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Old 01-12-2013, 07:55 PM
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Quadrajet Question

I was reading several articles online and got some conflicting answers.
The late model pre computer controlled Q-jets have the adjustable part throttle. A article I read says the APT also effects idle as by leaning it sets the metering rods further down in the jet. Well that doesn't make sense to me as the Q-jet, like all other street carbs have they're own idle circuit which is why they have idle mixture screws. What I'm dealing with is the Q-jet I have on my modified 4.3 V6. The idle is set low as it can go, in fact it's so low the idle speed adjustment screw isn't even touching it. It still idles higher than I'd like which is around 900 RPM. I've checked it high and low for vacuum leaks and there are none. I've also slowed down the spark timing from 24 degrees before top dead center to 18 degrees, that helped some but not much. I'm also positive the mechanical weights on the distributor isn't stuck or sticking open so I'm at the point to where I'm slightly confused on why it won't idle down. If the APT controls the idle any as the article states then I could just lower it's setting by lowering the metering rods further in the jets, but as far as I know they have no effect on the idle speed as I stated so I don't see that working.
Also I'll mention the throttle body on it is in great shape as it's already had the bushings replaced so any other ideas on why it won't idle down?

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Old 01-12-2013, 10:59 PM
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Possibly throttle plate hanging up in the bore or unequal throttle plates. One open a touch more. I had one do both.
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Old 01-13-2013, 03:37 PM
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Does your carb have a hot idle compensator? It will cause the idle to raise if stuck open.
Take the throttle body off and hold up to alight to check the throttle plate seal against the bore of the throttles. The screws in the shafts have to be tightened with the throttle held closed to force the plates to orient themselves to each bore.
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Old 01-13-2013, 07:56 PM
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Double check the fast idle screw isnt too tight.
Loosen screws on throttle blades and recenter them in the bores.
A faulty APT adjustment could keep the primary rods from fully seating in the jet, richening the mixture. Easy enough to check though with the air horn off. Idle mixture screws on a Q jet really dont adjust the A/F mixture, they simply adjust the volume of the available mixture below the throttle plates.
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Old 01-13-2013, 09:51 PM
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APT adjustment isn't the issue. What did the carb come off of originally? Barring any vacuum leaks, you're prolly going to find the idle bypass air orifices are too large for the 4.3L V6- the bypass acts like a controlled vacuum leak. If partially closing the choke blade or holding your hand over the primary side causes the idle to drop, then there's your answer. See what size they are now and close them accordingly. Again- no vacuum leaks!

Obviously you need to be real sure the choke/fast idle adjustment/cam/linkage isn't causing the primary shaft to hang open.

Has this carb ever run correctly? Warped castings or mismatched/torn gaskets can allow in too much unmetered air.

Last edited by cobalt327; 01-13-2013 at 10:02 PM. Reason: Add choke info.
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Old 01-14-2013, 10:24 AM
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This carb came off a 1979 chevy truck with a stock 350. However the throttle body came off a sean murphy carb. The throttle bodies between the two were identical except the SM body had the easily adjustable air mix screws while the factory body does not. There is no choke or fast idle assembly. I always remove them as down here where I live it don't get cold enough to need a choke. I did plug the vacuum coming off the side of the carb where the choke housing was. On the truck the carb was on originally the former owner told me when he drove it home it ran okay with the only issue being old gas.
When I got the carb from him it was still on the truck and hadn't been removed. He parked the truck when he found the frame was rotten.
When I disassembled it I was surprised at how good the condition was. It was clean with only a little fuel residue in the bottom which washed out with carb cleaner. How do I go about changing the idle orifices? Also the original throttle body wasn't worn out, it has very little play in the shafts I didn't use it only because of the difference in adjustable idle mixture screws. Thanks for the help.
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Old 01-14-2013, 11:11 AM
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Quote:
Originally Posted by DoubleVision View Post
This carb came off a 1979 chevy truck with a stock 350. However the throttle body came off a sean murphy carb. The throttle bodies between the two were identical except the SM body had the easily adjustable air mix screws while the factory body does not. There is no choke or fast idle assembly. I always remove them as down here where I live it don't get cold enough to need a choke. I did plug the vacuum coming off the side of the carb where the choke housing was. On the truck the carb was on originally the former owner told me when he drove it home it ran okay with the only issue being old gas.
When I got the carb from him it was still on the truck and hadn't been removed. He parked the truck when he found the frame was rotten.
When I disassembled it I was surprised at how good the condition was. It was clean with only a little fuel residue in the bottom which washed out with carb cleaner. How do I go about changing the idle orifices? Also the original throttle body wasn't worn out, it has very little play in the shafts I didn't use it only because of the difference in adjustable idle mixture screws. Thanks for the help.
The idle air bypass orifices are almost always in the base plate. There are holes above the baseplate connected to the orifices, but they are usually much larger than the orifices so the orifices in the baseplate are all that need to be enlarged or made smaller.

There are several different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them w/a drill bit.

Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off w/lead shot tapped into the casting. This can be fairly easily removed if it's encountered- but I doubt that's going to be your problem (no bypass air). They block them in various places, sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find the holes I'm talking about. Often these rebuilt carbs will use cheap unplated brass metering rods (unmarked) and might even have unmarket jets.

The older style I call the 'tunnel' type (circles):




The later type I call 'tab' type. Some have two like on the left (right tab would be where the circle is), most look like the one here; one orifice is in the left tab, the other is just inside the bore (arrow):

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Old 01-14-2013, 11:23 AM
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Thanks Cobalt. So I need to snatch the carb and figure out a way to close them up or should I just plug them entirely?
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Old 01-14-2013, 11:32 AM
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Thanks Cobalt. So I need to snatch the carb and figure out a way to close them up or should I just plug them entirely?
IIRC, my Q-jet originally from a 4.3L V6 (17086047) has no bypass orifices. So I would say to block them off entirely, then you can add bypass air only if it's needed. If your cam requires a lot of initial timing you might need some bypass air to keep from opening the primary butterflies too far. The bypass air is similar to drilling holes in the primaries.
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Old 01-14-2013, 11:33 AM
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Okies, much thanks Cobalt.
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Old 01-14-2013, 11:36 AM
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You're always welcome DV. I added an edit in case you missed it. No biggie, just some info you probably already know, but might help someone doing a search someday.
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Old 01-14-2013, 11:41 AM
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Since you used a different base plate with the old body maybe you've got a mismatched gasket covering something it shouldn't.
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Old 01-14-2013, 12:39 PM
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That's a thought Nuts, but I made sure I checked before installation and they all matched. I went on and removed the base plate I found the holes were open and they're quite big. So I shot some cleaner through them, found where the holes were on the opposite side and filled them both with solder that I melted with a hand torch. Here before long I'll reassemble it and give it another shot. Thanks again for the heads up Cobalt.
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Old 01-15-2013, 06:22 PM
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Cobalt , my hats off to you if i can get a Qjet to run 50% of the time im happy ,I have to stay with what i was raised with ,Holley.
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