Quadrajet APT Adjustment Confusion???

[savatreatabvr]

I bought a (supposedly) remanufactured 800cfm Quadrajet for my 1973 C20 454 and it was screwed up from the beginning. It never ran right (as seen in my other threads) so I tore it apart and fixed several things (as seen in my other threads) but I made the mistake of taking out the APT adjustment screw without counting how many turns it took before it was fully removed. I've read several articles on how to adjust the APT but one says one thing and the other says another! That's the only thing left that's stopping me from bolting it on so does anybody have any helpful hints on how to adjust it other than "YOU SHOULD HAVE NEVER $%^#@* WITH IT IN THE FIRST PLACE", lol!

It's the red arrow below!

[donut]

Does the carb top have the hole in it for external adjustment? I think I'd shoot for something in the middle and adjust from there.

Of course, I'm also wondering as my current carb has APT, but I haven't dealt with it before. AFAIK, the older ones didn't have it.

[Jake Wade]

They are normally 2-1/2 turns out from bottom.

[mrolds88]

Sounds like Jake may have something. I was going to suggest that if you, or a buddy had another Qjet, to pop the lid off and check it to see how many turns it took on that one. Sure would get you close.

[savatreatabvr]

Originally Posted by mrolds88

Sounds like Jake may have something. I was going to suggest that if you, or a buddy had another Qjet, to pop the lid off and check it to see how many turns it took on that one. Sure would get you close.

That is true but the only other Quadrajet I have available is a 750cfm and it doesn't have the adjustable APT. During my research I found one source that says 2 1/2 turns out from completely closed and another source says for the APT to be just above level with the gasket surface of the main body of the carb which would be pushing 6 1/2 turns out from completely closed! Lol, that's quite a significant difference! Whomever owned the 800cfm Quadrajet before me drilled out the plug in the airhorn so the APT is accessible with the airhorn on the carb. I'll adjust it to 1 1/2 turns out, bolt it on and do my final adjustments if it runs, I was just looking for a base setting (turns out) that I could set it to so it would run right off the bat because trying to get a freshly installed carb to idle can be difficult when there may be many other issues with why it's not idling.

[geezer#99]

THe apt doesn't effect your idle. It's part of the power circuit.
If you get a little stumble when you accelerate then adjust it richer. FWIW my 84 305 is set at 6 turns. I've found them anywhere from 2 to 9 turns out.

[Figuarus]

I took a picture of the carb number just in case that helps.

By the way, in researching this carb, the 2nd to last number is supposed to show what division this was made for (Chev, GMC, Pontiac, etc) but "9" doesn't show anything. Does that mean this was an aftermarket build?

http://i.imgur.com/lYGhhRt.jpg

[SkinnyG]

The APT sets the ~minimum~ size of the jet run by the needles. It effectively tunes "cruise."

I made mine adjustable by popping the plug out, threading the horn most of the way through, and trimming the end of a bolt to plug it (flush with the top of the air horn, and slotted so I can open it with a screw driver).

I set it the way it was when I disassembled the carb, and have been slowly turning it in about 1/6th of a turn just to see when I hit a lean surge cruising on the highway.

Some set it by turning up the idle speed to about 2000 or so, then slightly choke the engine with your hand and see if the engine speed increases about 50rpm (that means you're on the edge of lean). Allegedly that is good. If speed doesn't increase, pull a vacuum line and see if speed increases (means the APT is rich).

[savatreatabvr]

Originally Posted by SkinnyG

The APT sets the ~minimum~ size of the jet run by the needles. It effectively tunes "cruise."

I made mine adjustable by popping the plug out, threading the horn most of the way through, and trimming the end of a bolt to plug it (flush with the top of the air horn, and slotted so I can open it with a screw driver).

I set it the way it was when I disassembled the carb, and have been slowly turning it in about 1/6th of a turn just to see when I hit a lean surge cruising on the highway.

Some set it by turning up the idle speed to about 2000 or so, then slightly choke the engine with your hand and see if the engine speed increases about 50rpm (that means you're on the edge of lean). Allegedly that is good. If speed doesn't increase, pull a vacuum line and see if speed increases (means the APT is rich).

Sweet truck! Is yours the factory paint color? Reason I'm asking is because my 73 Chevy c20 Custom Deluxe is the same color.

[SkinnyG]

It's as close to the factory colour as Home Depot could colour match Tremclad (Canada's Rustoleum).

[savatreatabvr]

Originally Posted by SkinnyG

It's as close to the factory colour as Home Depot could colour match Tremclad (Canada's Rustoleum).

I've seen the Rustoleum paint jobs on YouTube and have considered trying it. I don't have the equipment or place to spray the paint so I was considering the roll on paint job. I'm assuming as long as the paint is thinned correctly and enough is applied some serious wet sanding should smooth it all out. How did you paint your truck, spray on or roll on?

[SkinnyG]

Follow the link in my signature - it will likely answer all your questions. Let's try to stay on topic for the original poster (grin).

[Flathead Smith]

Originally Posted by donut

Does the carb top have the hole in it for external adjustment? I think I'd shoot for something in the middle and adjust from there.

Of course, I'm also wondering as my current carb has APT, but I haven't dealt with it before. AFAIK, the older ones didn't have it.

Posted via Mobile Device

[Flathead Smith]

The older ones do have an APT, but it is harder to access
Posted via Mobile Device

[Gregski]

ok, I'll bite, what the heck is APT?, LOL

[geezer#99]

Originally Posted by Gregski

ok, I'll bite, what the heck is APT?, LOL

Adjustable Part Throttle

[donut]

I think it's adjustable part throttle.

Originally Posted by Flathead Smith

The older ones do have an APT, but it is harder to access
Posted via Mobile Device

Care to enlighten me? I was under the assumption that trucks didn't get APT until about 75-76.

[Gregski]

Originally Posted by donut

I think it's adjustable part throttle.

Care to enlighten me? I was under the assumption that trucks didn't get APT until about 75-76.

That's what I thought, maybe I didn't know exactly when they got it, but I don't recon my '74 half ton 350 V8 had one

[geezer#99]

APT was there right from the start of production. Located differently in the first ones. Still did the same thing.
Scroll down in this link to see.
http://www.crankshaftcoalition.com/w...4MV_carburetor

[Flathead Smith]

Originally Posted by donut

I think it's adjustable part throttle.

Care to enlighten me? I was under the assumption that trucks didn't get APT until about 75-76.

It's centered in between the idle mixture screws under a knock-in-disk in the base plate on the older Q-Jets. With the Gasohol stuff being sold these days you will probably need to access it if you want your engine to run correctly ... your fuel mileage will drop a little in the process.

[donut]

Thank you gentlemen, appreciate it. Never messed with the center plug.
My education involved keeping them running, that's it.

[Flathead Smith]

Originally Posted by Flathead Smith

It's centered in between the idle mixture screws under a knock-in-disk in the base plate on the older Q-Jets. With the Gasohol stuff being sold these days you will probably need to access it if you want your engine to run correctly ... your fuel mileage will drop a little in the process.

An update on this post... that I should have put in originally.

A slight problem that I have had with the carburetor on my truck that I never fully resolved was a part throttle surge that occurred while at speed. The truck idled perfectly and accelerated very nicely. The QuadraJet is a 1969 model so the APT was located on the front of the throttle body at the bottom behind a steel plug that had to be drilled and pulled out for adjustment. I turned the adjusting screw in about 1/16 of a turn increments until the surge stopped. My guess is that I turned the screw in about a 1/4 of a turn, if that.

The Truck runs perfect now.

[Firebirdjones]

Maybe this will help others in the future.

What I do when setting up the carbs is to drill and tap either the throttle body or the top air horn so I can run a plug in it and access the APT easily and make fine adjustments anytime it's needed without carb disassembly.

When building them I always start about 4 turns out as a baseline. I use a wideband and drive the vehicle and fine tune it from there for a comfortable part throttle cruise AFR. Best way to do it is with a wideband so you aren't shooting in the dark. With the removable plug I can make adjustments on the roadside and have it dialed in pretty quickly.