Can anybody tell me where I can purchase an assortment of Quadrajet main jets? Also, once I remove the airhorn, how do I identify what size jet is presently in the carb? Thanks,


Scott

 

Try Cliff Ruggles at http://www.cliffshighperformance.com/ for parts.

If the carb has never been modified, the carb part number can be decoded into what jets and rods you have. If it has been modified, the jet sizes are stamped into the jet.

Just be aware that the Q-Jet uses a main jet/rod combo. Changing just the jets can have unintended consequences. Since all main rods have the same tip diameter (0.026" unless using the fat truck M rods), changing the jet will affect both WOT and cruise mixture. If you just want to richen cruise, you need to change the rod, not the jet. Conversely, if you want to richen WOT but not cruise, you increase the jet and the rod so that the area difference is the same at cruise.

Remember, all Q-Jet main rods have the same power (WOT) tip. The rod number referes to its diameter at cruise. And, you cannot go from a 74/47 jet/rod combo to a 77/50 and keep the same cruise mixture because we are dealing with area, not diameter.

 

I can't just optimize WOT with jet changes, and THEN optimize part throttle with metering rod changes?

Scott

 

You can do that since the power tip is constant. I was just mentioning that you have to be aware of the jet/rod interaction.

 

Oh, I see, well thanks for the input. I don't really know what I'm doing, but I'm gonna try to get optimum a/f ratio with an lm-1 o2 sensor or equivalent. Wish me luck.


Scott

 

Here's a good place for Q-Jet parts:

http://www.carburetion.com/quadrajet.asp

 

OK, before you start, go to Barnes and Noble and get these two books and read them cover to cover. The Doug Roe book has good picks and the Ruggles book is full of important info.
http://productsearch.barnesandnoble....g=False&ugrp=1



Then, contact the author of the following thread via email and get his Q-Jet tuning paper. Lots of good info.
http://forums.corvetteforum.com/c3-t...available.html

One of the down sides of tuning Q-Jets these days is the lack of available rods/jets (especially the later carbs) and secondary hangers. GM had hundreds of combos available. We are lucky to find anything these days.

 

Edelbrock manufactures replacement parts for q-jets. They bought Rochester's tooling and produced the carburetor under their own name for a time.

They still have almost everything you need to tune a Qjet (including hangers, jets & metering rods)

http://www.edelbrock.com/automotive_...ss_q-jet.shtml

 

Be aware that the Edelbrock primary rods are the fat tip truck rods (they dont tell you that!) so you need much bigger jets to get the same WOT metering area. Cliff may be able to set you up with the right combo just from his experience.

 

I have successfully built and raced these carburetors, winning two national championiships. Roundy Round stuff, so driveability was king. www.jimboscarbs.com I am no longer in the business but would be glad to help you and answer any questions you may have.

I have read practically every book published on carburetion. The best Q-jet Book to date is Cliff Ruggles book. He is a extremely nice guy and the authourity on these carburetors for restoration purposes.

Things I can tell you from countless hours on the dyno with these carbs is they will outperform a Holley carburetor every time in HP and TQ.
They are very sensitive to air cleaner design as far as HP goes. Never and I mean never put a spacer under these carbs. The factory open plenum gasket makes the most HP to the tune of about 25 to 30 more at the crank than everything else I tested. The Gm engineers got this right.

If you are putting it on a square bore intake the best adapter with in 3 hp of anything I tested was the $26.00 Mr Gasket adapter you can get at autozone with a little blending you can pick up the other 3 HP.

The biggest single thing to making these carburetors friendly is to plug the leaky seal plugs in the middle casting bush the primary throttle shafts ( this is coverd in cliffs book) and resize all of the idle circuits they were very anemic from the factory to try and get through emissions testing.

A correctly prepaired Q-Jet will crank with out touching the accelerator with the choke flap removed. When these or any carbs are right they will completely change the tone of your engine. Just a note cliffs calibration in the back of his book for these carbs is very very close if you use it follow it to the letter.

I have nothing to gain in saying this so for what it's worth performance per dollar, a carburetor is the best buy you can make...

I will tell you that you will not get these carbs perfect unless you have access to a Dyno but you can get them close.
I have built hundreds of these carburetors and about one out of five I had to tune on alot to make good dyno #s. The desparity is in the castings the quality was poor.

Lastly I can not preach this enough "MAKE SURE YOUR IGNITION IS IN GOOD WORKING ORDER". I have preached this to every racer I sold a carburetor to. The winners either know this or they listened to me... LOL


FWIW Jimbo