[CastleBravo]

Hey, hope everyone had a happy Thanksgiving.

My switch from a YD 389 2 bbl to 4 bbl carb was very successful but one issue came up.
The transmission now shifts pretty hard from 1st to 2nd and it won't shift from 2nd to 3rd until 60 mph or so.
I'm really stumped with this one. I made the vacuum connection to the back of the carb just as it was before.
The only part I can see that's different is the kick down switch. There doesn't seem to be room for it with the 4 bbl so I left it off and I don't see how that would make a difference with the upshifts.
Before with the Rochester 2 bbl shifts were smooth as silk and almost imperceptible.
Is there a vacuum issue here? Wondering if the lines are the same on a 4 bbl. or is there a calibration issue?
I used a factory stock Carter AFB with factory manifold. It has the same vacuum connection port on the rear for the ignition advance, power brakes and transmission. Everything bolted right up.
Is there anyone else that's done this same mod? Do I need something additional for the tranny??
Hope someone here can help me with this one cuz I'm outta my league when it comes to a transmission problem.
Thanks, Kevin.

[CastleBravo]

Hey, hope everyone had a happy Thanksgiving.

My switch from a YD 389 2 bbl to 4 bbl carb was very successful but one issue came up.
The transmission now shifts pretty hard from 1st to 2nd and it won't shift from 2nd to 3rd until 60 mph or so.
I'm really stumped with this one. I made the vacuum connection to the back of the carb just as it was before.
The only part I can see that's different is the kick down switch. There doesn't seem to be room for it with the 4 bbl so I left it off and I don't see how that would make a difference with the upshifts.
Before with the Rochester 2 bbl shifts were smooth as silk and almost imperceptible.
Is there a vacuum issue here? Wondering if the lines are the same on a 4 bbl. or is there a calibration issue?
I used a factory stock Carter AFB with factory manifold. It has the same vacuum connection port on the rear for the ignition advance, power brakes and transmission. Everything bolted right up.
Is there anyone else that's done this same mod? Do I need something additional for the tranny??
Hope someone here can help me with this one cuz I'm outta my league when it comes to a transmission problem.
Thanks, Kevin.

[J_JAMNIK]

Just had a problem with an early shifting transmission and did a lot of reading the past few days. I looked at the service manual for both TH350 and 400 transmissions. Sounds like you have a weak vacuum problem. Try connecting the modulator vacuum line directly to the manifold vacuum port on the front of the manifold. The rear port may lose a significant amount of vacuum due to any number of problems. Especially if it has the brake booster and three or more vacuum ports comming off of it.