Here's a little info before I get to my problem. I have a 77 F100 with a 302 and C4. In May of 2009 I rebuilt the engine and had the transmission rebuit. At that time the trans went out, no forward, no reverse, nothing. So far everything has been fine.

Now with the problems. First off on cold starts the trans takes about a mile or so before it will shift from 2nd to 3rd. Low gears are fine, reverse is good. Dosen't seem to matter how long I let the truck warm up before I take off it does the same. I try to manually shift and it doesn't make a difference.

Once the trans makes the shift it will then up shift fine. Downshift like at a stop sign or what not sometimes it will sometimes it won't. If I burp the thorttle on take off it will downshhift then up shift fine.

The last problem i'm having is when at cruising speeds around 60 if I have to let out of the gas and try to get right back into it, the trans acts as if it's in neutral,get down to around 40 or manaully downshift it engages and all is well. Also if you try to keep it steady around 45-50 if wants to jump in and out of 3rd. Once engaged it olds, no slipping and if you stand on the gas to pass,it will kickdown and then shift back up again no slipping.This last problem started the other day after I rebuilt the carb. I checked the vacuum line to make sure i remembered to hook it up and it was.

Sorry so long just wanted to give as much info as I could.

Thanks Kevin

 

Not an expert but it sounds like a vacuum problem. The trans must be connected to full manifold vacuum. If connected to the carb the port must be below the throttle plate (throttle butterfly).
The rubber pipe connections on the vacuum line should be checked for splits/cracks.
With engine at idle if you pull the vacuum pipe off at the transmission there should be a noticable change in engine running if the vacuum system is good.