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Quadrajet APT adjustments and my theory

51K views 7 replies 5 participants last post by  RWENUTS 
#1 ·
Welcome to my world. fixing stuff that seems to be unfixable or unadjustable.

lets talk about the Quadrajet APT adjustments..

lets me describe my knowledge of these..

because the Quadrajet has a very small primary bore. it transitions to the primary mains very early in the throttle opening..

The APT adjustment is really not supposed to be adjusted.. its factory set . but we are hotrodders... we like the ability to snap heads back.. light the rear tires up at the touch of the throttle..

when the throttle opens lightly and passes thru the idle transition circuit.. the primary mains come into play and the air flow thru the booster venturi's starts pulling fuel up and out of them..

the APT adjustment is how far the manifold vacuum pulls down on the primary rod holder..

if the APT is set too low.. the primary needles will be too far into the primary jets and cause the carb to go lean.. this will require more throttle to reduce manifold to get the power valve spring to over power the vacuum to lift the primary rod holder.. to get enough fuel to make any power..

if the APT is set too high... it will be rich as the primaries start flowing.. but it will also be rich during low to moderate speed cruise where the manifold vacuum is pulling the primary rod lifter down as far as it will go.. step into the throttle.. the vacuum drops, the power valve spring lifts the rod holder and the primary goes richer... reach a steady speed.. the manifold vacuum comes back up and the metering rods go back down against the APT stop..

let me describe a friends Quadrajet i did a few years back.. it was a butcher job.. everything disconnected.. the plug missing from the top so somebody had been messing with the APT setting..

after i rebuilt the carb.. i started checking the APT adjustment. i happen to have a proper tool from kent moore that i have had for 3 decades... i found that from fully seated.. i could get the APT screw about 2 turns up before it would hit the pin on the front of the primary rod piston.. i knew that this would be too lean.. so i screwed it up 3 turns. i was holding the primary rod holder down with my thexton 370 dipstick..

i put the carb back on the truck.. got the idle mixture set using a vacuum gauge... lean best.. and got the idle properly set.. for those not familiar.. vacuum gauge hooked up to manifold vacuum. engine warmed up.. the choke should be wide open at this point. start by turning the idle mixture screws inward till the manifold vacuum starts to drop. then go back until the manifold vacuum does not increase.... do both screws... then turn them in until the manifold vacuum just starts to drop.. go back to the point where its right at the spot where it starts to drop.. using the vacuum gauge. i turn the screws in just slightly so with both adjustments i have reduced manifold vacuum by 1/2" of vacuum on the gauge. if you have assembled the carb properly.. both screws should turn about the same fraction of a turn to reach this. its usually 1/8 to 1/4 turn.. sometimes only 1/3 of a turn. this gives you lean best idle..

now.. with an educated driver behind the wheel.. lots of room behind and in front of the vehicle.. nothing to hit.. have them do a part throttle acceleration. i did this in the length of 2 parking spots. you only need a part throttle test.. if it backfires.. you need to raise the APT adjustment 1/4 turn.. back the vehicle up and try it again... if it backfires. give it another 1/4 turn upward.. continue this with 1/4 turns upward... soon it will be accelerating really hard.. you will find a point where the acceleration no longer gets better.. start turning it the other way.. you want to reach the point where the acceleration is the best..

your APT adjustment is now done.. its that simple..

because of so many variations in engine size and carb primary jetting.. you will have to play with this adjustment..

its far easier on carbs with the APT adjustment that is done thru the top of the carb...

the earlier versions are much harder to adjust.. as the APT screw is behind the tamper proof plug in the front of the base plate.. it bends a wire up that actually sticks up above the surface of the base plate. you have seen that if you have done any 65 to 75 quadrajets.. i bet you wondered why the early quadrajets have a pin that sticks out the bottom of the main body.. that is what hits the APT adjustment sticking up from the base plate.

there are a few quadrajets that have a very odd metering system.. they have 2 primary rod holding pistons.. instead of just one. i have not tried to deal with the APT adjustment on these..

you can fine tune the APT adjustment if you have an air fuel ratio display..

but please take your air fuel readings at 1400 to 1900 RPMs.. at a steady throttle. if its lean.. try raising the APT by 1/2 turn.. see if that cures your issue. if its rich.. turn it down by 1/2 a turn and see what happens..

a quarter turn makes a huge difference in fine tuning the APT.. Adjustable part throttle adjustment.




this is just a few thoughts..

and a few images.

the thexton 370 dipstick tool is 39MM from the tip to the beginning of the 1/64 scale..

that or something else can be used down thru the bowl vent tube. to depress the primary rod holder against the power valve spring and the APT adjustment.
 

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#2 ·
Good info Wayne!!
I've always adjusted mine to the 'step in stumble'
method.
I usually start at 2 turns.
When I first drive at low speed/rpm if it stumbles a bit upon light pedal movement, then I'm set too lean and adjust up 1/2 turn at a time until the stumble goes away then adjust back 1/4 turn until it stumbles again. Easy to find the sweet spot like that.
Of course ambient temperature will effect your adjustment too. Some might have a summer and winter setting.
And people need to realize every carb is different. Production tolerances is likely a possibility. Last quad I had I set at 6 turns from bottom. And I've had one that liked 1 1/2 turns from bottom.
Awesome carbs to work with!
 
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