Ive recently built and installed a mild 429 and 2000 stall tc in my 78 F-250 SuperCab 4x4 and now the tranny shifts oddly. When I drive easy I barely notice the 1st to 2nd shift (which is good) and 2nd to 3rd is slightly noticeable. BUT when I get on it hard at WOT 2nd to 3rd has a long delay. The engine revs but the tranny doesnt engage unless I back off the throttle a bit. Is it possible the engine isnt pulling enough vacuum to shift into 3rd gear?

 

It does require about 10 inches of vacuum to upshift.

 

So how do you get around this....besides not flooring it?

 

Make sure that the vacuum is manifold vacuum and not ported vacuum.

Vacuum to the modulator should drop when you increase the throttle.

When you say long delay into 3rd do you mean the shift is long and drawn out, or the shift timing is delayed?

 

Its definitely manifold vacuum. Basically it shifts out of 2nd and the engine revs. I back off the throttle before it hits 6k rpm then 3rd gear catches. I really dont want to let it go past 5500rpm. At WOT its not a very smooth shift. I think it should be a lot quicker so Im not sure what I need. Maybe a shift kit? For regular use this isnt really an issue since it shifts fine when going easy on the throttle. But if I got a wild hair to run this in the mud drags next summer I would like to be able to floor it and not worry about missing a shift.

 

make sure you have a metal tube most of the way to modulator. Rubber hose all the way can be bad and cause shifting problems. made this mistake once.

 

I agree with that and is your kickdown rod/cable adjusted correctly?

 

There is the original metal tube for the modulator and the rubber vacuum lines that connect it seem to be good.
Lazy, you might be on to something with the kickdown rod. The original rod is sitting loose right now and one of the things I need to get is a Lokar kickdown cable kit. I will have to get that and see if it makes a difference.

 

Good man. I bet it does make the difference.

 

I havent gotten my kickdown yet but Ive developed another problem. The tranny no longer shifts on its own. I can manually work it with the shifter but it wont go 1-2-3 on its own. When driving in 'D' and slowing down it down shifts fine but wont up-shift when driving. I have the kickdown rod wired forward so I know its not bouncing around causing weird issues.

 

Sounds like you have lost the the vacuum supply to the trans or have a faulty vacuum modulator on the trans. or the kickdown rod is wired to wide open throttle.

 

I need to get a vacuum gauge in my cab to see whats going on with the motor while Im driving. I found last night on my drive home that if I let it rev up to 3200 rpm it will shift on its own. This latest deal started after I made an idle air mixture adjustment to the carb. Its looking like every time I mess with the carb something different happens to the tranny. And yes the tranny gets its vacuum from the manifold.

 

Something funny. From your description it sounds like the trans. "sees" no vacuum at all, not reduced vacuum but none.

 

if the kickdown rod is wired forward or down that is holding the gears making the trans think you are flooring the gas cause this is where the carb kickdown rod attached. i would get or make a kickdown rod for that before you hurt the trans.

 

I have a question my 1977 F250 it has no problem shifting but when I’m going down the road I have to take my foot out of the accelerator and it will shift that’s only way it can get it to shift by itself if my foot is out of the Excelerator what should I do about this what can I add on I was told I have a Bing cam in my truck so that is what steals the vacuum away that way my transmission needs to shift by itself