Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

APT adjustment screw adjust to smaller or larger jet sizes

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GSP7:
Original Carb is a Rochester 17080513 on a 1981 chevy k20 truck Calif, has #72 jets, #51 rods, APT was 2 3/4 turns up(counterclockwise) from seated

Carb I rebuilt with Cliffs parts is a Rochester 17080213, had #73 jet, #49 rod ( dont know if it was off a 454 or what)

On the rebuilt 17080213 I installed new #72 jets, and (cliff's custom rods) #50c rods ( trying to match pretty close to the original 17080513 original carb on the pickup truck

So Im wondering if put the larger #73 jets instead of the #72 jet with the custom full tapper 50c rods so I dont have to turn the APT up so far(counterclockwise) .

Also is there a point turning the APT counterclockwise where its too far up (almost all the way up/ no more threads) and you should install larger jets and or smaller rods?

Is the APT screw turned up 5,6,7,8 turns to far ? I think 6, 7,8 turns and the APT screw is out of threads and removed...

PS I live in Idaho Now and do Not have DMV smog(emission) testing like calif does

.
 

lightning boy:
What size is the main air blleed?

novadude:

--- Quote from: lightning boy on September 22, 2025, 12:46:49 PM ---What size is the main air blleed?

--- End quote ---

I was thinking the same.  If main air bleed size is different in the new carb that could explain it. 

As Kenth pointed out, the idle circuit is also a player in that light load transition zone.  Is everything the same there? 

Finally, the power piston hanger arms could be bent a little different in the new carb, placing the rods lower in the jets for a given APT screw setting.  With as old as these things are, there is no telling who's been in it, and how many times that PP was handled, dropped, etc.  Doesn't take much to bend those stamped arms if you are a heavy-handed mechanic.

GSP7:
According to page 32 , this year car 1980-81 the main air bleed is only in the air horn.... Right?

I'll try to measure both carbs main air bleeds

I know I spent alot of time comparing both carbs and checking passages... I remember the idle bypass air holes were the same size etc

P.S. quick check the 17080513 carb air bleeds are at least .045 probably more like .050 ... Do you have references of air bleed size possibilities?

GSP7:
Measured both carbs main air bleeds .....  both .050..... tried a  .060 rod and will not fit in either carbs main air bleed

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