[Larry Davis]

I have a 1967 Firebird convertable with a super turbine 300 that I took out and had rebuilt at what I thought was a reputable old transmission rebuilder in town (Kansas City Kansas) and they seemed to know all about it when I talked to them and when I got it back they said it was all rebuilt and by the way all of the high speed springs were broke (whatever that meant I don’t know) but now that I have re-installed the transmission in the car it has a problem where it does a hard downshift everytime as I come to a stop. I discussed this with the rebuilder and took the car over so they could look things over and they said that my vacume is too low and that is why it is doing a hard downshift. They also put in another vacume Modulator on the transmission that they said worked better with low vacume and that did not resolve the problem either.
I have a 74-455 engine with an edelbrock performer intake and holly EFI Commander 950 throttle body in the car and am running 13lb vacume at idle and 21+ at 1000 rpm I notice that if I do a quick rev on the engine it might dip down to 8lb for a second but I don’t think that is a problem.
At this point I think there is not a vacume problem and I don’t know who knows the ST 300 well enough to have had this problem before and able to tell me how or what to tell the rebuilder or me to do to fix this. I put a half pint of the Lucas transmission stuff in and this cushions the blow but it is still doing a hard downshift.
Hard Downshifts did not occur before rebuild.
The shift from 1st to second is smooth.

Any assistance/Help will be appreciated!!

Thanks Larry Davis

[Crosley]

the modulator will raise the internal pressure.

the TV linkage can cause a harsh down shift if mis-adjusted.

I would also recheck the band adjustment

[Greg T]

You may want to check your idle speed. High idle speed will cause this problem. Also, does this trans have the variable pitch converter? If so, if the fuse is blown and the converter is engaged it could cause a harsh downshift.

[Larry Davis]

First of all what does TV mean? I don't think anything external is mis-adjusted all I have is the floor shift linkage attached and of course the cooling lines to the radiator , I don't use any kick down for a passing gear, electrical connection I believe (its not hooked up to anything)

I have it Idling at about 650rpm which I believe is proper rpm for that year 455.

[Crosley]

Originally Posted by Larry Davis

First of all what does TV mean? I don't think anything external is mis-adjusted all I have is the floor shift linkage attached and of course the cooling lines to the radiator , I don't use any kick down for a passing gear, electrical connection I believe (its not hooked up to anything)

I have it Idling at about 650rpm which I believe is proper rpm for that year 455.

You do not have the Throttle Valve linkage from the carb to the transmission hooked up?

[Larry Davis]

No I don't have any Linkage hooked up to the throttle body. Only the floor shift linkage hooked up to the floor shifter.

[Larry Davis]

This trans does not have the variable pitch converter it is actually the original transmission from this car in 1967 that was born with a 326 engine which broke and was exchanged years ago.