum lines as noted, back the idle screws out to fatten up the carb and see if it changes.

Try those things and get back with us.
 

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X2. Usually a surge at idle is due to an excessively lean condition. Vacuum leak or carb adjustment. See if it speeds up and smooths out if you drizzle a little carb cleaner into the carb or as stated, restrict the intake with your hand slightly.
 
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