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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Bullwinkle on April 05, 2023, 01:02:23 PM
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After replacing my mechanical fuel pump the old Quadrajet carb is idling way to high. I would guess around 2,200 rpm. With the adjustment screw backed all the way off. This is a 79 350, all original. The pump is the same as the one I took off except its a little later model, with the return line to the tank, for vapor lock protection. I'm thinking I should put a pressure gauge on the supply line going to the carb. I guess that means putting a T in that line?
Many thanks.
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Make sure that the choke fast idle cam isn't stuck on the high step. Then use some aerosol carburetor cleaner and spray for vacuum leaks.
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Do you have a return line?
Having one in the first place would suggest that there is an internal pressure regulator that routes gas back to the tank when not consumed by the carb/engine.
Or it just runs a higher volume to provide gas to the carb and return line.
So if you don't have a return line that may be the problem.
Why didn't you get a correct model?
A pressure test would tell you a lot, and yes, you can tee the line between the pump and carb.
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JP, the mechanical fuel pump with a return port incorporates a small diameter restriction in the return nipple of the pump to limit the volume of fuel returning to the tank(s).
Do you have a return line?
Having one in the first place would suggest that there is an internal pressure regulator that routes gas back to the tank when not consumed by the carb/engine.
Or it just runs a higher volume to provide gas to the carb and return line.
So if you don't have a return line that may be the problem.
How? :D
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Make sure you haven't inadvertently pulled a vacuum line off somewhere. That will cause a high idle.
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bd I know you're having fun with me, but assuming the pump has a return tube and there is no return line to the tank, what else is he going to do but cap it off?
Please enlighten us ;)
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Well, hypothetically, let's say the return line is capped off, ... or not. How might either result in a high idle on a carbureted engine? :)
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You probably knocked the choke wire off or a vacuum line popped off.
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Thanks for the great suggestions, I’ll check the choke. It idles the same speed hot or cold, so I didn’t check it but yes it could be stuck all right.
Yes there’s a return line to the tank, that’s why I got that type. I heard a guy say it worked well for him without any modification of anything else.
I don’t see how a return line could increase pressure unless the pressure is set higher on the supply line than on the older two-line pump.
If I do end up putting in a gauge, what pressure would that original quadrajet use?
Thanks again very much
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The reason I put in the later 3-port pump was the reason they designed it in the first place. Returning some fuel to the tank continuously keeps it from getting hot enough to vapor lock.
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Well, hypothetically, let's say the return line is capped off, ... or not. How might either result in a high idle on a carbureted engine? :)
X2
Should always replace parts that are designed for the year and model, especially if it is original engine!
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For the 1979 model year trucks, small block fuel pressure for engines using a return line is 5 1/2 - 7 PSI; small block engines without a fuel return line call for 7 1/2 - 9 PSI. Big block engines with a fuel return line call for 7 1/2 - 9 PSI.
Bullwinkle, fuel pressure isn't the cause of the high idle complaint. Most likely either the choke isn't releasing or air is bypassing the throttle (e.g., a vacuum leak).
JP, I was just trying to lead you to think the symptoms through. I know you have the knowledge and experience. :)
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bd=mentor
Thanks!