I went with a smaller jet in my q-jet and I was wondering if I can use the same metering rods

I went from a 750 to a 600 jet

thanks

 

¿Qué?

No such thing.

Q-Jet jets are in the range of about .060" to about .080". They have a number on them, that tells their size. What's the carb number, what size did you take out, what size did you put in, and why?

 

ok the carb is on a 4.1 and it was a computer controlled I replaced it with a non computer controlled carb and dissy

the carb that I put on was off a truck and it was a 750 according to my buddy and it was dumping way to much fuel

so I got creative and took the jets out of the cc carb and cut them to size and amazingly enough they looked the same as the non cc jets after bieng altered.

I put it together and it runs fine and is not running rich and no problems so far

I was just wondering if the metering rods in these carbs were different from carb to carb

one thing that I have to be truthfull about is the fact that the car is an 82 grand prix

I just really trust the members on this board and value thier input.

I hope that others can get a little better gas milage out of there cars with the creativeness that I have come up with.

the jets were definatly a smaller diameter that I put in after minipulation

sofa I know that you are a skilled builder and I hope this does not upset you too much

 

YES!!! I'm upset!!



OK, now that I've got that out of the way....

Q-Jets are all the same size, with one minor exception. There's no "750" or "600" one. They all will flow about 780 CFM. The only ones that are different, came on certain early 70s Buick 455s and a couple of other similar applications, and flowed about 850 CFM. Those have larger primaries. People call the ones on 305s "600" or "650" or whatever; but that's only because there's a little metal tab on the linkage, easily bent or hacked off, that limits how far the air valve opens. It's nothing intrinsic to the carb, like it is in a Holley; where the throttle bores are a different size, the venturis are different diameter, the boosters are different designs, etc. In that respect, Q-Jets are all pretty much the same, and are all the same "size", except for those rare few.

That being said however, there is an ENORMOUS variation in OTHER DETAILS about them, in how they're customized to a specific application. Besides just the jets and rods, there are several sets of air bleeds whose size (or even existence!) varies from one application to another, as well as different springs, settings, and so on. It's really quite a sophisticated design: the same basic thing can be made to fit almost any engine application from about 200 CID up to 500 CID, and applications from subcompact grocery carts to trucks to land barges to hot-rods (the Frod 429 Super Cobra Jet came with a Q-Jet on it!!!), with only those minor tweeeks.

Generally speaking, the worst kinds of them for performance, are the mid-late 70s ones (carb #s starting with 708 or 170-, except for 1708- which is the E4 computer-controlled model), especially ones used on trucks. They simply weren't designed for "performance" at all. Those in that time range have huge air bleeds or none at all, to diminish the "signal" that the carb develops that pulls fuel into the intake; and often, certain internal fuel passages that are cast or drilled so small, that even if the air bleeds were completely plugged off to maximize the "signal", not enough fuel could even be forced to flow to make the engine be out of emissions tolerance. That's something you always have to keep at the top of your mind when working on them: if they're going to make a mistake, the mistake is going to be in the direction of feeding TOO LITTLE fuel (apart from different calibrations for different vehicles). Which is different from a Holley, where all errors result in feeding TOO MUCH fuel. That's why Holleys didn't survive, on stock engines, into the age of emissions.

Likewise, I don't know where this idea that a carb with a higher CFM rating "dumps more fuel"; but it's wrong. Not that it applies in this case though. The fact of the matter is, if a carb feeds too much fuel for a given motor, one jets it down, regardless of its CFM "rating"; and if it feeds too little, one jets it up. Best to leave the "size" of the carb out of that picture altogether.

Without knowing exactly what you have, it's impossible to guess whether what you've done to it, is "right" or whatever. But if I can paraphrase Duke Ellington's comment about music - "If it sounds good, it IS good" - then, if it works good, it IS good ... or at least, good enough. Whether it's the best it can be, or can be made better, or you'll find some circumstance that it doesn't work good in (cold or hot weather, altitude, or something), is a whole other matter. That's up to you to find out.

You can actually buy jets for the non-CC ones, or at least, you could at one time; I used to keep a wide range of them in stock myself. Still have quite a few. Rods too; there were several designs, with either a single taper, "stepped" or dual taper, non-linear tapers, etc., to give a different fuel vs vacuum curve. There were quite a handful of power piston springs for them, which control how the rods move with vacuum. You can use a stiffer or lighter spring, to make their "crossover" point between cruise and power, occur at different points. You can set them up to just sort of "jump" from full rich to full lean, or to have a long slowly changing curve over a wide range of vacuum, or anything in between. And then, there's all the things you can do to the secondaries; different rods, hangers, spring tension, and so on. Like I said, ENORMOUS variations are possible even within the same casting. You just tune on it until it works like you want.

 

Just for clarification sake, all Q-jets (other than BOP big blocks) are 750cfm, CC or non-CC.

I don't even know what the jets in a cc-jet typically are. Pull them out and see what's stamped on them.

Remember the rods are very different from cc->non-cc setups. They are stamped with a number, 40-52 or so, and a letter, K, M, B etc. You may have 44K's for example. If you get fatter ones (higher number) they will fill the jet more, making it leaner - DURING CRUISE ONLY. During power the rods pull out to their "power tip" and all rods have the same diameter power tip (sigh, ok, some exceptions to that rule...). Either way:

-Only jets effect WOT usage
-Jet *and* rods affect cruise. ie. rods have zero effect on WOT usage. That's the key.

That's the primary side only of course, secondary rods only affect high throttle usage obviously.

 

Just for the record...

There are quite a few of the higher airflow 800 CFM carbs running around out there- including many Chevy style QJets. I have one on the motor in my Malibu, as a matter of fact. The only difference between the 800 CFM models and the more common 750 CFM units is that the primary main venturi diameter is larger on an 800 CFM model (easily identified becuase there is a pronounced "lump" on one side of the main venturi where a 750's main venturi is a perfectly smooth circle all the way around). Any other differences in airflow are, as stated above, usually the result of artificially limiting secondary air valve opening, although the basic carb is either the 800 or 750 casting and capable of that full CFM if the limiting tab is cut or filed down to allow 85-90* opening.

Most of the cc-QJets I have taken apart off of V8 applications (Camaros, Firebirds, Monte Carlos) have a primary jet diameter of .076-.077 and the 2-step rods are usually .056" x .026". You don't want to use computer controlled jets and rods in a non-computer controlled Qjet. Although, I must admit I never thought of cutting the "rod tower" off the top of a cc-jet before! How did you screw it in once you did that? What did your screwdriver grab onto to allow you to screw it in?

To the best of my knowlege, all computer controlled QJets are based on the 750 CFM casting design.

 

there is a hole on both sides of the cc jet by the base. I cut in the middle of those holes and used the half holes for my screwdriver