Author Topic: APT on a 75 q-jet  (Read 167 times)

Offline deanhickey

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APT on a 75 q-jet
« on: September 22, 2022, 03:20:10 PM »
This is my first rebuild so I am a little paranoid, Bought a carb on line and took the air horn off and was not what I was expecting. The spot for the apt was empty and no visible threads in the casting there was another apt unit on the right front side of the carb. Is this correct? or has some messed with it? photos attached
I have made more expensive mistakes.

Offline mcx

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Re: APT on a 75 q-jet
« Reply #1 on: September 22, 2022, 05:25:47 PM »
It’s correct…that is a AMJ (adjustable main jet) core.

Offline deanhickey

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Re: APT on a 75 q-jet
« Reply #2 on: September 23, 2022, 12:00:29 AM »
thanks for the info, have discovered it's an oddball one year only model and not very desirable for rebuild, It will be a good learner piece none the less.
I have made more expensive mistakes.

Offline mcx

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Re: APT on a 75 q-jet
« Reply #3 on: September 23, 2022, 04:03:47 AM »
What is the carb # on it? I have found it’s  a decent carb on the Pontiac applications, and it is  much better than the other 1975 (Oldsmobile)version, which has a complicated third primary rod set up. Those are a little difficult to assemble and tune, and no one wants that version.
Anyway, just a clean the passage way under the AMJ to the primary jets really well..use carb spray in both directions and shop air to remove all build up and debris.

Offline Cliff Ruggles

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Re: APT on a 75 q-jet
« Reply #4 on: September 23, 2022, 04:29:29 AM »
The two power piston models are for sure about the worst Q-jets out there.

I'm not very fond of any of those models as the later APT design is superior in every respect. 

Those 1975 models were discontinued quickly and for good reason.  They have a design flaw in them as well.  All of the cross-connected passages are in the bottom of the carb, right where all the crap is going to pile up after many years of use and drying up cycles.

It is also NOT a good idea to have one side the of fuel supply from the jet to the booster be able to see the other side.  Plus the distance from the APT jet/rod to each side is not the same.  I've ran into a number of these carburetors that didn't test well and nothing we could do about it. 

I stopped working on them at least 15 years ago and stick with the later design......Cliff

Offline deanhickey

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Re: APT on a 75 q-jet
« Reply #5 on: September 23, 2022, 04:57:13 PM »
Thanks for all the input, it seems to be in good shape and if I have flow in the apt passages I will give it a run on the dyne in the spring along with an eldelbrock carb just for sh@ts and giggles. It's been my first real world lesson in the Q-jet world, more to come I am sure. have just bid on a 79 Q-jet we will see how that goes. Living in the U.K.  options are limited. Every side inlet Q-Jet is advertised as coming out of a corvetteLOL.
Cliff rebuilt the Q-jet in my GTO around 2018 and it hasn't missed a beat on a stroked and bored 400 Pontiac engine.
This will sit on my Nova 355 400hp (projected) engine should be interesting and will post results.
 You will hear from me soon with more newbie questions. Be kind and if you can't be kind be funny.
I have made more expensive mistakes.

Offline Cliff Ruggles

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Re: APT on a 75 q-jet
« Reply #6 on: September 29, 2022, 04:56:50 AM »
It's about the same here, folks selling factory Q-jets like to attach the "Corvette" name to them.  I guess this instantly doubles their value.......

Offline tayto

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Re: APT on a 75 q-jet
« Reply #7 on: September 29, 2022, 06:29:36 PM »
make sure to not buy a previously "remanufactured" qjet, they usually have a metal tag under the front airhorn screw....