C3 Won't run under 1000 rpm

[studiog]

My brother in law just rebuilt his 71 small block and it runs fine on the highway and when you get on it but it won't idle under 1000 rpms. Checked all vacuum lines, timing, dwell, intake manifod for leaks, plug wires etc. It acts as if the carb idle circuit isn't functioning at all. When you try to drop the rpms below 1000 it is just like you turned the key off... it just totally dies. Anyone have any ideas? Thanks for any help.

 

[studiog]

I think it is a stock Rochester Quadrajet

 

[I_R_DA_ONE]

The problem sounds like one of two things......first a vaccum leak around the carb...or the idle jets in the carb. are to small,, they should be .57 or larger to make it work

 

[JohnZ]

Sounds like the idle mixture screws are turned all the way in and it's idling on the transition slot instead of the idle passages. Put a vacuum gauge on it and adjust the idle mixture screws properly.

 

[studiog]

Sounds like the idle mixture screws are turned all the way in and it's idling on the transition slot instead of the idle passages. Put a vacuum gauge on it and adjust the idle mixture screws properly.

John, We tried to adjust them but they seem to have no effect at all whether they are all the way in or out.

 

[studiog]

Thanks all for your suggestions, very helpful.

 

[JohnZ]

John, We tried to adjust them but they seem to have no effect at all whether they are all the way in or out.

Classic signal that the throttle blades are open too far at idle, exposing way too much of the transfer slot, rendering the mixture screws useless; close down the throttle blades (idle speed screw on the linkage arm) and the idle mixture screws will become active again.