Hot Rod Forum banner

reaming quadrajet idel ports ?

5K views 22 replies 6 participants last post by  LEROYDOZOIS 
#1 ·
so as you all who followed my before about the quadrajet i cleaned up again and such running super lean at idle, you guys said it needed to have the idle ports reamed out ( not the mix screw holes but the brass plugs in the carb base )

since im getting a solid 8 mpg with the suburban now i think this would be a good time to maybe see if i can get the quadrajet fixed and use it as a test??!!

ive drilled out jets for our methanol gokarts and such so as long as i got the correct ream i think i should do okay.

so does anyone know the start of seeing what ream i would use or info as how to do this kinda thing ?
 
#5 ·
my idle

when i got the truck the choke always stayed slightly open, and when i rebuilt it it didnt want to idle until it was warm. so in desperation i junked it and used my carter my uncle gave me, and the truck runs fine now

i have the afr meter on it and it wont idle because its off the charts lean, someone said to drill out the mix screws and i did( very tiny afr change )then someone said u can cut down the needles, so i cut down the needles with my lathe and that still didnt make much of a difference. the idle is like 16.5-17.5 afr and idles like a dieing cat anything past 1200 rpm its nice but 1000 and down its horrible. someone said i need to ream out the ( port plugs ) in the idle circuit. my quadrajet book vaguely talks about it, it shows a guy using a 6 inch ream thats very tiny


but idk if thats the issue or if the jets are wrong, its a 7045583CHEV & GMC Truck, 75-77 Calif.73427029862CP2.79993.6545

carb and has 73 jets/41b needles and the piston spring feels like my 9 inch springs for the eddy carbs


i cant say for sure but it seems like these arent correct
 
#6 ·
the mixture screws have to be backed out soo far they wiggle in the slot and even this makes the idle richer then it should.

food for thought, do some carbs have 2 gaskets under the air horn and not just 1 ? if soo then when my kit came with 2 identical gaskets and i only used 1... could this be a horrible mistake???!!!!
 
#7 ·
nice

my carb is a 75-77 and i have the needles for the pre 75..........


Incorrect Primary Metering Rods
Results in: Poor idle, rich idle, ineffective idle mixture screws, surging at cruise, poor mileage, poor cold-running characteristics, hesitations off idle and/or at cruise, poor manifold vacuum with resultant ancillary problems.
Comments: In addition to seeing incorrectly sized metering rods for the application, I am also seeing carbs with the WRONG rods installed. Pre-’75 (up through ’74) Q-Jets use metering rods that are approximately 2.47” long overall (total length from the metering tip to the extreme top of the rod). 1975 and newer Q-Jets use rods that are about 2.40” long. The early rods typically also have just a plain number stamped into them (like, “44”), or will often have the letter “B” after the number (like, “44B”). The later rods will often have the letter “K” or “M” after the number. If an early rod (long rod) is installed in a post-’75 carb, the actual shank (body) of the rod will be stuffed down into the jet, resulting in a severe lean condition with associated marginal drivability. Likewise, if a late rod is used in an early carb, there will be virtually no lean-out from the power piston at idle or cruise, and the carb will suffer a severe rich-running condition.
 
#9 ·
on the dizzy itsself or the line to the carb? i got mine hooked to the one where theres no emissions, because theres nothing on the motor anymore ( its a boat motor )

and yeah it wouldnt even humor the thought of roughly ideling less then 6 turns out

6.5 when i got it
 
#14 ·
i dont recall

i remember looking at info on here a few months ago and i set the truck to that, i think it like 3500 rpm it was like 8. idle checked out okay too

i probably sound stupid atm but i remember having my laptop out in the garage and doing step by step from a post here on hotrodders.

and the truck runs now, i drove it back n forth to work for like 2 weeks but it has the 1406 on it right now, im getting 8 mpg and everyone says q jets get better mpg, so i figured its worth a try
 
#15 ·
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.
 
#16 ·
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
 
#17 ·
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.
 
#18 ·
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
 
#21 ·
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.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top