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Hi all.
I have a 66’ Mustang with a 289. Motor was built in the 80’s and now in my 66’. Car that motor was in the 80’s got hit so I pulled motor out and it’s been sitting in garage ever since, till now. Put new seals on motor and upgraded the carb to a Edelbrock AVS 2 and a HEI distributor.
Problem I ran into is when car is warmed up it idles at 1250 rpm. Put C4 auto in gear and idle drops to 350-420rpm.
New gas tank. I checked compression and most cylinders were at 120-125psi. #1 and #8 were at 90psi. So I took another round of compression readings and a few of the 120psi now dropped to 90 psi. My only thoughts are the lifters are failing after sitting in engine 30 years plus.
Any thoughts would be greatly appreciated.
Thank you.
 

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What happens when you do a wet compression test? Have you pulled the valve covers and checked to see if all valves are moving as they should? May want to try a leak down test. Any vacuum leaks?
 

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Sounds like a few things. And as 70XL suggested, vacuum being one of them. Try spraying carb or brake cleaner around the base of your carb, intake edges, etc. The symptoms of an extremely high idle out of gear, and such a low one under any kind of load leads me to a vacuum leak around carb or intake. I would also do a leakdown test to see how long it takes for cylinder pressure to drop. The 80s were thirty years ago, and that is a LONG time to expect gaskets to holdup and not shrink harden, or retain their seal.
 
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