CLASS RACER FORUM - Q jet primary jetting?

JETTING
I posted a series of articles a few years back on the Quadrajet so for those of you who missed it , here's the section on primary jetting. If anyone would like to see the one on secondary jetting let me know and I'll post it. Danny @ Q-Jet Performance

Experimentation is the only way to find out how much fuel your engine requires. I wish I had some magical formula to share with you to save a lot of trial and error but I don’t. Everyone’s engine is different from camshafts, compression, cylinder heads, manifolds, headers, etc. To begin your quest you should have an ample supply of jets, primary rods, and an assortment of power piston springs when you go testing. Edelbrock has a good supply and GM still offers some. The only problem that I have run into is finding primary rods in the .038-.042 range for the late model Q-Jets. Remember that these are not inter-changeable with the ‘74 and earlier ones because they are .080 shorter. If any one knows of a source maybe they will post it. Let’s begin.

There are three ways to jet the primary side of a Quadrajet:

1. Conventional -- The way Rochester designed it with the primary rods being controlled by a working
power piston.

2. No Spring Type -- This is done by removing the power piston spring and allowing the large taper
portion of the metering rod to rest in the jet.

3. No Rod Type -- This consists of completely removing the primary rods and jetting “down”. The power
piston is left in to seal the vacuum hole.

Conventional -- I would venture to say that the vast majority of Q-Jets in class racing are using this method. One advantage to this way is that it allows you to control how much fuel is available to the idle circuits where as the other two methods are fixed. The power piston is pulled down by high manifold vacuum which places the largest part of the metering rods in the jet. When vacuum drops, the power piston spring raises it up to allow the small end of the rods to be in the jet for your richest mixture. These springs are available with different rates known as cut-in points. The most common one used by Q-Jet builders is a 6-3. This means that the power piston will start to raise at 6 in. of vacuum and be fully up by 3 in. This is a good “ fit all “ spring because most engines will pull 61/2 in. of vacuum. A small list of springs and their part numbers are listed in the Roe book. Just like the power valve on a Holley, you want a cut-in point lower than what the manifold vacuum is at idle so that the power piston is completely down. Ideally, you want to use the stiffest spring that will keep the rods down but will lift the piston quickly when the engine goes W.O.T. This is why you need to know for sure what the engine vacuum is at idle to be able to fine tune the system. While you have the vacuum gauge connected, see how much there is when you’re on the convertor at your launch R.P.M. Try to tailor the cut-in point so the power piston is just beginning to raise at this point. This would not be necessary for trans-brake or stick cars naturally.

No Spring Type -- With this method and the No Rod type, you are going to get the same amount of fuel available to the idle system as to the primary main metering circuits. When I first heard of these types of jetting I didn’t think they would work because there would be too much fuel available at idle and the spark plugs would be easily fouled. I was wrong. If you get your primary jetting correct, the plugs will be mid to dark brown but not black. At least that’s the way it is on my car. It really likes a lot of fuel down low.
The one advantage this type has over the no rod method is that it allows you to jet between steps allowing for more precise tuning. Between certain jet numbers, there are up to ten different combinations. Later on I will talk about how to figure areas and percentages of change but here’s one example of jetting between a .071 and .072. Remember that all primary rods have .026 tips ( except GMC’s motorhome #50D which has .036 ). These areas are figured with the larger taper in the jet when the spring is removed :

x.071 jet w/ .026 Tip = .0034283
.079 w/ .043 = .0034494
.078 w/ .041 = .0034581
.076 w/ .037 = .0034612
.077 w/ .039 = .0034621
.074 w/ .032 = .0034966
.075 w/ .034 = .0035100
.079 w/ .042 = .0035162
.076 w/ .036 = .0035186
.078 w/ .040 = .0035217
.077 w/ .038 = .0035225
x.072 w/ .026 tip = .0035406

No Primary Rods --Based on the carburetors that I have seen and the racers I have talked to, this is a more popular way of jetting than the No Spring type. Maybe it’s because it’s easier to calculate than the No Spring method. I use this type on my car but I never tested it against the No Spring method. I tried them both in the garage one day and noticed the No Rod idled better and had better throttle response than either the No Spring or the Conventional types. So I decided to back to back Conventional against No Rod. As I said in an earlier post, the No Rod was .04 quicker. The rule of thumb for the No Rod type is to jet down five sizes when you remove the primary rods. If you were using .072’s then the replacement would be .067’s. When the areas are figured, the No Rod type is always slightly leaner. Should you decide that this type is best for your engine, here’s a tip. Snip the arms off the power piston that holds the rods. This will make it easier to service the carb because you won’t have to slide the air horn gasket under them when it is removed or installed.

When figuring areas, first find the area of the jet then subtract the area of the metering rod that you will be using. This will either be the tip ( .026 ) or the large taper.

Example for Conventional Jetting a .072 w/ .026 tip :

A = .072 x .072 x .7854 - .026 x .026 x .7854 = .0040715 - .0005390 = .0035406

Example for No Spring Jetting a Conventional .072 set-up :

A = .077 x .077 x .7854 - .038 x .038 x .7854 = .0046566 - .0011341 = .0035225

Example for No Rod Jetting a Conventional .072 set-up

A = .067 x .067 x .07854 = .0035256

As you can see the No Rod and No Spring types would be slightly leaner than the Conventional set-up. The Roe book has an area chart that makes the calculating a lot easier. You may be wondering how I knew what jet and rod combination to use that is close to the .072 set-up. I took some time ( a lot actually ) and figured the areas for all combinations of rods and jets for the three ways to jet the primary. I also did the secondary rods from .030 to .070 in .001 increments with the stock .135 pressed-in jet. I had all this printed, laminated, and put in a spiral-bound notebook. I think I just about wore out a calculator. It really saves time when I or a customer needs to figure the total jetting areas or wants to proportionally change front to rear or vice-versa. I’ll wait until the end of the next thread to show how to do this. There is actually a fourth way to jet that would use a combination of all three types if you wanted to stagger jet but unless someone really wants to know I’m not going to go into it. That’s it for now. Questions or comments ? Thanks for reading this. Danny