carnut114

Hello,
I have a 1972 454/400 rochester carb that is leaking gas. I think there is something wrong with the secondary linkage too since it has never had that rochester 4 barrel kick in. I am looking to get a stock repleacement rochester carb for it. What is the cfm on a original rochester carb? What would be a good replacement for it? Maybe summit rebuilt ones?

Thanks Eric

SteveG75

Rebuilt carbs are generic. GM designed yours specifically for your engine. Any generic rebuilt carb will be a poorer performer than the stock carb.

Now, as for the problem.

Remember, the Q-Jet is a mechanical secondary carb with an air valve on the secondaries that opens with demand. The mechanical secondaries are linked to the primary throttle shaft but are locked out when the choke is engaged.

So, first, make sure that your choke disengages when the car is warmed up. Now, with the engine off, and choke disengaged, have someone floor the pedal for you. Manually open the air valve and make sure that the secondary thottle blades are opening fully.

If that all works, the next area is the air valve. It is springloaded and also has a vacuum pull off that prevents it from opening too fast. The spring preload should be set to about 3/4 turn past zero, no more.

One thing to take into consideration is that the air valve only opens with demand. So, when I floor my engine at 1500 rpm, I don't get much kick. When I floor it at 3000 rpm, big kick.

I would suggest getting a copy of Cliff Ruggles book on Q-Jet's. He also does rebuilds that will be custom tuned to your engine. http://www.cliffshighperformance.com/

BTW, all Q-Jets are 750 cfm except for some very rare Pontiac 455 SD carbs that were 800cfm.

Rickman

Eric,

I am no carb expert, but there are some on here, most notably Lars. Do not go the refurb Summit route. As I recall they are rebuilt by Holley and imo, well, let's just say, not great. Ask me how I know.

Do some research on here. There was just another carb thread the other nite on CF specifically about replacing your Q-Jet w a Holley and how not just any Holley is the best answer.

Personally, I would try to find a regional expert and see if your Q-Jet can be rebuilt.

Rickman

Lemans Blue 69

If that all works, the next area is the air valve. It is springloaded and also has a vacuum pull off that prevents it from opening too fast. The spring preload should be set to about 3/4 turn past zero, no more.

Interesting. What would be the results if the spring preload were reduced? I would assume the air valve would open sooner and how would that effect performance?

SteveG75

If you reduce the air valve spring tension, the air valve opens too fast. Q-Jets do not have an accelerator pump on the secondary side. So, with the valve opening too fast, you have too much air and not enough fuel which results in the dreaded Quadrajet bog. This is the kind of bad tuning that resulted in the Q-Jet getting a bad reputation as a performance carb.

IMHO, the Q-Jet is the best all around carb made. The down side is that the supply of carbs and parts is drying up fast.

Lemans Blue 69

Thank you for the reply. Just thought reducing the spring tension might allow for a better response from the secondaries. Presently when I go WOT, there is a hesitation in the engine for less than a second then she crows right up until I take my foot of the pedal. Runs fine otherwise. Will have it checked out.

SteveG75

Now, this might sound counterintuitive, but try tightening up the secondary air valve spring just a hair. And I really mean just a hair. IF you imagine a clock face, think about 1 hour or even less. Literally, like 1/12 of a turn. This will slow the air valve slightly and the bog will be reduced.

Good page here with some pics of the spring.
http://www.73-87.com/7387garage/drivetrain/myqjet.htm

Lemans Blue 69

Steve, Took carb off and will attempt rebuild. Have some friends to assist me if necessary. Read the post about setting the air valve spring tension. Got a lot of info from posts on Corvette Forum to make me want to attempt it it on my own. My major problem is the stripped filter housing on the right side of the carb. A good machinest probably could make the proper repair. Thanks for the reply. John

airtime

i ordered a custom 800 cfm q-jet from smi carbs in california. it is off a 76 corvette and has an electric choke. i have a 489 stroker in my 71 corvette and the carb was set up for my engine. i can say it idles at 750 and runs perfectly. i would put yours on a shelf and just order one from him. nothing worse than a poorly tuned carb.

lars

I can fix those correctly. You can e-mail me for info if you need help with it:
V8FastCars@msn.com
Lars

Faster Rat

Is your carb correct and date coded for the car? If not, you will be better off in the long run looking for a replacement. I had a similar problem with mine...fuel leaking on the intake manifold. This is not a good thing. I found out the carb was from a '67 passenger car. Decided it was best to get a new carb, rather than fool around with leaks, poor performance, age, wear, doubtful results after rebuilding, etc. Found a NORS Q-jet still in the original shrink wrap/tray and just upgraded the accelerator pump. I plan on driving my car for years and decided to bite the bullet now, rather than later. No rebuilt carb will perform as well as a brand new carb.

lars

That might be a bit of a generalization... A more correct statement might be that many rebuilt carbs do not perform well. A correctly rebuilt, set up, and tested carb will perform as good as, or better than, a new carb. However, it's getting tougher to find good, unmolested cores to build these carbs.

Lars

metal man

Off topic here but one of my co-workers says that he met you at a AWS function in Ontario last year. Are you the Lars he was speaking of? If so small world isn't it.

Ric

ripforrey

I replaced a Carter (came with the car) with a Holley; worked great just couldn't close the hood-thanks Jegs. Went to a Jet reman Rochester. Life is good.
Experience is the best teacher, but it has the highest tuition

lars

That's funny... yes, I was at the Annual International AWS Conference in Ontario last year.
Lars

78MahogL82

Lars,

Are you still accepting Q-jets and repairing them?

Lemans Blue 69

Lars, I appreciate your offer and if I run into a bind, I will certainly email you. Thank you.

Faster Rat, I believe the carb is correct dated code for my 69 L46. 7029207 DA.
7= Rochester
0 = Non A.I.R.
2 = 1960s
9 = 1969
2 = Quadra Jet 4 Barrell
0 = Chevrolet
7 = L46, 350 ci, 350 hp. manual Transmission
DA = Corvette
Its a keeper and will be rebuilt. As Lars said, if done right, it will perform.

I do appreciate your alternative solution.

I love working on my baby with my own two hands and I'm sure most of you share the same sentiments.

Faster Rat

I also enjoy working on my car but know my own limitations. 42 years ago I worked in a service garage and rebuilt (simple auto store kit) the Q-jet on my '66 GTO and it worked fine. These carbs are now a lot older and tend to have wear issues that only an experienced set of hands and eyes can rectify correctly. For most of my restoration I have decided to do the remove/re-install thing myself and leave the outside sourcing or rebuilding to the experts. Costs more up front but hopefully pays off by not screwing something up or having to do it more than once. Just my 2 cents.

lars

On a limited and discretionary basis.

78MahogL82

If one were to include a bottle of your favorite drink along with the Q-jet, would that grease the skids sufficiently?