Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
idle help
73ss:
I started out with the above specs. .077 LIAB size. I closed up the LIAB's to .063. This resulted in a very rich idle and no control of the mixture screws, Very little difference with them fully seated. engine would not stall. By closing up the bleeds this provided more than enough fuel at the transfer slots to keep things running, correct? Engine would load up if I closed up the choke flap.
I then opened them back up to .070. This resulted in regaining control of the mixture screws. Engine will stall fully seated. If I close up the choke it will stabilize the idle somewhat and pick up about 30 rpm.
Any idle circuit mods also effect the amount of fuel at the transfer slots correct?
I think I need more bypass to get it off of the transfer slots and also slightly more idle fuel. I have quite a bit of transfer slot exposed, but not opened so far as to have any nozzle drip.
With the holley, I was able to ditch additional timing from the vacuum advance at an idle and have a nice steady 12" of vacuum. Tryin to get the Q-jet to do the same.
Cliff Ruggles:
You were on the right track with the smaller LIAB's, just too much transfer slot exposed and probably DCR's a little to generous.
That engine should EASILY be pulling 2-3" more vacuum than it is without a butt-ton of timing at idle, at least by looking at the specs you provided. I'd be real tempted to advance the cam 2 degrees, I'll bet that will make the process a lot easier.
In any and all cases we still have to give the engine what it wants to make it happy. Believe me I've tuned engines effectively that had 5-6" vacuum at idle and struggled below 1000rpm's! You will get there, it's just going to take exactly the right combination of timing, bypass air and idle fuel
73ss:
--- Quote from: Cliff Ruggles on June 14, 2021, 09:29:49 PM ---
That engine should EASILY be pulling 2-3" more vacuum than it is without a butt-ton of timing at idle, at least by looking at the specs you provided. I'd be real tempted to advance the cam 2 degrees, I'll bet that will make the process a lot easier.
--- End quote ---
I did call Howard's last week for their feedback on this cam. Said it should be pulling 13", especially in a near 500 inch engine. Asked what my cranking compression was, 160 psi. (forgot to mention that in my original post). The tech said that's good but could be a tad higher. He asked if I degreed the cam. No. He suggested advancing it 2 degrees......
Also suggested getting some miles on it before getting to carried away.
I've got to give Howard's some credit. Answered the phone right away, didn't get put on hold, didn't treat me like a idiot, and didn't even bash on the Q-Jet like so many of them like to do. I went with Howard's because they had the largest selection of Gen 6 cams. All of comp's stuff for this generation was the extreme thump your mama stuff and we all know how that works out.
I do appreciate help on this. Thanks
Cliff Ruggles:
Last time I looked it was 2021, but I still hear folks "bash" Quadrajets and regurgitate old, inaccurate and outdated information about them.
I woln't touch one made in 1965-66, and try to avoid most of them made before 1969. By the time Rochester corrected the main issues with them it was too late and they had a BAD reputation for bogging and leaking bottom plugs. To this day folks still think all of them are leaking and dab a big glob of monkey chit over the plugs when the "rebuild" them.
The same idiots that tell you to throw it away will tell you to put a Holley Street Avenger on it in the same breath. What a complete and utter POS those are. I've had locals bring vehicles up here using them and despite my best efforts I can't get them to work for chit.
Judging by what I saw at the Car Show at First Friday in town couple of weeks ago Edelbrock Performer carbs are the "go-to" for most. There were at least 150 cars there and at least 80 percent of them had one of those AFB clones sitting on an adapter or aftermarket aluminum intake. Didn't really see too many Holley carbs, and not surprised and the newer varieties aren't all that great.
If I mess with a Holley carb it's going to be from the 70's or early 80's, downleg boosters and rear metering block. My favorite is the 4781-2 850 DP and early 750/780 cfm vacuum secondary models.
I avoid the Edelbrock AFB clones like the plague. It's cool to see what folks do to install them. Cut the factory fuel line with a hacksaw blade, run a long piece of rubber hose around to the left rear fuel supply and secure it with a couple of hose clamps. Talks about a fire waiting to happen....YIKES!.......
old cars:
Single plane or dual plane intake . Idle Rpm ? Vacuum readings in and out of gear.
Optimal idle advance is typically about 32 degrees. So you could have 12 initial and 20 vacuum advance.
You shouldn't need those huge idle tubes and down channel then.
I run a similar cam with 17080213 Quadrajet in my BBC that idles 900 rpm with 14" vacuum in neutral and 750 rpm 12" vacuum in gear. relatively smooth idle.
Next I converted it to DMAB. Idle fuel ratio 13:1 , cruise fuel ratio 15.5:1 with .031" idle tube and .046" down channel.
If you want throttle feel/response like that 750 holley you will need to work on the linkage of that quadrajet to get the secondary butterflies opening much quicker. Then you'll never look back.
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