Quadrajet Carb idle


#1

I have an 86 chevy with quadrajet carb. When it’s warm or cold the idle is correct. If I drive it and let it sit for a little while and it’s not at normal operating temp but it’s not fully cold, the idle races and will not drop until it’s at temp. If i try to shut it off it will run on. What can I adjust to bring the idle down for this condition.


#2

Is there a fast idle adjustment on this or the choke pull off working?


#3

The fast idle seems to be working, but it won’t kick down when below temp but above cold.


#4

Whatever kicks it off, vacuum diaphragm or temp sensor might either be weak or need to be adjusted a little leaner. On my 59, I finally just disconnected the dang thing because it had trouble kicking off. Wait for a real mechanic though to respond.


#5

It could be a binding choke mechanism. Have you cleaned it?


#6

I had that problem on my truck – not a quadrajet but an autolite 2100. But my experience may apply to you too. Idle speed was fine cold, and fully warm, but didn’t transition from cold to warm like it should, idling too fast at the mid-warm point. I fixed most of the problem by adjusting the various idle rpm & choke linkage adjustments, several iterations of this, narrowing in on the best combination. I had to read the carb rebuild instructions and look at the diagrams for quite a while before I figured out how it is supposed to work and could come up with a strategy that would converge. I decided I didn’t like it to idle too fast anytime, so this was quite a challenge. But eventually I got all the screws adjusted so it behaved like I wanted, idle rpm vs engine temperature-wise.

Even after that it would still occasionally idle too fast at the mid-warm point. How frustrating. I finally figured it out, exactly what @texases says above, parts of the choke linkage wasn’t moving freely, binding up a little at times. I bent one of the rods a little to provide some more clearance, and sprayed that bit with wd40, and that finally got it working like I wanted. I have to say the adjustments are not obvious what you need to do and are very sensitive. Being 1/8 turn off on just one the screws can make it perform poorly.

If you got an electric solenoid for the throttle that pops out when the engine is running (or the key is on), and pops back in when you turn the engine off to prevent dieseling, make sure that is working consistently too.


#7

I’d also check for vacuum leaks in those hoses and fittings.


#8

Thanks for the replies. Hopefully I’ll get a chance to play with it Friday. It’s supposed to rain tomorrow.


#9

What vehicle is this carburetor in? Is there an electric idle stepper on the throttle?


#10

86 chevy 305 C10 pickup. Has electric choke.


#11

It no longer has the plug in stepper.


#12

Is your carburetor the feedback model?


#13

Texases reminded me I was thinking of the vacuum choke pull off not the fast idle pull off. If its electric though, I think those were troublesome. At any rate next time just take a look at the choke position to see where it is and if the fast idle is still holding the throttle open. Then you’ll know which one is the problem.


#14

It was the feedback model, but I swapped it with one without the feedback. It has electric choke. I guess I’m a little ignorant on carbs.


#15

You can’t replace a computer controlled carb with a mechanical one.

First. It just won’t work.

Second. You tampered with an emission control device…

How much money did you waste on this carb?

Tester


#16

The original carb died because of the ethanol. It runs fine except the fast idle during non operating temp start. There is no computer. It had a spark advance module that plugged into the distributor. I replaced the distributor with an earlier distributor. I only had to pass emissions in Ga until the truck hit 25 years old. I wasted $200.


#17

That’s a Rochester carb, right? My local Barnes and Noble bookstore carries a workshop book on those, how they work, & how to tune them. Next time you go to B&N, take a look in the transportation section.

Not sure it applies to you, but fyi, my truck has these five adjustments designed for setting idle rpm

  • warm idle speed (affects throttle plate angle when engine is warm, choke totally disengaged, and idling)
  • fast idle speed (affects throttle plate angle when choke is at least partially engaged and idling)
  • rich/lean choke (affects choke plate angle vs temperature curve)
  • fast idle cam adjustment (fast idle rpm vs choke plate angle curve)
  • choke pull off (affects choke plate opening angle when engine is cold & running)

hmmm …Of those the choke pull off fast idle cam adjustment seems like what would affect your situation. Still, I’m wondering if you got some kind of problem with the choke heat riser tube. The whole kit and caboodle won’t work if the choke doesn’t know how warm the engine is. The choke heat riser tube is how the carb knows the engine is warm. If it wasn’t working correctly, even tho the exhaust manifold was still warm, the carb might not realize it, and would therefore idle too fast. I had to replace that part on my truck last summer. Suggest to take a look. On my Ford truck, here’s how it is configured: there’s a metal tube from the carb or air cleaner – forget which – to the exhaust shield, and another tube from the exhaust shield back to the carb. Creates a thermal loop, aided by engine vacuum, and that heat is what heats up the spring in the carb’s choke housing.


#18

George.

The choke on this carb doesn’t use a choke riser tube.

Carbs since the mid-70’s use a heated choke spring powered by the alternator to open the primary choke.

Tester


#19

Yes, those two tubes in Tester’s photo above are what they look like. My early 70’s truck uses both the heat riser tube method and an electric (alternator) heated choke spring. Ford engineers weren’t messing around taking any chances the choke function didn’t turn off quickly after start-up I guess. If the OP’s quadra-jet carb uses only the electric choke method, then that part should be suspect. Actually the electric choke on my truck hasn’t worked for a while, I use only the heat riser method now. High resistance measured in the electric choke. I think the heating coil burned out or the thermo-controlled contacts have corroded.


#20

So here’s a question - did it EVER run right?