That does sounds a bit low. I don't have specs in front of me, and maybe someone with a '78 can tell ya, but i would imagine the correct idle speed to be around 650rpm, set with an automatic trans in "D"(with parking brake firmly set!).
Does your car still have the Federal emissions decal on the firewall? That will give the correct idle speed for the car, if it is still there...and readable.
For adjustment info, and other great tips on the Quad carb, check out Zen-Master Ken's
Carb Article here.
Click on the link, click on "Ken's Quadrajet". You'll learn more about a Quadrajet than you ever wanted to know!!
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"