I’m asking for advice on my ‘70 Cutlass Supreme with stock 350 and 7040250 quadrajet. The idle is set at 650 rpm, however, after driving for any length of time, it’ll stay at around 1000-1200 rpm. It seems to not be returning to normal idle. Any suggestions welcome!

 

Is the 650 RPM idle when the engine is fully warmed up?
Have you verified the choke is fully open / disengaged?
Does the idle speed drop if you blip the throttle?

 

When it is not returning to idle, look at the fast idle cam on the passenger side of the carb. If the cam is not fully in the down position, this will cause the idle to be held high. The other thing to look for is the gasket between the throttle body and the float bowl. I've had this happen on one carb, and it turned out that the gasket was a little small at the throttle bores, allowing the throttle plates to hang up on it occasionally.

 

Idle set at 650 rpm at operating temp. I will check aforementioned items.

 

Also disconnect the throttle linkage to make sure its in the carb only.

 

If previous post don't reveal a problem, put a timing light on it. Something in the distributor may be sticking causing unnecessary advance at idle.

 

IN my many years of working on these cars, it seems that a throttle setting that gives 1000 in Park turns into 650 in Drive with the AC on. Hot or cold, AC or not, in gear or park all make a difference with a carb. Need more info.

 

Car is a non-A/C, 4 speed so that should keep it fairly simple. I discovered one of the idle mixture screws was quite a bit different than the other. I screwed them all the way in and currently have them 4 1/4 turns out. After driving it a while, that seemed to correct the issue.

 

Mine has the same problem. The choke is sticking slightly on. I can get it to drop back down by stomping the accelerator but as soon as I get on it it sticks again (I can feel the car slightly bucking while driving). It's a brand new rebuild by a reputable rebuilder. I already went through resetting the idle mixture screws, lowering the fast idle, and just about everything else I can think of. I've talked to the rebuilder and he tells me its the electric choke but I've moved that in every possible position and it hasn't helped.

 

I had the same issue on the carb in my '84 GMC; FWIW, replacing the electric choke thermostat solved the issue.