This a 1980 GM carb, my papers don't tell what make it was on.

Factory primary jet, GM #7031971 (.071") marked 71
Factory primary rod, GM #17056948 (.048") marked 48M
Factory secondary rods, GM #17053659 marked DR

A 76 jet would be really rich, look for a 71 or 72 jet. You can ream out the 68 with a good reamer, its not 100% accurate, but it will get you really close. Just a size or two will make a big difference, so go a little at a time.

A 48 rod is on the lean side of the middle. Keep the rod and change out the spring to a softer one if you have it. You will notice a hesitation when you go from cruise to power circuit if the spring is to soft. When you get the right one you will not notice the change. To stiff will cause a rich condition all the time and your fuel mileage will suffer. This is a tough one to iron out, it takes a lot of drive time to get a feel for how its operating. If your engine is stock like mine, it should make a lot a vacuum, mine has 20" at idle, but since it has little horsepower, I am always in the throttle some for hills or passing. This is where your fuel mileage goes, its like going from a 71 jet to 93 each time the vacuum drops below the spring pressure on the metering rod holder.

Any rod will work as long as its what you want. The later three digit rods like this one (48M) are the same as a older two digit 48 rod. The difference is in the taper from the big end down to the small .026" tip. The older ones use a single taper, the later ones are dual taper so they stay leaner longer.

The secondary circuit probably won't effect much. Just play with the air valve spring to keep from bogging when you hammer down on the throttle. Keep it on the tight side, you should hear it open and feel the engine start to pull, if you feel a quick snap or bog, its to loose.

Just for fun, I figured the square inch of each rod and jet,
76 jet & 48 rod = .00272 sq in opening
71 jet & 48 rod = .00215 sq in
68 jet & 48 rod = .00182 sq in

76 jet & 44 rod = .00303 sq in
71 jet & 44 rod = .00243 sq in
68 jet & 44 rod = .00211 sq in

At full open,
76 jet with .026" rod tip, .00400 sq inches opening
71 jet with .026" rod tip, .00342 sq in
68 jet with .026" rod tip, .00310 sq in

So you see, you can get the same effect with a rod change or jet change, where you get in trouble is the amount of fuel you get through the smaller jets at wide open throttle, remember, the tip of each rod is still .026".
So to stay in the factory air/fuel ratio at full throttle, you need to stay close to the .00342 sq in opening (71 jet). By using a fatter rod, you can lean out the cruise circuit with out hurting performance.

Joe

Last edited by Joe H; 09/07/10 12:29 AM.