General Category > Quadrajet Carb Talk and Tips

AFR dropping just off idle

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Cadman-iac:
  What would you do to correct a lean condition at just off-idle, or just as the throttle starts to open?
 Would a larger down channel restriction fix this, or a bigger upper idle air bleed?

 Running the mixture screws out until the AFR is rich still doesn't correct the lean condition at throttle opening, nor did raising the APT screw.

 So which would you change, upper idle air bleed, down channel restriction, or idle tube size?

 This occurs in gear when opening the throttle to check the AFR during the transition to the main circuit. Idle speed set at 550 rpm in drive, and the lean condition starts at 650 to 900 rpm, too low for the main circuit to begin working.

 Thanks for any advice, Rick

Kenth:
I would set idle speed at 650 rpms in drive, then adjust the idle needle screws for best vacuum/rpm.
Reset the idle speed if nessecary and check the idle needle screws last, both the same amount of turns out when finished.

If that does not help, start with open the idle tubes .002", downchannels .003" and check for at least .086" idle needle holes.

HTH

Cadman-iac:
  I increased the idle in drive to 650 and adjusted the mixture as suggested but it made no difference.
 So I increased the idle tubes by  .003" and the down channel restriction by .004" and left the idle ports at .065", then dropped the idle back to 550 in drive. This improved the off-idle lean condition quite a bit but it's still a little lean yet between 600 and 850 rpm in drive.
 I didn't want to change the idle ports as it's not reversible if ineffective, the idle tubes and channel restrictions can be reversed if necessary.
 So with that done, do you think if I increased the two areas I've already changed would help eliminate the problem or should I try increasing the idle ports?

 Would bigger bypass air help here?  That is currently at .039". My thought is it wouldn't as it doesn’t increase the fuel availability any, only the air so may make the problem worse, but I thought I should ask anyway. What do you think?

Kenth:
I have found that .086" is the needed minimum size on the idle screw ports for most performance build to not have the screws falling out of the threads.

How many turns out are the idle mixture screws?
Does the rpm decrease when turned out further from best vacuum/rpm setting?

77cruiser:
When I put the IAB in the top they can be changed without pulling the carb apart. Makes it handy for these kinds of cases. But if you want to start modifying to that extent I understand.
It's easier if you have a couple of junk cores to practice on before hand.

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