[LEROYDOZOIS]

Originally Posted by DoubleVision

If the spark timing is too low, the idle has to be opened up so far to get it to idle that it pulls fuel from the boosters and not the idle circuit.
You have to remember, you only have so much room for the butterflies to be closed enough to run off the idle circuits. If the timing is set too low then the further the idle speed has to be opened to compensate. When it's off the idle slots in the base of the carb it pulls fuel from the boosters to idle, and this makes the idle mixture screws unresponsive. Thats why I need to know what the base timing is set at and if the vacuum advance is in use.

the truck is running right now... i have a eddy carb on it and ive been driving it like this for the past few weeks, im interested in the qjet because of more mpg.

i dont think timing is off, i went through those steps i found on here and many said it was a great thread. the truck atm idles at 600 rpm and i get 8 mpg, the motor is really smooth with the eddy, which is why i feel my issues is with the 1975 qjet

[cobalt327]

Info on timing and setting up an advance curve is here.

The '75 Q-jets VERY often used the "aneroid" set up or a spool affair that replaced the aneroid as a part of the APT (adjustable part throttle) calibration. These carbs are not what I would pick for performance OR economy- but if that's all you have and you cannot or don't want to get a different core, you're options are to either use the aneroid set-up or to disable it and tune it like a "normal" Q-jet.

Some info on the aneroid is here.

Good luck.

[75gmck25]

If your Quadrajet is from a '75 truck, don't get confused by reading information about the '75 Quadrajet used on cars. Carburetor changes in the trucks lagged behind, so your carburetor is equivalent to a pre-75 carburetor used on cars. That is not necessarily bad, just confusing when you read articles about Quadrajets tuning.

GM continued to delay truck carburetor upgrades for several years. I decided to not rebuild my '75 Quadrajet, and use a core from an '86 1 ton GMC truck. This is an APT carb, and the last version before they added any electrical connections. However, my carb is equivalent to what they started using in 1980 on cars, since by '86 most Quadrajets in cars had at least one electrical connector.

Bruce

[LEROYDOZOIS]

Originally Posted by cobalt327

Info on timing and setting up an advance curve is here.

The '75 Q-jets VERY often used the "aneroid" set up or a spool affair that replaced the aneroid as a part of the APT (adjustable part throttle) calibration. These carbs are not what I would pick for performance OR economy- but if that's all you have and you cannot or don't want to get a different core, you're options are to either use the aneroid set-up or to disable it and tune it like a "normal" Q-jet.

Some info on the aneroid is here.

Good luck.

theres nothing like that on my carb... it even looks scarry lol

[LEROYDOZOIS]

Originally Posted by 75gmck25

If your Quadrajet is from a '75 truck, don't get confused by reading information about the '75 Quadrajet used on cars. Carburetor changes in the trucks lagged behind, so your carburetor is equivalent to a pre-75 carburetor used on cars. That is not necessarily bad, just confusing when you read articles about Quadrajets tuning.

GM continued to delay truck carburetor upgrades for several years. I decided to not rebuild my '75 Quadrajet, and use a core from an '86 1 ton GMC truck. This is an APT carb, and the last version before they added any electrical connections. However, my carb is equivalent to what they started using in 1980 on cars, since by '86 most Quadrajets in cars had at least one electrical connector.

Bruce

i gotcha, i figured it wasnt the rods after i thought it through, ill start with the stuff i ordered and see where the truck sits. i just want MPG, no need for extra power when ur talking 454 lol

[BlackStrat67]

So I'm no guru or anything, but I have successfully set up a q-jet for a street performance small block. I ran into the same problems with the idle circuit you're having, instead of large displacement causing the issue it was a larger duration cam (242@.050). You've already drilled out the the base plate where the idle mixture screws set in right? How big did you make those holes? I did mine at .090. You might have to shorten the springs on the idle adjustment screws if you can't seat them all the way. That alone won't really fix anything, the real restriction is further up stream. Take the top of the carb off so you can see down into the fuel bowl etc, remove the accelerator pump and then lay the gasket back down. Look in front (towards the front of the car) of the primaries and you will see 2 kidney shaped spaces in the gasket in front of each primary bore. There are 2 brass restrictions in each kidney. These act as the idle channel restrictions. The hole further towards the back is the idle down tube, the one towards the front is a press in restriction. Measure the restriction and drill it out .004 or .005 bigger, don't go nuts bigger. Take the baseplate off and clean out those filings you just made. Put it back together, go 2 turns out on the idle adjustment and see if it idles any better. If not you might need bigger idle tubes and need to enlarge the idle air bleeds. Try this first though, it should help richen the circuit up.

[cobalt327]

Originally Posted by LEROYDOZOIS

theres nothing like that on my carb... it even looks scarry lol

Then your carb will look similar to the one below (there are many small variations- what I'm referring to is the basic castings):



And not like the carb below (note the conical looking casting on the passenger side of the air horn- the aneroid would be under it on '75 carbs, although the casting remained this way even after the aneroid was discontinued):



The "early style" carb at the top was used until the '90's in marine applications and was also used at the same time as the "later style" casting at the bottom in some applications.

[LEROYDOZOIS]

u hit the nail on the head, i got the one ur finger is on.