Thread: Need advice on tunning a carb.

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I have a 750 cfm Rochester Qaud. carb. and I need any suggestions how to tune it. The carb. is on a '72 454cid egine with a automatic trans. I can tune the idle fine, but beyond that I dont know the steeps to make other adjustment. My attempt is to get a better reaction from idle to flooring it.

Originally posted by DennyW
Do you have a picture you can post showing the carb?

Sorry, but I dont have a piture of my carb., are thier no basic steps or process to tune a carb.? The carb. has an automatic choke, and works well, thier are three screws for adjustment, basic idle, wich is adjusted fine. If you need further information about my carb., I can give a discription, of any particular part.

Does it look like this, this is a picture from JEGS of their Jeg's Remanufactured Carbs Rochester Q-Jet Carbs that are available in 600, and 750 CFM. These look like stock replacement style carbs and are listed for the 305ci-545ci engines so this may a picture of exactly what you have?

Originally posted by ChevyPower
I have a 750 cfm Rochester Qaud. carb. and I need any suggestions how to tune it. The carb. is on a '72 454cid egine with a automatic trans. I can tune the idle fine, but beyond that I dont know the steeps to make other adjustment. My attempt is to get a better reaction from idle to flooring it.

if its the Q-JET that you've got ChevyPower you have the 2 hole pump rod to work with, but the main thing on the Q-JET is the secondary butterfly adj. there is a spring adj. to it and if its to tight you wont have full 4 brl. use, and if the butterflys are to loose then you are gonna bog the motor down when you floor it. the back butterfly's shouldn't work at all with the choke on so you need to worm the motor up and then tap on the secondary butterfly's with your finger and if they fill loose or sloppy then you will need to tighten them up. to get the max. out of the Q-JET. you can loosen up on the secondares untill you bog the motor down and then tighten it back up a little and that should be the max. fuel that your motor will burn. i wont go into that now until you check the tension on them, and let us know. make sure the choke works good, get the pump rod adj. properly, and adj. your secondery butterflys right and thats about all you can do to a Q-JET externally. JMO not trying to steal the thread.

Originally posted by lt1s10
if its the Q-JET that you've got ChevyPower you have the 2 hole pump rod to work with, but the main thing on the Q-JET is the secondary butterfly adj. there is a spring adj. to it and if its to tight you wont have full 4 brl. use, and if the butterflys are to loose then you are gonna bog the motor down when you floor it. the back butterfly's shouldn't work at all with the choke on so you need to worm the motor up and then tap on the secondary butterfly's with your finger and if they fill loose or sloppy then you will need to tighten them up. to get the max. out of the Q-JET. you can loosen up on the secondares untill you bog the motor down and then tighten it back up a little and that should be the max. fuel that your motor will burn. i wont go into that now until you check the tension on them, and let us know. make sure the choke works good, get the pump rod adj. properly, and adj. your secondery butterflys right and thats about all you can do to a Q-JET externally. JMO not trying to steal the thread.

Thanks, for the help. This sounds like my main issue, because my egine does bog down when I floor it.

Originally posted by ChevyPower
Thanks, for the help. This sounds like my main issue, because my egine does bog down when I floor it.

check those back better flys.

Originally posted by lt1s10
check those back better flys.

Ive adjusted the secondary spring to the best setting, it still has a hesitation when I floor it. I may have to rebuild, or get a new carb . Thanks for all the advice.

Q-JET BOG IS A TRATE OF THAT CARB. I'VE LEARNED MANY YEARS AGO TO GET RID OF IT WAS TO RASE THE FLOAT LEVEL, JUST ALITTLE. IT WORKS FOR ME ALWAYS.

Here is a site that explains how to tune/adjust it.
http://www.recarbco.com/technical/

Hey Chevy Power, I got a QJET on my Monte. I've been thru 3 of them over the years. For some reason when they get old, they start to hesitate, bog the motor, all that fun stuff. I've just rebuild'm and that fixes a lot of the problem. I also ran an Electric fuel pump that help out by suppling constant fuel pressure. On some the Vac secondary if equppied can be opening to quickly, or too slowly. Usually they had mecanical secondairies. right now I had a vac secondary, and it works great, the best so far.
If It's a lighter car or lower gearing u want them to open faster, but if it's over 4000lbs or so, having it open a little slower will keep the powerband steady over the curve...

also, dirt is its worst enemy.... just a little between the needle and seat can hose you... So A good way to clear it is to rev it up , then cup your hand over the primary butterfly till the engine almost stalls, this causes a great deal of Vacuum to be pulled thru the carb, in most cases clearing out dirt and other crap in there.

I thought but could be wrong, but if the acellerator pump is out of adjustment, or has weak spray, wouldn't that cause hesitation from idle to wide open throttle?

Originally posted by jramshu
I thought but could be wrong, but if the acellerator pump is out of adjustment, or has weak spray, wouldn't that cause hesitation from idle to wide open throttle?

yes!