Quadrajet Adjustment Question

[Godwinaustin]

I currently have my idle screw all the way out , such that it is not tensioning the cable whatsoever or putting any pressure on the little plate at the end of the screw.

my question is , am I supposed to tune the air/fuel mixture with base (minimal) tension on the cable (where the idle screw contacts the little plate) or does it not matter what the position of the idle screw is prior to adjusting the air/fuel mixture screws to highest possible vacuum?

 

[1987 GMC Jimmy]

Setting idle speed and timing are both best done in gear with a tach showing 500 RPMs. If you’re in that neighborhood where you’re set now, that should be fine, and that’s where my Q-Jet is, screw all the way out or damn close to it. Mixture on mine is set 3.75 to 4 turns out from lean, but still use the vacuum gauge. You want the highest AND the most stable reading. I look at the idle speed as a coarse adjustment, and the mixture as a fine adjustment.

 

[DoubleDingo]

When I swapped carbs on Saturday I had the same thing. Yes you can adjust everything like that. For me, the idle wouldn't go down at all, turned out to be timing was way advanced. Last night I did some more fine-tuning, and still, the idle screw is not touching the throttle lever.

 

[Godwinaustin]

When I swapped carbs on Saturday I had the same thing. Yes you can adjust everything like that. For me, the idle wouldn't go down at all, turned out to be timing was way advanced. Last night I did some more fine-tuning, and still, the idle screw is not touching the throttle lever.

I’ve got a high idle too ... won’t idle down after warming up ... something may be sticking.

I checked the timing and it is spot on at 14 degrees with the advance off.

 

[DoubleDingo]

I’ve got a high idle too ... won’t idle down after warming up ... something may be sticking.

I checked the timing and it is spot on at 14 degrees with the advance off.

That's too much. As soon as you hook up the advance it is advancing even more. That's about where mine was. Even doing 10 base timing it advanced too much. I have to time this truck with everything hooked up and just time it to 10 btdc. On mine, anything more and the idle goes up too high. My old carb I could adjust to my heart's content as long as everything was hooked up for the timing part. This carb, the only adjustment is choke, secondary air flaps, and idle mixture. There's no vacuum leaks, it holds a steady 17.5.

 

[Godwinaustin]

Also found out I’m missing the vacuum brake linkage/rod that connects to
The secondary plates ... could have something to do with it

 

[DoubleDingo]

Yeah that's not helping anything

 

[Godwinaustin]

Alright guys ... so went down the vacuum port rabbit hole a bit ... attached is the diagram from the airbox. According to this diagram dizzy should be hooked up to the other side of the carb closest to the cab, which seems to be a manifold vacuum source (It’s currently connected to the ported connection on the drivers side closest to the grille)

This vacuum fitting (circled in the second photo) was capped and when I pulled the cap a small amount of gas came out. Is that supposed to happen ? Should I connect the dizzy per the diagram to this port?

in the third photo , what I believe to be the diverter valve is hooked up incorrectly (according to the diagram) to manifold vacuum on the passenger side of the carb instead of ported vacuum on the drivers side

finally in the last photo , what I believe to be the EFE valve is completely disconnected with a broken dry brittle old vac hose, but there is still a vacuum hose connected at the Thermo switch ( along with the yellow line from the carb with the the check valve in line on it)


And finally does any of this even matter haha

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[DoubleDingo]

I know it matters what is hooked up to which port. Discovered this by swapping on a carb for an '85 c20 onto my '81 c20. Carbs look about the same but they're not when it comes to the vacuum ports and such. The throttle lever actuator used to work, now it doesn't and it was never unplugged electrically.

Hopefully your diagram is correct to your truck and your carb too.

 

[73c20jim]

I use total timing to set timing. About 34-36 degrees I found to be about right.

 

[Godwinaustin]

Since I don’t have an air pump or a
Canister or any of that other emissions stuff what does the diverter valve (Dvtr valve on the schematic) do ? Should it get ported or manifold vacuum ?

according to the schematic my dizzy should be hooked to manifold vacuum but as far as I know the newer Quadrajets pulled from ported vacuum...

 

[DoubleDingo]

I'm leaving that one for the experts that know more about the various smog components on these things. Mine has minimal on it from the factory so my knowledge is little at best about that stuff.

 

[AuroraGirl]

That's too much. As soon as you hook up the advance it is advancing even more. That's about where mine was. Even doing 10 base timing it advanced too much. I have to time this truck with everything hooked up and just time it to 10 btdc. On mine, anything more and the idle goes up too high. My old carb I could adjust to my heart's content as long as everything was hooked up for the timing part. This carb, the only adjustment is choke, secondary air flaps, and idle mixture. There's no vacuum leaks, it holds a steady 17.5.

17.5 inches of mercury? Just curious on what unit, i have no idea on normal or good vaccum, i am not familiar with the art of using a vacc gauge, which im told is very precise and handy and not hard to learn to tune it just right. Old method that seems to go by the wayside My grandpa had a vacc gauge with no face and just had tape over it with an arrow that said "GOOD". im guessing he wanted it to read there.

 

[DoubleDingo]

17.5 inches of mercury? Just curious on what unit, i have no idea on normal or good vaccum, i am not familiar with the art of using a vacc gauge, which im told is very precise and handy and not hard to learn to tune it just right. Old method that seems to go by the wayside My grandpa had a vacc gauge with no face and just had tape over it with an arrow that said "GOOD". im guessing he wanted it to read there.

17.5 is good vacuum. It's right in the middle of the "green" on my vacuum gauge. If you have access to your grandpa's gauge use it.

 

[Godwinaustin]

I’m only at 15 pounds of vacuum but I’m guessing that’s cause the idle is high , until I manage to get the idle down from where it is around 1000 (guessing) I won’t be able to pull a proper vacuum reading for fuel mixture...