59BlueSilver

It seems to me that it takes an exceptionally strong spring to get the primary throttle plates to close. If I use an ordinary spring, then I get a fast idle sometimes. Anybody else have trouble like this, or do I have something adjusted wrong?

Shark Racer

If your front throttle blades bind, it could cause this. Binding can either be maladjusted plates or worn/bent throttle shafts.

A basic test I do is check to see if the throttle blades will shut completely with the throttle springs disconnected, cable disconnected and engine off. The accelerator pump return spring should be able to return the primary throttle blade to closed. If not, you've got a bind somewhere.

Shark Racer

Additionally, I used twin springs on my Q-Jet, as per OEM:

7T1vette

You have binding somewhere. It could be in the carb, in the linkage, in the throttle cable, in the accelerator pedal or in any combination of them. Test the resistance of each without the others (if possible). Remove cable from carb and check linkage/mechanical resistance to full range of throttle movement. Then check resistance of cable movement, now that it is not connected to the carb. Get in the car and move accel pedal by hand.

Hope you find the problem.

P.S. It could be that the carb is not lined up 'true' with the gasket and intake manifold. This can be a source of binding (between throttle plates and gasket and/or intake inlet bores).