1975 Quadrajet APT adjustment 7045222

[dmruschell]

I'm restoring a 75 Corvette and daily drive a 79 Corvette. I bought a rebuilt 75 Quadrajet for the 75, and had the original carburetor rebuilt to try out a Quadrajet for my 79.

Everything went well, except that I'm getting a (I'm guessing lean) surge at cruise. I figured I would adjust the Adjustable Part Throttle, but I can't find any information how how to actually do this on a 75 Quadrajet. All that I can find is that the 75 APT is a one-year only design and generally one to stay away from (lucky me).

So, experts, how do I adjust the APT on a 1975 Quadrajet?

Thanks in advance!

I've rebuilt WCFB and AFB carburetors, but haven't messed with Quadrajets too much, so things that may be obvious to the more experienced may not be immediately obvious to me

[Frank400]

I'm quite positive '75 carbs don't have APT.  I did one of those lately and cancelled the aneroid and adjusted the part throttle by changing primary metering rods.  Takes more time plus you have to have many rods to chose from, but ends up working fine.

   If you have to have that carb for numbers matching purposes, it can be made to work but it's much more time consuming. 

   If you don't mind a different number, '76 and up carbs are so much more fun to play with...

[dmruschell]

I thought the APT was adjusted by adjusting the aneroid. Any tips?

I ordered Cliff's book, but it won't be here until Saturday.

I didn't need it for numbers matching purposes, but I just spent $400 having it rebuilt before I knew about the 75-only APT. I didn't realize it didn't have the bowl vent, either, and paid to have the rebuilder add the bowl vent. I probably wouldn't have put that much money into it if I knew about the unique APT. I would have just gotten a 76-79 carb and had that built.

[dmruschell]

I re-read part of the tuning paper that the rebuilder (Lars) sent me. Since all of the metering rods have the same power tip, and WOT seems to be good with this carb, I can replace the metering rods without affecting WOT. I just need to know/figure out what metering rods to put in. It's listed as having 46 rods right now.

[Cliff Ruggles]

1975 Chevrolet passenger car carburetors have an APT system which is adjustable.  It adds fuel to the primary side of the carburetor thru a single jet/metering rod located in the left front corner (as viewed from the front) of the carburetor.  The adjustment screw has a brass "cup" plug over it.

I don't use, like or recommend those particular units as a general rule, they are HORRIBLE.

The concept of that APT system is OK, however it is an unequal distance from the APT jet to the two main jets in the bottom of the float bowl, and those passages are in the bottom of the carb and can get clogged up over the years and not easily accessed for cleaning.

I've ran into those that didn't work correctly no matter what you did for them.  Most likely the interconnecting passage between the main jets can cause issues in fuel flow to the booster/venture. 

It's no surprise at all to me they only used those very briefly and quickly went to the later and much better design APT that uses an adjustment screw that raises and lowers the power piston to precisely locate the metering rods (tapered) in the jets........Cliff

[dmruschell]

I popped off the plug above the APT adjustment and 1/2 turn counter-clockwise seemed to eliminate the lean surge I was having.

I have smaller rods and larger jets on the way if I eventually need them.

Right now, this horrible carb runs better than any other carb I've put on it, and is preferable to an EFI system I tried out lol.

Is there any way to convert these carbs to use the later style APT? My rebuilder modified it to have a 77-79 style bowl vent because the casting provisions were all there, so the casting itself couldn't be that different. Since it's working correctly now, I probably won't bother, but it would satisfy my curiosity and maybe help someone else who has one of these carbs.

[Cliff Ruggles]

Yes, they convert to the later style.  You have to thread the hole for the APT screw, and put a "slot" in there for the later APT style power piston with the pin on it.

Keep in mind here than many of those had an auxiliary power piston in the front hole and single rod on it with a single jet in the main casting (those are really horrible) that will need to be blocked off for the conversion.

This still does NOT eliminate the cross-connecting passages between the main jets from the early APT system, which I am not very fond of and have seen quite a few of those units that simply don't work well no matter how much time you spend on them.

The early APT system is actually quite flawed, as it adds fuel under the jets, which will effect not only part throttle A/F, but everywhere else as well.

It's just a poor design all the way around, however, if you spend enough time with one and arm yourself with enough jets and metering rods you can usually come up with a pretty good end result.

As stated earlier, I typically avoid those units, as the later APT design is far superior in every respect, and far less labor intensive when custom tuning.........Cliff