Tuning a 1975 APT Q-jet

[Glutenfreecarbs]

I received a 7045202 in a cloud of dog hair and farm grunge a while back, and I thought it looked interesting, so I cleaned it up and threw Cliff's SR rebuild kit at it.

I set it up with the 46K primary rods that came with it, and after doing a bunch of algebra on the back of an envelope, I chose to install .067" primary jets to accommodate the extra fuel supplied by "third jet" of the oddball APT system.

This idea backfired (literally) as soon as I put the car into gear for a test drive. By raising the APT rod (?) all the way up, I was able to drive around. It was still waaaaay lean at light throttle, but runs great once the PP rises up. I'll put some normal-sized jets in it tomorrow and continue my research...

[Cliff Ruggles]

I don't like, use or recommend messing with those units unless it's a factory restoration and the owner just has to have it in place.

They have a design flaw and the factory knew it and why they had a very brief lifespan. 

Even if you block off the APT at the jet hole they can be difficult to effectively tune.  It's just a crappy design all the way around.  The best way to set one up is to completely get rid of the APT system all together and size the jets in the normal fashion depending on carb number and application.  Then find a metering rod, (J or L will work the best) about 33 to 35 numbers smaller than the jet size chosen and work from there.  You can even get really "fancy" and slot the partition and install an APT screw and PP with a pin on it to move the rods up and down like the later models.

Even with that move I've still had issues getting them dialed in effectively.  I suppose the common passage connecting both jets to the boosters may be part of the problem, not sure and never exploited one enough to find out.

I've ran into more than one of those that would NOT tune no matter how much effort you thru at it.  Probably doesn't help that all those cross-connected passages are in the bottom of the carb where all the dirt and crusty stuff would gather and they are sealed off so you can't get to them.

Not trying to put a big black cloud over the project.  I've had some come right in and work fine, and others have had to come off and apart dozens of times only to end up making that sound I LOVE to hear from Holley carbs we've replaced with Q-jets.......the "tink" when they hit the side of the dumpster!.........Cliff

[Glutenfreecarbs]

Yikes! I remember you discussing these units in your book, but didn't realize they were so notorious. It must be my lucky day; I put .072" jets in mine this morning, drove it around town on some errands, and it seems perfectly happy now. Nice stable idle, and very responsive all the way up to WOT.

Personally, I'm not a fan of the larger "stepped" metering rods like 46K, and prefer a nice taper like 38J or 51M. And you're right; this early APT system is bit convoluted, and inherently too rich at heavy throttle. But, this carb was just a rebuild job for beer money...

[Cliff Ruggles]

I have full tapered rods for the later model carburetors to replace the pretty much useless "K" series rods most of those models came with.

I also make full tapered rods to replaced the "M" series rods in the SMAB models.

I wasn't trying to put a big black cloud over all the 1975 and early 1976 early APT models, just letting everyone know that you will run into a few that will NOT work well no matter how much effort you throw at them.

I personally believe that the flaw lies in the cross-connected passages between the single jet and different length passages leading to each booster.  Those passages and not the same length and that is where ALL the crud is going to gather after many years of use, drying up cycles, etc.

The cousins to them with two power pistons are even WORSE!  I cringe when someone sends one in here to be rebuilt, and have learned to avoid them like the plague!......FWIW......