700r4 rebuilt, troubles bogging down!

[shortchevy84]

ok, i got a 383 405hp crate from blue print engines, and it has a 670 holley street avenger carb matched to it. its still in the break in phase, about 500 miles on it so far. i got a built 700r4 with a slightly larger stall converter than stock, and its made to handle the 400hp. shifts smooth and everything, however, during WOT play time,  , it shifts good and crisp into 2nd gear, but then just fall flat on its face, as if it had less power than stock. i know what it should shift like, as i have an exact same truck with a not built up 700r4 with a milder engine that works just perfect. anyway, when the pedal is on the floor and it is bogging down, it also only gets to about 25 -3000 rpm before it shifts to 3rd again, again a smooth crisp shift, and then operates properly again.

we thought maybe a carb issue with secondaries opening up too early or too late, and since it was a quick swap of springs that came with the carb, i tried the one stiffer spring, and the one weaker. but the bog down goes unchanged. as i am no expert in transmissions by any means, does anyone know what would be causing this? 1st, 3rd and 4th all seem to operate fairly well. i could be mistaken that it could be 3rd bogging down a little too. its somewhere in the 40mph range. everything seems to be pointing to me tearing the transmission out and sending it back, but i really don't want to do that. i just got the darn thing on the road!! my journeyman in my shop doesn't seem to think its the torque locking up at a wrong time either, but it hasn't been ruled out. anyone else had this issue, or know what i can look for? its an 86 shortbox 383, with 700r4 trans, dynoed at 405hp and 440lbft.

thanks, andrew

[SUX2BU99]

So it does have a lockup converter then? I had a couple of Dakota R/T's that had a 4 spd overdrive auto trans and lock-up torque converter. At WOT, right after it shifted into 3rd, the TC would lock-up and would bog the motor. Well not really bog but it wouldn't accelerate for a second or two I guess as the additional load was being added on, then pick up. I even got it on video once. Maybe if there was some way to deactivate your lock-up and then retry it.

[shortchevy84]

i'm kind of thinking its something along those lines. we tried changing the springs for the secondaries, and no change. also tried holding the secondaries closed and no change. next we are gonna try manual shifting into each gear and getting it up to speed that way. i'm thinking, over all the reading i've done, i remember someone mentioning that the stock torque locked up with something like 7" vacuum? and a better one for a big cam and stall engine would be something like a 3 or 4" one? does this sound right to anyone? it really makes sense, as it happens in 3rd gear, and from what i've read, 3rd does a have a lock up as well as 4th. let me know! thanks