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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: st3vomettler on June 02, 2015, 06:05:56 am
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I have a autozone brand carb for a 77 350 and i was wondering how to adjust the fuel/air mixture screws on it?
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There should have been instructions with it, but what kind of carb is it?
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It is a autolite c9394 made by Rochester and when I bought it it was only on a car board wrapped in shrink wrap
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Hi,
It's "Auto LINE" (not Autolite, like the spark plug) just in case you did some searching for it.
It's a remanufactured Q-jet without the electronic mixing solenoid. The C9394 model usually has a $100 core charge. You're supposed to take your old one in and get $100 credit, like a car battery.
The attached pic is the underside of the carb. The adjustment screws are like most typical carbs. They are located in recessed ports and spring loaded but you can see the flat slot on the screws which are located right close to the intake manifold.
A lot of Q-jets in my experience will almost not go rich enough to idle smoothly. Yours may be different. There is a modification where you can drill out the ports for a richer mixture with less CCW turns of the screw, but try adjusting it correctly first. With a screwdriver, turn the adjustment screws in (tighten) until they seat, and then turn them out 3 turns each.
Crank engine and look at a tach or hook a vacuum gauge to the manifold. Decrease your idle speed to 500 or 600 RPM with the idle speed adjustment screw on the right side of the carb. Make sure the engine is up to normal operating temperature and the fast idle cam isn't kicked in and the choke plate is open.
Turn one screw CCW (loosen) and see if the engine speed increases. Keep adjusting until it increases to maximum speed. So the same for the other screw until maximum speed or maximum vacuum is obtained. They should be very close to equal in number of turns in and out. Some folks will lean the mixture just a tad by turning each screw in about 1/2 of a turn, or until the engine speed drops 40 or 50 RPM.
Hope this helps.
Mack.
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Here's the link to an article I ran across a while back about "drilling out" the idle ports for the Q-jet. I was on the edge of trying it on my Q-jet before I changed it out to an Edelbrock 600 CFM. My Q-jet would NEVER adjust too rich, even with the screws backed out almost all the way until they fell out.
http://www.73-87.com/7387garage/drivetrain/qjetidle.htm
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Thank you. I'll do that once I get some more gas for it.