The Quadrajet Carburetor has a bad reputation...mainly because a lot of people don't know how to make that particular carb work well on their engine (me included). I've driven a car with a well tuned Q-jet for a while now though and can attest to how well they perform once tuned properly. Switching from an electronically controlled Q-jet to a mechanical Q-jet is a big step and I wanted to make sure it was done properly...so I enlisted the help of a friend that is an expert at them.
The first trick with a Q-jet is to get the idle and cruise Air Fuel ratios right, at idle you want the primary throttle blades closed with just the proper amount of air and fuel going through the bypasses and enough adjustability in the idle air screws to fine tune per the engine. Same with cruise AFRs... How much vacuum the engine makes (largely cam relative) combined with cubic inches figure in to this equation.
The carb I acquired for this project is a 1977 small block passenger car carb that had a Holley reman sticker on it. It was in good overall shape but once disassembled we found idle passages epoxied shut and some "unusual" jets and bypass sizing. My friend Bob contacted Cliff Ruggles who is one of the few real Q-jet gurus and between the two of them they developed a plan of action to make this carb work correctly with my engine. Bob is in Pennsylvania but has an engine in his car similar to my build. His is a 427 ci vs my 383 ci but has a similar cam that builds tons of vacuum and tons of low end torque. Once he got the basics on the carb done, he's been running it on his car to fine tune. He got it close enough and it has lots of idle screw adjustability each direction for me to fine tune it once on my engine.
Here is the initial build sheet on the changes he made to the carb...
The carbs spread out on his bench...
All put back together...
I think he's had the carb on and off his engine at least 10 times trying to get it right. Here's how he described the tuning to me via email on Monday...
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Got the carb done, installed on my engine Sat. Immediately had a problem with bad nozzle drip at idle. Turned the idle psi down to 3 psi, lowered the float, changed the needle and seat, this eliminated those three as not being the problem, still a dripper. No control of the idle mixture. Car is not driveable.
Took the carb off the engine AGAIN, decided I would open up the idle mixture screw orifices, .088" to .093", and increase idle air bypass from 078" to .086". The carb I run, .mixture .096" and 098" for bypass air. Those changes on your carb now gave me control of the idle using the mixture screws, idle rpm was about 700, vac about 17", no nozzle drip. Time for a drive,
Sun I picked a stretch of highway close by where I could get to 60 and had minimum salt, not a lot of them right now in the great NE. Car drove OK, no secondaries due to my air cleaner housing it the air valve linkage, stopped and tweaked the cruise A/F, back to the shop. Stacked 5 gaskets on top the carb, tweaked the cruise again, if I adjust the idle to my usual 750-800 very slight nozzle drip. Run up the road again, secondaries only in 3rd gear, air valve at 5/8 turn, I run a little over 3/8. Well wouldn't you know it my Innovate decides to get stupid, it has been acting dumb for most of your carb tuning, now it just want's to throw an ERROR 8, O2 sensor. So knowing what I really ended up with after my last trip up the road not sure yet. Back in the shop it idled fine with full control using the mixture screws, as long as I kept idle slower no drip.
I pulled the carb for the 10th time, dropped the float level from Ruggles "1/4" is good" to 5/16" which is still higher than factory spec, probably 12/32". This affects "pullover", should reduce nozzle drip potential. Reset air valve to 1/2 turn. New air horn gasket installed after double checked everything, new carb to manifold gasket, 600 rpm good idle, no drip, idle mixture screws out about 2 1/2 turns. Ran out of time Sun for another run, and got no A/F meter.
Today was a snow/rain/slush storm, now it's low 20's with 40 mph winds, lots of ice tonight. Can't see getting the car out for a few more days.
I think the carb is good to go. I know if I opened up the idle air bypass a little more I could get the results I want for my engine. But I feel a 383 with 10% less "suck" and less cam, but still idling at 17-18" vac should be able to get the throttle plates closed at 700 rpm, no drip and with .086" bypass.
Here's some good reading. Kinda give you an idea of tuning a carb. Ignition timing mentioned, importance to getting the throttle plates closed. http://www.wallaceracing.com/qjetidle.php
I was concerned about the size of the primary rods Ruggle sent me, stock/Holley installed were 52K, he said run 44K. It appears they will be OK. Will make the trip to the shop again tomorrow. Will recalibrate the A/F meter and hope to get good idle readings, maybe a road test. Would like to see this ship very soon.
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Then last night he sent me this...
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YEA!!!!!!!!!!! Boxing the carbs tomorrow.
Nice day today, almost 40. Went to the shop and first thing swapped the O2 out for one of the used ones I had, recalibrated, Innovate back to normal. Gauge giving good steady readings.
Before starting to tune again I called Ruggles, another 1/2 hr chat about qjets, racing and managed to get his thoughts on where I should be at on hole sizes, I think we're good to go.
Started the engine, warmed it up, a steady 14.0 at idle, 700 rpm, no drip. Two trips up the road to check response, cruise, WOT. A/F gauge now working correctly, set cruise at 15.2, lot of + and - adjustment left on the ATP to play with. Very responsive, but no secondaries in 1st and 2nd, 3rd gear they opened and blew the tires off at 60. This is just a matter of adjusting the air valve windup lighter to get 1st and 2nd. You'll need to tune that on your engine.
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This is just a little look into the science of a Q-jet Carb. They are VERY engine dependant for idle and cruise characteristics and if you don't get them right, they just won't be happy on the street. Once right though, they'll give you great street characteristics, 18-20 MPG in my car, and a full 800 CFM of flow with the proper Air Fuel ratios for those full throttle blasts on the track. I'm happy and fortunate to a have such a capable friend willing to help with this project and I can't wait to get it back running again with this new setup.
My steering column will get here tomorrow so on Saturday I can reinstall it in the car and begin fabbing up the seat mounts to get the new Recaro seats in the correct position. Then when the carb gets here next week I'll finish up wiring the MSD, bolt the carb on and see if she makes heat once again. Then it's off to the fab shop for the roll bar install.