Quadrajet, the misunderstood carb

[Z28SSMAN]

When the Quadrajet was introduced in the 60s, I thought they were a fantastic carb. The first time I rode in my brother's new '67 442, I fell in love with the wot sound.

I never started to hear negative things about them until the early 70s, when people yanked them for the Holley replacement carbs. I was also guilty of that when I replaced the one on my '71 SS-454 with an 800 cfm spread bore that was marketed as a replacement carb specifically for cowl induction Chevelles.

After all the Quadrajet bashing, I saw car after car run competively with these carbs, and after I rode in a guy's '65 GTO, I had to look under the hood to see if a Quadrajet was making this car perform as it did. I then was convinced that most Q-Jet haters didn't really understand how to get the most out of them.

Here are a few interesting articles about the mysterious carbs....

http://www.carcraft.com/howto/57178/index.html

http://www.circletrack.com/techartic...ech/index.html

http://members.shaw.ca/crussel/quadr...ing_Paper.html

http://www.buickperformance.com/qjet.html

[MR.hp]

Ya Q-jets are great carbs. I remember when I raced a guy in an AMC and got beat with the Q-jet on my bird years ago and I found later that the secondary rods I had in it were too rich [came out of a turbo t/a] I replace them with some leaner ones and smoked the guy on the second meeting. Needless to say he was not a happy camper then after that, Then I blew the doors of a 74 camaro a friend of mine had and he could not believe that a Q-jet carbed car could beat his highly modified engine with a holley on it.

Those GM engineers are pretty dumb for making all this bad stuff over the years ehh.
Look at the LS1, fErd guys say thier junk, but the fErd guys still keep getting thier asses handed to them LSX cars. I suppose we should get rid of the new LS3 now. LOL

[Z28SSMAN]

That's pretty funny. There's nothing worse than getting beat and then looking under the hood to see a Q-Jet.

The one article pointed out how critical tuning the primaries are. I guess I only concentrated on the secondaries. I loosened up the tension spring on the secondaries and wondered why I got the Q-bog syndrome.

I saw a Corvette at Brainerd leave the line and ride on the back wheels for about 60ft. I went over to look at his car and saw a Q-Jet on there and just scratched my head in disbelief. I talked to him fo a couple minutes and he said it was critical to maintain the bowl with fuel since one bowl feeds all four barrels unlike Holley.

[big block fiero]

There is a feature that q-jets have that few people take advantage of. This all correlates with the secondary spring adjustment and vacuume dashpot restriction tunning. I am refering to the fuel pullover enrichment holes located just above the vacuume secondary butterflys.
The static position of the butterflys in relation to the holes is critical when a guy smashes the throttle open. The primaries have an eccelerator pump but the secoundarys only have these holes for quick fuel enrichment. If the secondary mixture curve correlation is spiraticly lean then a guy will naturally cover it with an overly rich primary pump shot. I'm shure you get the idea but another symptom of this same mal-tunning error is when a guy is in gear running a highish rpm but the throttle barely open, then when he smashes it wide open he gets a noticable hickup before the car lunges forward.

[smooth_358]

I know alot of guys that have tricks for these things, mostly because they are olds guys and have figured them out over the years. . after seeing how the give away that I gave my friend for his enduro car performed.. I was kind of disssapointed that I didn't keep it

[Jumpercables]

How many different style gaskets do they make for the q-jets? I have seen several different patterns.

[Red Rocket]

There is a feature that q-jets have that few people take advantage of. This all correlates with the secondary spring adjustment and vacuume dashpot restriction tunning. I am refering to the fuel pullover enrichment holes located just above the vacuume secondary butterflys.
The static position of the butterflys in relation to the holes is critical when a guy smashes the throttle open. The primaries have an eccelerator pump but the secoundarys only have these holes for quick fuel enrichment. If the secondary mixture curve correlation is spiraticly lean then a guy will naturally cover it with an overly rich primary pump shot. I'm shure you get the idea but another symptom of this same mal-tunning error is when a guy is in gear running a highish rpm but the throttle barely open, then when he smashes it wide open he gets a noticable hickup before the car lunges forward.

Once I figured that out for myself years ago, the Qjets weren't that bad. I have one that ran like a scalded dog and still got 18 MPG on top of a 355 in a K10 Chev... Mixed and matched springs and rods from a few different carbs, set the dashpot correctly, and let the float run high. You could also get away with a wayyy loose air door spring setting if the vacuum to the dashpot is metered correctly along with the correct linkage adjustment. You had to bend the linkage just right to get the initial opening to expose the holes. One other common mistake was choke adjustment, namely the little dog leg shaped dealie that disabled the secondaries when the choke is active.. More often than not the linkage would lock out the secondary butterflies regardless of choke position..

Even tried adapting a larger set of primary blades from another carb (Mopar Super Slant Six two barrel IIRC) and boring the throttle plate to increase flow.. It worked to an extent but we never got it quite right. Didn't get the "closed" fit correct, and had idle and part throttle issues.

Man what a learning experience for a 17 year old making those things run good!! I miss working on them, but not enough to actually WANT to work on them again... LOL!!

Look what the Super Stock and Stock guys can do with a Q jet. Pretty spectacular.

Then there was the guy down here that had two Q jets on a tunnel ram... Boy was that laughable!!! The locals should remember him..

[MR.hp]

I remember when I did the 30 degree secondary throttle pickup to a Q-jet. If you have never done the mod then you should try it makes them run hard. I got 3 or 4 Q-jets left, I should get one going again.

[thedillybar]

Mixed and matched springs and rods from a few different carbs, set the dashpot correctly, and let the float run high. You could also get away with a wayyy loose air door spring setting if the vacuum to the dashpot is metered correctly along with the correct linkage adjustment. You had to bend the linkage just right to get the initial opening to expose the holes.

Glad I found your post. I have been tuning the Quadrajet on my boat (Chevy 350). It does 4000rpm w/ good performance & throttle response. When I lock the secondaries shut, it performs exactly the same...so I've been playing w/ the air valve spring & secondary hanger. I discovered that if I have the hanger rich enough, it performs well even if I have no preload on the air valve spring.

After seeing your post about running it wayyy loose...I'm thinking the dashpot is so tight it's not letting the secondary open. Set the spring at 3/8 turn, going to start loosening up the dashpot rod & we'll see what happens.

[MR.hp]

Glad I found your post. I have been tuning the Quadrajet on my boat (Chevy 350). It does 4000rpm w/ good performance & throttle response. When I lock the secondaries shut, it performs exactly the same...so I've been playing w/ the air valve spring & secondary hanger. I discovered that if I have the hanger rich enough, it performs well even if I have no preload on the air valve spring.

After seeing your post about running it wayyy loose...I'm thinking the dashpot is so tight it's not letting the secondary open. Set the spring at 3/8 turn, going to start loosening up the dashpot rod & we'll see what happens.

Depending on which dashpot you have they do come in different rates, i.e.
some of the older plastic ones have different holes drilled in them so you can taylor the the spring to be used in different rates. I had always bought them for '67 '68 '69 and some 1970 models, Sure did help to get rid of the dredded bog syndrome.

There is a good book on q-jets that has a lot of trick you can do to them for performance, I an not sure but I think it was written by David Roe. See if you can find it as it is good reading.