Quadrajet help please. What’s this adjustment!?

[Itali83]

Rebuilding my Q-jet on my new truck. I can’t find any videos telling me what this screw is. It has a spring under it so I’m assuming it’s an adjustment of some sort. Every video of Q-jet rebuild shows a carb that doesn’t even have this at all. Maybe a truck specific thing? This is a stock carb original to my ‘76 gmc K25. Thanks!
Ben

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[Itali83]

Figured it out. It’s the adjustable part throttle mixture screw. I didn’t know what it was during disassembly so I didn’t count the turns it was out from bottomed out. Seems the googler says 3 turns out is a good starting point. I pushed the plug out of the air horn, tapped for 1/8” pipe plug and now I can adjust it easily with no disassembly.

Ben

 

[bedwards]

That is your mid range jet. Most say don't adjust it. I drilled and tapped my carb body and slotted a screw diver to fit it. Got it adjusted through trial and error then put a hex plug in it to seal the carb.

Oh well, I'm a little slow typing. Glad you figured it out.

 

[Itali83]

That is your mid range jet. Most say don't adjust it. I drilled and tapped my carb body and slotted a screw diver to fit it. Got it adjusted through trial and error then put a hex plug in it to seal the carb.

Oh well, I'm a little slow typing. Glad you figured it out.

how did you go about adjusting it?

Ben

 

[bedwards]

You won't like this but I ran it in untill you could tell it was too lean then opened it up till it took the throttle good and ran good. The plugs had always been black since I bought the truck now they look real good.
Its similar to idle mixture except at partial throttle.

 

[Matt69olds]

You won't like this but I ran it in untill you could tell it was too lean then opened it up till it took the throttle good and ran good. The plugs had always been black since I bought the truck now they look real good.
Its similar to idle mixture except at partial throttle.

That’s exactly how you fine tune it.

Pick a road where you can cruise at highway speed. It needs to be level, and several miles long, with the weather typical for your area. Don’t try to tune it during a cold snap, not during a heat wave. Keep leaning out the APT screw until you get a lean miss, that’s the lean limit. All you need to do at this point is slightly richen the mixture.


Once you find the happy point, they make springs with different tension. If you start to crowd the throttle and you get a soggy throttle, you need a spring with slightly less tension. The lighter spring will allow the metering rods to come up out of the jet with less vacuum.


The Holley guys say a Q-Jet is too complicated, these are the things that make a Q-Jet shine in a daily driver. You will never equal the throttle response and fuel economy of a well tuned Q-Jet.