Retro-fitting modern, overdrive transmissions into early musclecars is nothing new, and with the rapid raise in gas prices (only two dollars a gallon as this is being written) the trend is sure to grow. The most popular swaps continue to be the 700R4 and the 200R4, both of which are good choices and have their advantages. The physical swaps are straightforward, but the one component that still mystifies many people is the throttle-valve (T.V.) cable setup. If the T.V. cable is not set correctly, the transmission can operate poorly, and its life can be greatly shortened. When connected to the throttle linkage mechanism, a correctly set up T.V. system will pull the cable, causing the part throttle/detent valve inside the transmission to immediately start moving with even slight carburetor-linkage movement.
Designing a pulling arm on your carburetor's linkage that pulls the T.V. cable this fixed distance is the easy part. The challenge really begins when you want the transmission to behave in a specific manner. The physical dynamics of this cable pull can vary radically with different pulling-arm designs which control the rate of plunger travel speed just off idle and determine the shift timing and shift feel of the transmission.
Bow Tie Overdrives has just released a quick, easy, and very precise throttle valve control system for correctly setting up this relationship in minutes, not hours, and allows a wide range of adjustments for fine tuning the transmission's shift timing and shift feel. The system is available for Edelbrock Performer series, Carter AFB, Demon and Holley 4100 series carburetors.
We started by removing the carburetor and gaskets and cleaning the intake surface. We then installed the supplied open-bore gasket and the new, square-bore base plate with the four, small, threaded holes positioned to the driver's side rear. The gasket with the four throttle-bore holes was next.
The new T.V. cable-mounting bracket was then bolted over the four holes in the base plate using the four 10/32-inch pan head bolts (Do not install any spacers between the base plate and the cable-mounting bracket). The new T.V. cable to the transmission was attached, routed up to the carburetor, and snapped into the T.V. cable-mounting bracket.
While the carburetor was off the engine, we installed the new linkage adapter. We pre-assembled the three-piece adapter to see how it went together. It will sandwich over the carburetor's stock linkage, indexing over the lower, stock cable-attachment hole with the raised lip.
We used the adapters front plate as a drill guide and drilled a hole approximately half the way through the carburetor's linkage with the supplied .144-inch (#27) drill bit. (Note: We didn't drill all of the way through the carburetor linkage at this time because it would have damaged the back plate's threads.)
The three-piece stack was placed over the stock, raised-lip, kick-down stud location and to the linkage using two 6/32-inch pan head bolts through the two unobstructed holes in the adapter stack. Only two of the three 6/32 pan-head bolts can be installed because the carburetor's linkage prevents installing the pan head bolt in the upper left hole. A hole needed to be drilled through the carburetor's linkage to allow the third pan-head bolt to be installed.
Before we removed the three-piece stack, we looked into the curved slot and found where the carburetor linkage was protruding slightly into the slotted area (arrow). We marked this protruding section so we could remove it when we had the stack back off to finish drilling the hole we started. The three-piece stack was removed, and we finished drilling the hole through the carburetor linkage. Using a file, we removed the marked material on the carburetor linkage and any burrs caused by filing or drilling so the stack would fit snugly together.
We installed the three-piece stack back onto the carburetor linkage using all three pan-head bolts and installed the cable attachment stud into the slot, positioning it at the bottom-most end of the slot.
The modified carburetor was installed back onto the intake manifold, and the plastic T.V. cable clip was snapped onto the cable attachment stud.
To adjust the T.V. cable we started by attaching a 0-300 p.s.i. gauge to the transmission's hydraulic pump diagnostic port located just above the manual shifter shaft. A 1/8-inch NPT plug needed to be removed to install the 90-degree fitting. The gauge should have a hose long enough to reach the engine compartment. We checked the transmission's fluid to ensure that it was at its proper operating level.
With the engine running, we unlatched the T.V. cable's slider lock (arrow) and pulled the slider towards the front of the engine until the transmission's pump pressure stopped decreasing. This pressure reading was recorded.
Once the pressures were stable and would not decrease any more, we slowly pushed the slider back into its housing until we saw the pressure start to rise again. We then locked the slider back down. This reading should be within 2 p.s.i. of the previous reading. This is the most critical part to these transmissions, so if you have any doubts on setting up this system, redo it until you feel 100 percent comfortable it's correct.
We had now established a correct setup of the throttle valve system. We positioned the T.V. cable attachment stud at the bottom-most position of the adapter's slot. This position will communicate to the transmission in a very aggressive way just off idle.
We can easily change this personality by moving the stud position along the slot. If we raise the stud position in the slot to soften the shifts (moving it up and to the right as viewed from the driver's side of the carburetor), we must correspondingly raise the cable-mounting bracket an equal amount using the spacers provided (arrows). This will maintain the correct relationship between the stud and the cable-approach line. By raising the cable-mounting bracket the same amount as the stud was raised, we will maintain the correct pre-engineered relationship between these two components, which is the cable running parallel to the bottom of the carburetor base.
Each time you adjust this relationship, you will need to reset the cable slider, using the pressure gauge. This will ensure instantaneous pressure response with the new setup. This is critically important. Operating the transmission with low hydraulic pressures can very quickly result in transmission failure.
Bow Tie Overdrives
17359 Darwin Ave., Unit C