Author Topic: Quadrajet 7029282 non adjustable APT  (Read 506 times)

Offline Kavesh

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Quadrajet 7029282 non adjustable APT
« on: June 14, 2021, 02:28:10 AM »
Hello from South Africa again

My carb for whatever reason after running fine for a while will develop an erratic idle, everytime I strip and assemble and its fine again, no dirt found internally.

While looking at my spare carb of the same number I realised that I have never paid any attention to the height of the wire on the APT which on this carb is  non adjustable.

So I removed the base plate of the carb and found that the wire is actually flush with the base height, compared to the spare which was about 0.5mm higher. Please see pics attached.

When I swapped the base plates and ran the motor it was idling way too high. I had to back off the curb idle screw more than 2 full turns and now it just makes contact with the screw.

I also had to turn in the mix screws from 2.5 turns to 1.75 turns each.

I have not taken my truck for a drive as yet.

Does the above sounds about right or an indication of too rich a condition?

As always really appreciate the advise.


Offline Cliff Ruggles

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Re: Quadrajet 7029282 non adjustable APT
« Reply #1 on: June 14, 2021, 08:03:53 AM »
Is the donor baseplate from a carburetor with the same part number?

APT is there to fine tune the height of the metering rods when they are in the down position.

Typically it has little if any effect on the idle fuel to the engine but I've seen a few cases where raising the APT would allow a little more fuel to the mixture screws especially if the metering rods were pretty "low" in the jets right to start with.

The APT system is however a BIG player with fuel delivery off idle and in the "normal" driving range.  It is ALWAYS best to stick with the original baseplate and APT height as the factory fine tuned them for the jets/rods being used and exactly for the application.......Cliff

Offline Kavesh

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Re: Quadrajet 7029282 non adjustable APT
« Reply #2 on: June 14, 2021, 09:49:34 PM »
Yes Cliff the donor baseplate is off a carb with the exact same number 7029282.

Based on your comments I can only assume that the rods were sitting too deep in the jets which affected the off idle when driving.

I went for a short drive yesterday and I must say that the truck felt very happy light rolling, if you understand what I mean.

Previously the truck always felt very heavy and needed a bit more gas to get going.

I hear you Cliff regarding the height of the APT set at factory. I have 2 spare carbs of the same part number and both the spare ones had a slightly higher APT compared to what I had on my carb. I am just wondering whether the previous owner or carb builder at some point may have changed the height of the original baseplate APT.


Offline Cliff Ruggles

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Re: Quadrajet 7029282 non adjustable APT
« Reply #3 on: June 15, 2021, 03:16:25 AM »
Why are you swapping baseplates in the first place?

It is always best to use the original one that was on the carburetor unless you plan on removing the plug and adjusting the APT to duplicate its original position.

Here I remove the APT screw and replace it with an adjustable one so we have full control with the APT system.......

Offline Kavesh

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Re: Quadrajet 7029282 non adjustable APT
« Reply #4 on: June 15, 2021, 09:51:23 AM »
The reason I swapped the base plates was because I seemed to develop an erratic idle after a period of driving. I read a little about the APT and when the carb was apart I never gave it much of a look.

When I compared it to my spare parts they were not the same, measured with my veneer to be 0.5mm difference in height.

I am just trying to fix the carb so I do not have to open it up any more. My gaskets are getting ruined with the frequency of opening up to look for a possible issue with the cause of erratic idle.

Cliff are you saying that I must put the original base plate back or should i give it a run like this and see how it goes?


Offline Kavesh

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Re: Quadrajet 7029282 non adjustable APT
« Reply #5 on: June 15, 2021, 10:43:48 AM »
Btw I like the idea of the screw to control the APT system.

Offline Cliff Ruggles

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Re: Quadrajet 7029282 non adjustable APT
« Reply #6 on: June 16, 2021, 03:19:38 AM »
The original baseplate was used when the factory set the APT height.

Installing a different one changes all that, sometimes not for the better.

There isn't anything going on with a baseplate that would cause "erratic idle" unless there is a LOT of play at the throttle shaft (vacuum leak).  ALL of those older units need bushings installed anyhow, which would take that out of the equation.  They had too much play brand new and it isn't going to be any better half a Century later!

I would recommend rebuilding the baseplate with bronze bushings and installing an external APT screw at the same time.  This will get rid of any potential issues there and allow for fine tuning in the normal driving range without having to take the carb apart and change metering rods......FWIW....

Offline Kavesh

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Re: Quadrajet 7029282 non adjustable APT
« Reply #7 on: June 17, 2021, 12:53:56 AM »
So if I understand you Cliff, even though the carb number is the same, when the carbs were put together at the factory, each carb was set up individually and therefore parts should not be interchanged? I shall try the original baseplate again.

The primary shaft bush has a minor play but does not seem to have a vacuum leak. I tried spraying carb cleaner around it and the idle did not change.


Offline Cliff Ruggles

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Re: Quadrajet 7029282 non adjustable APT
« Reply #8 on: June 17, 2021, 03:55:24 AM »
Side play at the shaft is a vacuum leak.  I haven't seen an early carburetor that didn't need bushings installed for the primary shaft.

In addition to leaking, play in the primary shaft can and will cause idle issues as it may not settle down to exactly the same spot each time it closes.  This may be part of or all of your issues with idling problems......

Offline Kavesh

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Re: Quadrajet 7029282 non adjustable APT
« Reply #9 on: June 17, 2021, 11:36:30 PM »
Thanks Cliff

The bushing is something I will need to get done.

On a side note....
A kind fellow car enthusiast from the USA had offered to and I accepted to send me some bushings in 2019. He used the US postal service which in turn would use the South African postal service to get it to me. I waited 6 months and went to my post office weekly and tried various call centres to track the package. NO LUCK.

Eventually John messaged me saying that the package found its way back to the USA as it was unclaimed. Its a sad reality here with any government operated business. Nothing works as it should.

So now if I need anything I would need to use a private courier but as you know they are extremely expensive so I hesitate to order just a small low value part.

Thank you again for all your invaluable advice.

Offline Cliff Ruggles

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Re: Quadrajet 7029282 non adjustable APT
« Reply #10 on: June 18, 2021, 02:38:23 AM »
A pair of bushings woln't do much good without a reliable way to bore the baseplate to install them.  You can't just stick a drill bit in there and open up the hole, it must be done directly on center.  I make and sell a kit with a self-guiding drill bit to install them.  It is very easy for the do-it-yourselfer, all you need is a hand drill, no drill press, vise, etc.

There are ways to ship reliably anyplace.  I ship overseas daily and have very few issues.  Sometimes it is best to ship to a business, to a Post Office, etc rather than a residential address.

In all the years doing this the only place I was not able to ship to was a remote location in the Amazon region of Brazil.  The shipment was gone from here several months then came back.  It really did look like it traveled around the World several times but never found it to the customer........