Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Quadrajet 7029282 non adjustable APT

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Kavesh:
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.

Cliff Ruggles:
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

Kavesh:
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.

Cliff Ruggles:
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.......

Kavesh:
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?

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