tuning a quadrajet

lkothe · Feb 9, 2016

Love hate relationship! ! They are some of the best and some of the worst............. they don't have the huge bowl slosh that some of the others have (Holley) if I remember right. Set up properly they are very good carbs. Can't say I'd be any help, it's been 25 yrs since I messed with one.

swampratt · Feb 9, 2016

I have one on my 57 that i drive almost every day 336 gears th350 and a 350 chevy engine ..18 MPG
AFR is 18.5 at idle and power is in the 13.5 afr and cruise is 14.5.

Almost perfect.. lots of tweaking can be done to these and MPG is really no better than any other carb once you get the correct AFR numbers..

I have tested many carbs back to back for MPG and many different sizes.. all fall within 1/2 MPG of each other once dialed in..
But big power differences in different carbs..And the slosh is very important..My Q jet gets slosh issues when I get her sideways really quick and then come to a quick stop.
Drive it like you stole it.

mightymouse · Feb 9, 2016

I'd much rather have a Holley. Easiest carb in the world to tune. Even back in the 1970's, it was hard to find a guy who could make a Q-Jet work well. Not saying it can't be done, but guys that can are few and far between.

Toney · Feb 9, 2016

If you don't open up the secondaries the qj gives you good gas mileage.

The dead spot is usually the vacuum pot letting the secondary air butterfly open too fast.

I've rebuilt hundreds of carbs, I like the qj, the hollies are ok, the carter's are the ones that really have a dead spot in them...

vvvvvvv · Feb 9, 2016

Larry Morgan said:
I've heard them called that before too. Seems people either hate them or think they were pretty ingenious. The vacuum adjusted metering is kind of a cool idea and generally gives them better fuel economy.

The haters are lazy and would rather bolt something on that does 80% of the job for 20% of the work. Just having the wrong rod and jet combination for the current conditions can make them really lack in performance. Get it right and there's no better feeling than when those big ass secondaries open up (especially if you're running one of the 800cfm carbs).

rlongnt · Feb 9, 2016

People remember things so much better than they really were.

swampratt · Feb 9, 2016

I never quit running them..I remove the vacuum pot and tune and you can eliminate the bog.
Well that and all my engines have had high compression and small well ported intake runners to keep velocity up.

I actually liked the frowned upon carter ones..No problems with the Rochester ones either .. just make sure you can tune.
A lot of the bog was the anemic timing curves.. couple that curve with vacuum advance that drops out when you floor it.
WOW 28 total timing when i most need some extra power..I really hated GM engineers for some of the odd ball stuff they pawned off on us.

You guys that run the Q jet Look at where the secondary fuel shot comes from.
The 2 tiny holes above the rear air valve door.. You know you have many different tops and some of those have the hole way above the door and others the bottom of the hole is just below the air valve door.. the fuel shot on that one comes in sooner.. helping to cover up the so called bog.

You can fill it and drill it lower.
You all must also be familiar with the lettered secondary hangers.. you can drill extra holes in those also.

You can have a lot of fun with the Qjet.

Rooster1971 · Feb 9, 2016

I really like the quad. Small primaries for good gas mileage.

Usually the bog or hesitation is in the adjustment on the spring for the air flap on the secondaries. Adjusted properly it's a smooth transition.

Sometime the throttle shafts get loose and suck air causing a lean condition at idle causing a rough idle. They can be re-bushed though.

I'm no carb guru but have them on my GTO and C10 and work great. I did put an Eldebrock on my 71 firebird after pulling my hair out trying to get that quad right.

RidgeHunter · Feb 9, 2016

I'm one of the lazy guys. I run an Edelbrock.

I do love a quadrajet tho.

I'd take swampratt up on his offer if I were you! Cool learning oppurtunity.

71buickfreak · Feb 9, 2016

QJs are definitely hard to tune. I am by no means a tuner. I know just enough to make it harder for my tuner to get it back to normal! I have rebuilt many QJs, and they always seem to run well. I don't use QJs on performance stuff though.

Holley carbs are easier to tune, but they have their issues too. Edelbrocks are essentially aftermarket versions of the QJ- bolt it on and go, very little tuning.

tuning a quadrajet

lkothe

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swampratt

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mightymouse

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Toney

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vvvvvvv

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rlongnt

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swampratt

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Rooster1971

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RidgeHunter

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71buickfreak

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