APT adjustment screw adjust to smaller or larger jet sizes

[GSP7]

If I have to adjust my APT many, many turns up, like 5-6-7 etc etc turns , to try and get rid of hesitation/"boog" ... Does it need a bigger primary jet ?

In other words: Will a  larger jet size need less APT screw turns up than a smaller jet size?

My original 17080513 carb(1981 chevy K20 truck)APT was set at 2 3/4 turns up. Ran perfect

My new rebuilt 17080213 carb with the same size primary jets and rods installed as the above carb is requiring alot more APT turns up, am still  trying to get rid of a slite hesitation/ Boog off idle throttle acceleration.

I would think the APT adjustment would be about the same as the original carb

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[GSP7]

[GSP7]

Checking in for any answer after 90 views

 

[Frank400]

well, the apt controls the light load cruise mode.  So if you're lean in that area and turning the apt screw counterclockwise still does not work (I am assuming a later carb with the apt screw accessible from the air horn), then you have to go to a smaller metering rod.

[Kenth]

The answer to your question is, yes.

And, even if the same jetting there is often different idle/lowspeed calibration between Calif. and other units especially if one is Rochester made and the other Carter made with the same application number on it.

The "tip-in" test will tell you what the engine needs.

HTH

[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

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[lightning boy]

What size is the main air blleed?

[novadude]

What size is the main air blleed?

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

[GSP7]

Im at 5 turns up on my APT screw,,,,, seems like the boog off idle throttle position maybe gone,,, maybe... maybe not.. (I only drive the truck occasionally down the hill from my house )

Is 5 turns up on the APT unusually alot more than average adjustment???

That "tip in" procedure mentioned above didnt do anything when I first installed the carb and did some tuning

[novadude]

I wouldn't worry about 5 turns.  In my experience, 3 turns +/- 2 turns is in the range if the jets and metering rods are right.  If the tip-in test does nothing, it may be rich.  I wonder if your problem might be elsewhere?  Does the accelerator pump give a shot of fuel with a small amount of throttle movement?

[GSP7]

I wouldn't worry about 5 turns.  In my experience, 3 turns +/- 2 turns is in the range if the jets and metering rods are right.  If the tip-in test does nothing, it may be rich.  I wonder if your problem might be elsewhere?  Does the accelerator pump give a shot of fuel with a small amount of throttle movement?

Im now at 5 - 4 1/2 turns and seems like the bog is gone. I only drive the truck once in awhile and have to go down a hill 2 1/2 miles leaving my house to test on flat road or back up the hill

I had cranked the APT all the way up to 7 to get rid of the bog then back it down bit by bit and test drove. over many days.... was down to 3 1/2 and thought the bog was gone, nope. Now im at 4 1/2  from 5 . I'll do several more test drive to see if the bog/hesitation is stiil gone or not



.

[lightning boy]

IF it runs well at 5 turns that's not too much.

[quadrajam]

  Power piston spring could be weak.Get a new spring and put the 73 jet back.

I assume you've double checked the acc pump function and proper hanger arm
geometry as previously posted....??

QJ