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Discussion Starter · #1 ·
When adjusting the mixture screws on my Rochester quadrajet four barrel, as I turn the screw either in or out , the rpm never changes. I can seat them or turn all the way out with no change in rpm. The funny thing is it runs fine. Just curious if this is an indication of something more sinister lurking.
 

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I'll assume the choke is open and it's not running on the fast idle cam...

You have the idle speed screw turned in too far, it's idling on the transition circuit..

I bet you either don't have the vacuum advance hooked up or have it hooked up to a ported vacuum source. Hook it up to a port with full manifold vacuum and turn the idle speed screw out until the throttle blades are closed, then in about a half turn...
 
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I agree with whats been said That's a common mistake.^^^ On the passenger side of the Q-jet is the fast idle cam with a screw that's only for the engine speed when the choke is on.After the engine is hot, that screw shouldn't be holding the throttle open at all.
So with the engine fully at operating temp try backing the curb idle screw,the one that's on the drivers side of the carb all the way off,then you should get some engine changes with the 2 mixture adjusting screws in the front of your carbs baseplate.You may have to adjust your timing at this point.
 

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Discussion Starter · #4 ·
It's been awhile since I read this post. I have had the engine at operating temperature and backed off on the curb idle till it just about she down. Still no change when adjusting. The fast idle is adjusted orrectly as far as I know. Next step is........
 

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Ok when discussing idle settings you'll need to accurately check the idle speed and the timing settings.If there is no response when you turn the mixture screws it means the carb is getting its idle fuel from somewhere other than the idle circuits.
Turning the mixture screws all the way in should get the engine to at least start to stumble. `
 

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How fast is it idling?
 

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What year Quadrajet? I remember the early carbs would leak fuel from the two plugs in the bottom of the carb body which plug the holes that were drilled in the two circuits,there is a well in the base plate for clearance of these two circuit passages.I know if these plugs leak fuel it can cause a rich condition @ idle and idle mixture issues.Most quadrajet carb kits come with a little neoprene sponge which fits into the well in the base plate and resolves the leak issue.My two cents worth,I've rebuilt a lot of quadrajets over the years.
 
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