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Old 04-30-2011, 12:49 AM
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APT Adjustment

ive recently rebuilt an 81 apt quadrajet. it was working pretty well, but today it seems to have a really crappy idle and nearly died when i gave it gas. upon inspection, i noticed the primary rods were letting gas pool up on the top of the primary butterflies. i turned in the apt which seemed to help, but i cant quite get the idle mixture perfect. ive ordered cliff ruggles book, but it doesnt explain a lot when it comes to apt, just to put it back where you found it. are there any other qjet guys on here that could help me out? thanks in advance!

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Old 04-30-2011, 05:54 AM
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I have played with a few Qjets but I am not an expert.

The fact you are seeing fuel on the butterflies and an adjustment to the APT had an effect on idle tells me the carb is pulling fuel out of the main boosters at idle which it is not supposed to do. This generally means your idle circuit is too lean. Look straight down into the carb while it is idling and you should be able to see if fuel is coming out of the main booster or not. If it is then the APT is not the direct cause of your issue.

This is a common issue when running a qjet on an even mildly modified engine especially when using an 80's era qjet which have an especially lean idle calibration designed to reduce emissions.

If when you disassembled the carb you did not establish how many turns the APT screw was out from the factory, there is a way to re-establish the APT baseline setting but you have to disassemble the carb to do it.

I would look at and sort the idle circuit first before messing with the APT any more.
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Old 04-30-2011, 01:25 PM
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Quote:
Originally Posted by TommyK
I have played with a few Qjets but I am not an expert.

The fact you are seeing fuel on the butterflies and an adjustment to the APT had an effect on idle tells me the carb is pulling fuel out of the main boosters at idle which it is not supposed to do. This generally means your idle circuit is too lean. Look straight down into the carb while it is idling and you should be able to see if fuel is coming out of the main booster or not. If it is then the APT is not the direct cause of your issue.

This is a common issue when running a qjet on an even mildly modified engine especially when using an 80's era qjet which have an especially lean idle calibration designed to reduce emissions.

If when you disassembled the carb you did not establish how many turns the APT screw was out from the factory, there is a way to re-establish the APT baseline setting but you have to disassemble the carb to do it.

I would look at and sort the idle circuit first before messing with the APT any more.
alright, ill give it a try. and how many turns would you suggest i take the mixture screws out after turning them in? i was told to mess with them until it idles the highest, but i can turn them almost all the way out and it idles the same, never drops.
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Old 04-30-2011, 05:09 PM
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Can you take a look and see if in fact fuel is dribbling out of the main boosters? This is important to know in trying diagnose the rest of the problem.

Also, do you know how much vacuum the engine is pulling at idle?

Can you tell us a little more about any modifications done to the engine?
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Old 04-30-2011, 05:53 PM
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Info is in the Knowledge Base
http://www.hotrodders.com/kb/carburetor-articles-2
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Old 04-30-2011, 06:01 PM
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NIIIIIIICE! I keep forgetting about that cache.
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Old 04-30-2011, 06:14 PM
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Thumbs up

That's why I refer to it when ever I can
It's hard to keep up with all the info out there
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Old 04-30-2011, 06:17 PM
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wow just like the old days its George! Good to see you posting man...

Sorry for the off topic post,

K
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Old 04-30-2011, 09:29 PM
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Quote:
Originally Posted by TommyK
Can you take a look and see if in fact fuel is dribbling out of the main boosters? This is important to know in trying diagnose the rest of the problem.

Also, do you know how much vacuum the engine is pulling at idle?

Can you tell us a little more about any modifications done to the engine?
ill check that in the morning

im not sure on pulling vacuum, idle vacuum is between 9 and 14

edelbrock performer intake, msd ignition box (6a), crane energized cam .440 lift 266 duration, and glasspacks.
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Old 04-30-2011, 11:27 PM
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[QUOTE=Ricer Eater 78]ill check that in the morning

im not sure on pulling vacuum, idle vacuum is between 9 and 14

edelbrock performer intake, msd ignition box (6a), crane energized cam .440 lift 266 duration, and glasspacks. oh, and roller rockers
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