1958-61 348 TRI-POWER PROGRESSIVE LINKAGE

1958-61 348 TRI-POWER PROGRESSIVE LINKAGE


Originally, the tri-power intake option was available on 348 engines in 1958-1961. However, the factory tri-carb linkage was not truly progressive in that the center carb handled engine operation up to about 60% of power demand and once that was reached, the other two carbs kicked in at full fuel flow The factory linkage setup didn't allow for smooth power transfer, which led to the development of our aftermarket progerssive linkage. With our linkage installed,you can adjust it so that the front and rear carbs are activated at whatever point you choose.

In this tech articlewe will show you how lo install and calibrate our progressive linkage.Our kit is a universal-type linkage, so there is some modification required to hook everything up for various Chevy engines. The kit is availabel as PIN 40-168269-1.

The photos that go along with this article show an original intake manifold and carburetors. All of the photos are taken from the passenger-side of the manifold, thus the left side of each photo will be toward the rear and the right side will be to the forward part of the manifold. So that the photos are not cluttered, they are devoid of everything but the carbs and manifold. The following is a basic procedure; your setup may be a little different.

Parts List
1958-61 3 x 2-Barrel Progressive Throttle Linkage Kit

Tools Needed:
5/16" Wrench
3/8" Wrench
7/16"Wrench
1/2" Wrench
Straight Screwdiver

Time Frame
2 Hours

Image #1: The butterfly cross-shaft on the middle carburetor must be replaced with the new one provided in the kit. The basic procedure is to undo the linkage rod that connects to the cross-shaft bell crank.

Image #2: Turn the carb over and remove the three screws that secure the base to the carb body.

Image #3: Remove the four screws that secure the two butterflies to the cross-shaft.

Image #4: Tum the cross-shaft so that the butterflies are accessible and then slide the two butterflies out of the cross-shaft and base.

Image #5: Slide the cross-shaft out of the base.

Image #6: Use a die-cutter to carefully grind the metal that secures the cross-shaft to the bell crank. Install the bell crank onto the new brass cross-shaft with the Truss­ head screw and lock washer provided in the kit. Make sure that the orientation of the shaft and the bell crank matches the original. Re-assemble the middle carb.

Image #7: Start by installing the three carbs onto the manifold. The forward and rear carbs have built-in return springs. The middle carb will use the original return spring that is on the drivers-side of the manifold. There are three swivels and two rods that come in the kit. Two of the swivels will be connected to one rod and one swivel will be connected to the other rod. Two swivels have normal threads and one swivel has reverse threads. One end of the double-threaded rod has normal threads and the other end has reverse threads. You need to figure out which is which and mark the rods and swivels accordingly. I used "N" for normal and "R" for reverse threads.

Image #8: Two of the swivels will be connected to the forward carb. (I suggest using both of the "normal" threaded swivels at this end.) We found that the long screw, which comes with the kit, was not quite long enough.

Image #9: As a result, the nylon sleeve must be shortened. How much it will be shortened will depend on the thickness of the bracket on the carb. For our setup, the sleeve was shortened to about 7/8-inch

Image #10: Install the two swivels as seen in Photo #10. Notice the order of the pieces: the head of the screw goes to the outside, then one of the swivels, then the plastic sleeve, then the second swivel, then the carb bracket and finally the self-locking nut This screw and nut should be tightened.

Image #11: The lever on the middle carb has six holes for six possible locations for the slide attaching screw. For our setup it was secured in the 4th hole from the axis. Notice the orientation of the attaching screw and self­ locking nut. We shortened the bolt because it was too long and it hit the carb. This screw and nut should be tight enough so there is no wobble, but not so tight that it does not swivel. If you want the throttle response to be quicker, the slide attaching screw can be located closer to the brass shaft. We suggest starting with the 4th hole and changing it later if you want.

Image #12: Now install the lever onto the middle carb. The lever will point toward the front of the car and the slot where you will install the lever will be vertical. Using a small box wrench, tighten the screw and nut that secures the lever to the middle carb.

Image #13: Install the swivel (the one with "reverse" threads) onto the rear carb with the screw and self-locking nut. (For a better appearance,we trimmed he length of the screw.)

Image #14: Now determine the length of the long rod that runs to the forward and rear carbs. Hold the swivels so that they face each other and measure the length between them. (The distance on our setup was very close to 9-inches.) Add about I-inch for threads and cut the rod so that there is an equal number of threads on each end of the rod.

Image #15: Install the rod onto the two swivels on the forward and rear carbs. Start to thread the rod into each swivel at the same time, that way the number of threads at each end of the rod will remain the same. As you start threading, one of the carbs will be "open" because the rod is too long, but as you continue to thread the rod, the carb will return to "closed". There will be a point where you have threaded the rod the correct amount where both front and rear carbs remain "closed". If you continue to thread the rod (shortening it), one of the carbs will begin to "open". There is a place where there will be a little "play" and that should be the length where both carbs are "closed".

Image #16: Tighten the jamb nut at the front carb swivel.

Image #17: Tighten the jamb nut at the rear carb swivel.

Image #18: Slide the short rod through the swivel in the middle carb lever. Thread the rod into the outer swivel on the forward carb and then tighten the jamb nut. Slide the lock assembly for the forward and rear carb "kick down" onto the non-threaded end of the short rod. For now, temporarily snug the set screw so the lock assembly is about 2- inches away from the middle carb lever. Do not tighten the set screw on the lock assembly at this time.

Image #19: Now we are ready to set the lock assembly for activating the forward and rear carbs. "Open" the middle carb to about half-throttle and then slide the lock assembly forward until it is in contact with the lever on the middle carb. With everything in this position, tighten the set screw on the lock assembly. By setting the screw in this position, the forward and rear carbs will begin to open at the half-throttle position. You can adjust this to your liking. If you want the front and rear carbs to kick-in earlier, then go back and put the set screw at a location when the lever is at approximately 1/4-throttle.

Image #20: When you are all done, check out all of the connections and test.