1981 Pontiac Firebird, 1970 Pontiac GTO - Tech Q&A

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.

It isn't advisable to simply swap Quadajets around. I strongly feel it's when hobbyists try to install, for example, a Q-jet originally calibrated for a standard-performance 301ci engine onto a high-winding 474 stroker without proper modification that it lends to its negative reputation. We've had great success using any Quadrajet once properly modified for the combination, and it sounds as if you took the right approach, properly modifying yours prior to installation.

The black plastic cup you're asking about is one of those pieces that can be removed without worry. Its purpose is to fill the void that was left behind when the small altitude compensator known as an aneroid-metering rod was removed from the M4MV casting in the mid-'70s. I'm unaware of any negative issues resulting from its absence, and it actually increases float bowl capacity slightly, which may be enough to ward off fuel starvation in otherwise marginal conditions.

It sounds as if you have the carburetor operating as intended. If it's calibrated correctly, you'll find out just how versatile a Quadrajet can be. It can offer a smooth feel that mimics EFI under normal driving conditions yet provide you with the infamous "four-barrel moan" that hobbyists love.

Stuff A '76 455 In A '70 GTO
I have been a loyal subscriber to HPP since the early '80s. My '70 GTO convertible came with a 455 and has since been replaced with a 400. I have my hands on a 455 that came out of a '76 Grand Prix or Catalina. Will this engine work in my Goat (bellhousing and motor mounts)? Can I put on older heads, intake, and exhaust manifolds from a '70 series? Thanks! Bryan Plummer Jackson, MS

Paul Spotts responds: I've also been receiving HPP since the beginning days of Thunder Am. The answer to your question is yes, as all 455s are drilled and tapped for all the engine mount combinations. In other words, the 455 accepts the early two-bolt and later three-bolt mounts.

You need to research which engine mounts are on the 400 block right now. The '70 455 used the three-mount system, which was new for '70. The '70 400 used the early two-bolt system. Not only are the mounts different, the frame brackets are different as well, and do not interchange with each other. You can still use the two-bolt mounts on the 455, if the 400 has them installed. In other words, you can use the system in the car, as long as everything is lined up correctly. Pontiac came out with a stronger mount system with the new-for-'70 engines that had more torque.

Also keep in mind, you do not need a 455 block to get more cubic inches anymore, as the stroker kits allow a 400 block displacement to increase to 495 ci!

The basic Pontiac block from the mid-'60s has the same parameters to allow interchanging of the timing covers, heads, and so on. With your car, seriously consider either the correct WA (manual trans) or YA (auto trans) coded '70 block or a service replacement block. These are hard to find and can be expensive. Your GTO was originally a four-bolt main 455 with #64 casting heads and the only high-compression 455 Pontiac produced. A '70 GTO convertible with the optional 455 engine is quite collectible these days, and most owners are keeping them as original as possible.

Another suggestion is to use your '76 block and use the Felpro head gaskets as a template, then machine or grind the block chamfers. This allows the heads to flow more as the '76 block may not have those. If the block is bored 0.060, it is not as critical, but will help. You can look at a '68-'70 350 HP 400 block to see what the chamfers look like.

I would also install forged rods, dished KB pistons, and the stock top end (#64 heads, '70 intake, Q-jet, and so on). The dished KB pistons (now Icon pistons) will allow pump gas with the high-compression heads. For this combination, I usually recommend a Comp Cam XE hydraulic-roller 236/242 cam, but the Q-jet needs to be modified for the duration used. Keep in mind this is for the dished pistons and lower compression, and is borderline pump gas for certain areas (quality and octane of pump gas).

For the budget-minded and definitely pump gas, the typical RA IV type cam would work well. That also needs the Q-jet modified for the larger duration. I've also used both the 4.250 and 4.500 stroke (both cast and forged for the 4.250-4.500 is forged) with dished pistons (if you use the 64 heads) to make your 455 a 461 to a 495-CID engine, depending on final bore and stroke. More stroke is more torque for your heavy (over 4,000 pounds) GTO. So as you see, you have many, many choices.

Shaker For A 403 Olds
My '79 runs a 403 Olds, and I have some shaker ideas for this Olds motor. I prefer an Edelbrock 7111 over a 3711, and I have an earlier shaker scoop, a Barnum 491982 that lines up on the stock 403 air cleaner. I may need a shorter filter element, however. Has anyone used a similar setup with a shaker before? Chris Hohenfels Poughkeepsie, NY

Rocky Rotella responds: Welcome to the wonderful world of Pontiacs! The Edlebrock Performer intake manifold for small-block Olds (PN 3711) is generally considered a stock replacement unit, allowing the use of the factory Shaker assembly and positioning it in the correct location. According to Edelbrock's website (www.edelbrock.com), it's most effective when used up to 5,500 rpm. The Performer RPM for small-block Olds (PN 7111) is roughly 2-inches taller than either the stock unit or the Performer, and it's designed for operation as high as 6,500 rpm. As you've found, it does not allow the use of the stock Shaker assembly without modification.

There are a few drop-base kits on the market specifically designed to retain the Second-Gen Trans Am's Shaker in the stock location. To the best of my knowledge, however, both are designed for use with the Pontiac V-8 but can be altered to fit the Olds V-8. You can learn more about them by contacting Blocker's Performance at www.blockersperformance.com or WFO Performance Works at WFOTECH@comcast.net. If you believe you've already purchased everything you need to replicate such a kit, then you might use either as an example to gather ideas on how to fashion yours.

In stock form, the 403 Olds isn't generally thought of as a high-winder. I don't know what modifications you've made to the engine, but if it's close to stock, there might not be much gained when using the Performer RPM over the basic Performer. If your combination is such that the engine will continue making power above about 6,000 rpm, then a Performer RPM might be for you. If your engine will be run at a maximum of 5,500 or less, then the effort installing a Performer RPM and modifying an air cleaner to fit may not be worthwhile.