63 won't hold 3rd gear after Edelbrock carb swap

[Steve1963]

I had to ditch the Carter carb on my '63 after it was deemed non-rebuildable by a mechanic I trust and a Lincoln expert who verified. So I've got a Edelbrock 1406 on the car now. After the new carb was installed, anytime the car's under load it drops to 2nd gear. When I let my foot off the gas paddle it shifts to 3rd until I press down again, then back to 2nd. So far the only fix has been to disconnect the downshift rod (red circle in the picture). The problem with that is I've got an overly soft gas paddle now that's sometimes to much or not enough power and difficult to maintain a steady speed.

The linkages all appear to be set correctly. I thought it could be the vacuum modulator, but a trans shop said there's nothing wrong on that end.

I'm likely going try switch out the springs on the linkage and see if that helps.

I'd be grateful for any suggestions since my mechanic couldn't come up with anything.

[Lee]

Two possibilities:
Did the vacuum line routing to the modulator get changed during the carb swap? It should be direct to the intake manifold, and there should be no other possibly vacuum signal decreasing accessories on that line. If the vacuum signal to the modulator is decreased for any reason (including poor tune or vacuum leaks incurred from the swap), it will cause much later shifting. If it is routed correctly, check your idle vacuum. Should be in the 17-20" range.
Second: if you disconnected the downshift rod, you must somehow prop or tie it back up to normal height. The weight of the rod is enough to partially open the downshift hydraulic passages, and the transmission is always expecting a downshift. Also, the mechanic could very well have goofed up the kick down adjustment.

Edit: one other thing to check...if the effective throttle arm length (distance of the throttle rod from the center of the throttle plate shaft) on the new carb is longer, then you may not be getting full throttle, even though you have the pedal more depressed. So even with only partial actual throttle application, you are dipping into kick down range. You should check that the linkage is adjusted so that when the pedal is just getting to the floor...the throttle plates have just reached fully open, and simultaneously, the kick down rod is fully applied (and not before). I've had to drill new holes in the throttle arms of certain carbs to make this all happen the way it's supposed to.

There is a section in the manual on setting up the linkage correctly. Run through that routine, and see if maybe the new carb throttle geometry isn’t goofing up the kick down application.

[Steve1963]

Thank you for the suggested places to start looking.

Nothing changed on vacuum line routing to the modulator. The line branches in three directions off the manifold. First line goes to locks and HVAC, and there are check valves on this line. Second goes to the modulator. The last and largest goes to the brake booster and front left reserve tank. The transmission shop indicated I had the proper vacuum at the modulator.

As part of the first diagnostic for this issue, after fixing one vacuum leak on the carb itself, we verified the linkage adjustments based on the service manual. The gas peddle is the correct distance from the firewall and everything else appears to be set correctly.

I mentioned with the downshift rod disconnected the peddle is overly soft. With it connected, it's rock hard and difficult to take off from a start slow and smooth, more like jump to warp speed. A video on YouTube of a '64 getting the same Edelbrock carb has much different springs which had me wondering if the springs I've got are creating too much tension on the linkage causing the shifting issue. I'm just not sure what the correct springs would be. Universal springs are available from O'Reilly and Autozone for under $10.

I'll recheck the throttle arm length to make sure that isn't an issue.

[Lee]

It really sounds like something is up with the kick down linkage. Do you have a manual to do the adjustment?

Here’s the way it is “supposed” to work: as you apply more and more throttle, you eventually get to the point where the throttle plates are fully open, and the throttle arm will move no further. The pedal should still be about a half inch or so above the floor. Now as you continue to push on the pedal…that starts the kick down linkage to begin pushing the kickdown rod toward the transmission, and then forces the tranny to shift down a gear. You will also encounter increased resistance to your foot.

You can easily check this operation with a helper to push on the pedal. It needs to happen in this order. The way you describe the rock hard resistance with the kick down rod attached suggests to me that something is wrong in that area.

Run through that routine and let me know what you find.

[Dan Szwarc]

The throttle rod angle to the new carb and the throttle bellcrank is incorrect and the kickdown is being activated.

You can observe this condition if someone pushes the throttle while you watch the linkage yourself.

I had this very same problem. Drove me nuts. Here's my old thread.

[Steve1963]

Thank you, Dan!!