Self-Taught Carb Lesson
I have a slightly modified '81 Firebird. The Q-jet I built for the engine runs way too rich-black smoke rich. I followed the "bible" of Q-jets by Cliff Ruggles, How to Build and Modify Q-jets, to the letter in selecting a good core (PN 17083234). Yes, I know it's off a Chevy and was originally calibrated for a 305/350, but it was the closest to the Edelbrock Quadrajet casting I could find.
I selected the "recipe" for a modified Q-jet from Mr. Ruggles book and modified it as follows: idle bypass air, 0.100 inch; lower and upper idle air bleeds, 0.070 inch; main air bleeds, 0.080 inch. This was done to try and get the engine enough air (pull off the brake booster and it idles up 1,000 rpm and leans out, but the idle screws have no effect, and I can close the throttle plates completely). It's not dripping fuel. Other mods were increasing the idle tubes to 0.036 inch. I did make a small boo-boo and accidently drilled the idle channel restrictions too large at 0.080 inch. I have 0.074-inch jets and 0.049-inch primary rods in the carb now, however, no matter what I try it is still too rich.
The engine is a '66 389, bored to 400 and running '68 #16 heads from a 400 (too much compression?) and an Edelbrock Performer dual-plane intake, as well as an Edelbrock Performer cam. It had a stock rebuild many moons ago and has stock 400 pistons, from I believe Speed-Pro, with four valve reliefs, an MSD box firing a HEI distributor, and an Edelbrock fuel pump (too much pressure?)
I am an ASE Master Tech with L1 and 20-plus years with Chrysler (sorry it's Chrysler, not GM) I came in as carbs were going out, so I didn't get to "play" with them much. We replaced carbs, as we rarely rebuilt them.
Any ideas? Did I screw up something? Would a different casting be better? If so, can you give me an idea which one to look for?
UPDATE: I solved the "mystery" when I rebuilt the carb. I had this and another older one apart. It was from '75 or '76 with the adjustment for the power piston in the well.
I bet you can guess what I did by accident when I rebushed the base plates-I mixed them up. While they look the same, when you very carefully examine them, there is a hole drilled through the base plate of the older one. It's just under the power piston, exposing it to full vacuum and pulling fuel from the power piston uncontrolled. The original base plate had the hole, but it was not drilled through, so the power piston gets its signal elsewhere.
I do have one more question. How important is the cup that takes up space in the "well" to the left of the power piston? I don't remember my carb having one, but I can't find any definite answer on whether all had this or not. I still need to tune this carb and don't want any other "mysteries" to solve.
William Readon
Via Internet
Rocky Rotella responds:
Don't feel as if you're the first to have jumbled Quadrajet parts. Any of us who've tinkered with them for any length of time have learned lessons the hard way-even if we don't wish to admit it!
When Rochester designed its Quadrajet carburetor, each unique casting was specifically calibrated to match the exact fuel/air requirements of a certain sized engine at every rpm point under every driving condition. Even though two Quadrajets sitting side-by-side might visually appear identical, they are in fact, very different. Each particular casting number has its own specific characteristics for a certain application, and you've found firsthand that subtle differences in Q-jet throttle bodies (or "base plates," as they are more commonly known) exist.