The APT (Adjustable Part Throttle) Screw
Thread Starter
Instructor
Joined: Jan 2006
Posts: 223
Likes: 14
From: South Florida
The APT (Adjustable Part Throttle) Screw
Lars, or anyone else that might know.
What is the factory setting for the APT adjustment screw? I have heard 8.5 turns out. This is for a Rochester Q-Jet Model 17059216 with all stock parts. I have no idea
where it was when I started...
Thanks!
What is the factory setting for the APT adjustment screw? I have heard 8.5 turns out. This is for a Rochester Q-Jet Model 17059216 with all stock parts. I have no idea
where it was when I started...Thanks!
Thread Starter
Instructor
Joined: Jan 2006
Posts: 223
Likes: 14
From: South Florida
Originally Posted by stingr69
You just have to count the number of turns when you remove it or you are screwed.
I did it too.
-Mark.
I did it too.
-Mark.
Le Mans Master
Joined: Mar 2004
Posts: 7,510
Likes: 1,519
From: Little Rock AR
I do not know but I would consider trial and error. The adjustment affects part throttle mixture so I would assume lean is one direction and rich is the other direction. One day I might try using my wide band 02 meter to play with it. Just not enough hours in the day recently.
Otherwise I may just pull it off and send it to Lars. (wave white flag here)
-Mark.
Otherwise I may just pull it off and send it to Lars. (wave white flag here)
-Mark.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
It's a very finicky adjustment.
The service manual recommends you buy a new or professionally rebuilt carb if you mess up the APT.
It requires a lot of specialized equipment to reset the APT once it has been messed with.
I suppose you could guess.... but I tried that on my Silverado Q-Jet and it didn't work too well... ended up replacing it with a Holley 80555-1. Had to replace the computer controlled distributor as well.
The service manual recommends you buy a new or professionally rebuilt carb if you mess up the APT.
It requires a lot of specialized equipment to reset the APT once it has been messed with.
I suppose you could guess.... but I tried that on my Silverado Q-Jet and it didn't work too well... ended up replacing it with a Holley 80555-1. Had to replace the computer controlled distributor as well.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Here... I'll save him the trouble...
"All Chevy passenger car Q-Jets from 1975-up have the APT - not just the ECM-controlled carbs. There are 2 versions of the APT: One is a brass adjuster located just forward of the power piston. This one sets a lower stop limit for the power piston, thus setting part throttle (cruise) mixture. It has no effect on WOT mixture.
The other configuration has no piston lower stop limit, allowing the primary rods to pull fully into the jets. Instead, there is a second metering jet in the forward passenger side corner of the float bowl with a large, round insert above it. This insert is a spring loaded "metering rod" that is adjustable from the top. The position of the rod in the "jet" changes mixture across the entire operating range.
For emissions purposes, the only way to adjust either of these settings is with a 4-gas analyzer. If emissions are not a concern, you can knock the cover plugs out of the carb airhorn and gain access to these adjustments from outside the carb. You can then set them up for best throttle response and drivability, which will not be best emissions..."
"All Chevy passenger car Q-Jets from 1975-up have the APT - not just the ECM-controlled carbs. There are 2 versions of the APT: One is a brass adjuster located just forward of the power piston. This one sets a lower stop limit for the power piston, thus setting part throttle (cruise) mixture. It has no effect on WOT mixture.
The other configuration has no piston lower stop limit, allowing the primary rods to pull fully into the jets. Instead, there is a second metering jet in the forward passenger side corner of the float bowl with a large, round insert above it. This insert is a spring loaded "metering rod" that is adjustable from the top. The position of the rod in the "jet" changes mixture across the entire operating range.
For emissions purposes, the only way to adjust either of these settings is with a 4-gas analyzer. If emissions are not a concern, you can knock the cover plugs out of the carb airhorn and gain access to these adjustments from outside the carb. You can then set them up for best throttle response and drivability, which will not be best emissions..."
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Thread Starter
Instructor
Joined: Jan 2006
Posts: 223
Likes: 14
From: South Florida
Originally Posted by Tom454
Here... I'll save him the trouble...
Alternate “Quickie” Procedure
If you don’t have the tools to perform the measurements described above, and you just want to get the carb “into the ballpark” for some good performance, you can do the following quick verification and setup:
With the airhorn removed off the carb, push the power piston down to the fully seated position with your finger. Observe the relationship between the top “lip” of the plastic retaining ring and the top edge of the power piston cylinder. In its correct position, the power piston cylinder top edge should be about .020” above the top lip of the plastic ring. You can raise or lower the piston from this position ½ turn at a time to fine-tune your idle & cruise mixture (pop the silver plug out of the airhorn to gain access to the adjustment with the airhorn installed – you can turn the screw with a pair of long, thin needlenosed pliers.
Thread Starter
Instructor
Joined: Jan 2006
Posts: 223
Likes: 14
From: South Florida
Originally Posted by Tom454
I did that numerous times... was able to get the truck runnig well, but it flunked emissions.
By the way, did you pop the silver plug out of the airhorn so you could adjust the APT while on the car and running? If so, what did you put in place of the plug? A screw? Rubber plug?
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Originally Posted by Eric79-L48
Ahh Yes...I see... Well, fortunately for my APT settings, here in Florida we have no emissions...maybe not so fortunate for the air though...
By the way, did you pop the silver plug out of the airhorn so you could adjust the APT while on the car and running? If so, what did you put in place of the plug? A screw? Rubber plug?
By the way, did you pop the silver plug out of the airhorn so you could adjust the APT while on the car and running? If so, what did you put in place of the plug? A screw? Rubber plug?
Yes... you are lucky in that respect.
This topic is an old one that I've been part of for several years on this board. Since it began Lars has made a lot of progresss in defining the parameters we can all work with... Kudos.
On the plug issue, I removed the top and popped the plug out, then I tapped the hole with a 1/8" pipe tap (thanks Clem for that suggestion). I bought a 1/8" brass slotted (screw type) plug from Lowes and installed it. The trick is to tap the carb only enough so that your plug is semi-tight & flush with the top of the carb to avoid interference issues (which I had on the Silverado). This involves trial & error & patience. I.E.- if you go too far with the tap, a pipe plug will just fall inside the carb when you try to install it.
Last edited by Tom454; Feb 16, 2006 at 08:57 AM.
Racer
Joined: Nov 2004
Posts: 485
Likes: 1
From: Oakville ON
Originally Posted by Eric79-L48
Lars, or anyone else that might know.
What is the factory setting for the APT adjustment screw? I have heard 8.5 turns out. This is for a Rochester Q-Jet Model 17059216 with all stock parts. I have no idea where it was when I started...
Thanks!
What is the factory setting for the APT adjustment screw? I have heard 8.5 turns out. This is for a Rochester Q-Jet Model 17059216 with all stock parts. I have no idea
where it was when I started...Thanks!
Thread Starter
Instructor
Joined: Jan 2006
Posts: 223
Likes: 14
From: South Florida
Originally Posted by Rich's'78
The total thread length is 8.5 turns. Turning CW lets the needles sit lower in the jets and leans the mixture. Get a piece of tubing and squish it so one end has 2 flat sections to match the brass screw flats. Screw it all the way in / down and then back out 3.5 turns. Drive the car - if it surges under steady cruise at 60 mph, back it out 1/2 turn and repeat cruise test. I fit feels good at 3.5 turns out, try turning it in 1/2 turn at a time and check cruise. I have found that if you can determine where you can just feel lean surge, 1/2 turn out (richer) will be good (having an O2 sensor system really helps).
Racer
Joined: Nov 2004
Posts: 485
Likes: 1
From: Oakville ON
Originally Posted by Kalway
The car will suddenly speed up when the throttle is held in the same spot.
1st Gear
Joined: Jan 2025
Posts: 1
Likes: 0
Qjet apt adjustment with vacuum gauge
It's a very finicky adjustment.
The service manual recommends you buy a new or professionally rebuilt carb if you mess up the APT.
It requires a lot of specialized equipment to reset the APT once it has been messed with.
I suppose you could guess.... but I tried that on my Silverado Q-Jet and it didn't work too well... ended up replacing it with a Holley 80555-1. Had to replace the computer controlled distributor as well.
The service manual recommends you buy a new or professionally rebuilt carb if you mess up the APT.
It requires a lot of specialized equipment to reset the APT once it has been messed with.
I suppose you could guess.... but I tried that on my Silverado Q-Jet and it didn't work too well... ended up replacing it with a Holley 80555-1. Had to replace the computer controlled distributor as well.
Works great
Tech Contributor
Joined: Aug 1999
Posts: 14,385
Likes: 6,412
From: At my Bar drinking and wrenching in Lafayette Colorado
You're responding to a post from almost 20 years ago, and the original poster hasn't been on this website for 3 years.
...and you can't use a vacuum gauge to set the APT. It requires a wideband.
...and you can't use a vacuum gauge to set the APT. It requires a wideband.
