High idle after installing carb spacer

[gizmoduck81]

Hi all,

Short version: I installed a 0.5" carb spacer, no other changes, and I can't get it to idle below 2500.

Long version: 1963 GMC with a 454 out of a 1988 Suburban. Edelbrock 1411 750cfm carb, Edelbrock Performer RPM intake, and "RV" cam from prior owner. I'm assuming the heads are stock, but I haven't checked. It was running great, fast idle at 1500 and normal idle in D at about 650-700. I was having problems with vapor lock, so decided to try installing a 4-hole spacer to see if it helps. I didn't adjust anything and put everything back as it was. Turn the key and it climbs to about 2500 rpms. The throttle isn't stuck, I don't detect a vacuum leak anywhere, adjusting the fast idle screw does nothing.

Any ideas?

Thanks,

Ryan