I just put my rebuilt 79 460 back in and the current problem is that the c6 tranny doesn't downshift. After much kickdown rod adjustment I've got a situation where upon the first time (after starting) it will shift normal 1-2-3. It will not however, shift down (like when I come to a light) so I end up launching from a stop in 3. Not good!

Here's the deal:

A) Edelbrock carb (with adapter for kickdown rod) on an edelbrock intake.

B) Mallory unilite distributor (vacuum advance)

C) Modulator Vacuum connected with a T between right side of carb and distributor.

D) I definitely bent the kickdown rod when removing/installing engine (hence the adjustmenting I decribe above) so further adjusting could be in order.

E) I've replaced the modulator valve.

F) Tranny shifted fine prior to engine rebuild

Questions:

1. Is it vacuum? Should I obtain a "clean" source of vacuum off of the intake?

2. Is it related to the kickdown rod? I noticed that depending on the placement of the rod I get different shifting characteristics.


3. Bad or incorrect modulator valve (they said it called for three different ones... they gave me one with a "gray stripe")

4. If not any of above what should I be checking?

Thanks in advance and sorry for the long post,

Bob

 

I have never seen the modulator hooked up to the same vacuum port as the distributor before. Normally, this port on the carburetor does not see vacuum until the throttle is opened up a little; the transmission should have a continuous source of vacuum such as from the brake booster line.

Try disconecting your kickdown completely. The lever at the transmision should move freely with only a little internal spring tension. It is ok to drive with it off, it just won't downshift into the next lower gear when you floor-it. You can later put it back on after you find out what the problem is.

Neither the vacuum or the kickdown would typically cause this kind of problem, it sounds more like something wrong with the governor or valve body. How long was the truck sitting while the engine was being worked on?

 

Thanks for the response.

My truck sat about a month (I'm guessing) while I was playing with the motor.

I actually did disconnect the kickdown today. It shifts fine on the way up 1-2-3 but never comes down. If when I stop I put it in reverse or park and then slap it into drive it will go 1-2-3 fine.

I'll see if I can get some vacuum from the brake area. I'll need to get creative with some tees.

 

:-) :-) :-) I tapped into the power brakes vacuum line and now it shifts up and down!!!

Thanks for advice.