# heres a strange one th400 issue



## Boomstick (Sep 13, 2021)

so ever since i got my 69, it hasnt felt right. it always felt like it should have more power and more "4 barrel sound" at WOT. Well over the weekend i swapped out the speedmaster dual plane for an edelbrock rpm. i had the rpm on before and it just ran better with it IMO and the speedmaster leaked antifreeze at the water pump seal. anyway, when i hooked the throttle linkage up, i got to wondering if maybe it wasn't opening all the way and going to WOT. (i saw an episode of roadkill where this happened). i set my phone up to video so i could watch the carb linkage as i gave it gas and sure enough, it wasn't opening the carb all the way. so i moved the linkage on the carb from the top hole to the bottom smaller hole and videoed it and now it opens the carb all the way. i took it for a test drive and wow what a difference that made on off the line pull!!!! but now the car shifts real early on the 2-3 shift, like almost immediately after the 1-2... 1-2 winds out to where it did before (i dont know what rpm, tach doesnt work) but the 2-3 shift is real early. even if i manually shift it, it still shifts early. now if i let off the gas after the 2-3 shift and floor it, it kicks down and pulls right to where it shifted before i moved the cable. so i put the cable back where it was and the car shifts like it did before. it just seams like it doesnt want to wind out at WOT on the 2-3 shift... any ideas what could be causing this?


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## ponchonlefty (8 mo ago)

my guess would be the vacuum signal to the modulator changed.


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## lust4speed (Jul 5, 2019)

Yes, to the point where you might not have connected the trans vacuum line back up. Also pretty easy to pull the hose out of the vacuum modulator down at the trans while fighting things getting the manifold and carb back on.


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## Boomstick (Sep 13, 2021)

modulator is hooked up and getting plenty of vacuum. the issue only come up at wide open throttle. if i hook the throttle cable up so it only opens 3/4 full, it shifts fine, if i move the throttle cable so it will go wide open the 2-3 shift is very early.


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## Boomstick (Sep 13, 2021)

with the cable here, it shifts like it supposed to but its not opening the carb all the way













with the cable here, it goes wide open but it shifts 2-3 way too early











in both pics the pedal is on the floor


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## michaelfind (Dec 14, 2018)

Isn't this why some vacuum modulators have a screw inside for adjustment? I think clockwise increases RPM shiftpoint.


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## lust4speed (Jul 5, 2019)

My reading and comprehension skills seem to be lacking today. Ignore what I said above because lack of vacuum would delay the shift and not make it sooner like your problem. Only thing I can think of is moving the cable to the lower position speeds up the ratio of carb opening and shortens up the accelerator pedal movement. Maybe check and see if you are reaching full throttle far enough from the floorboard that the kickdown switch isn't being activated. Doesn't seem like the kickdown switch not engaging would alter the shift point that much, but all I have for now.


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## 1969GPSJ (Feb 26, 2020)

does the trans have a shift kit installed I have one installed and car comes out of first at 10 mph


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## Greek64GTO (Dec 13, 2015)

michaelfind said:


> Isn't this why some vacuum modulators have a screw inside for adjustment? I think clockwise increases RPM shiftpoint.


Try backing off on the modulator 1 full turn and see what that does for you.


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## Boomstick (Sep 13, 2021)

I ordered a new modulator. I don't know the history of this car. If it does it with the new one, I'll look into adjusting the governor. It doesn't make sense it only does it at WOT and only from second to third...


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## armyadarkness (Dec 7, 2020)

michaelfind said:


> Isn't this why some vacuum modulators have a screw inside for adjustment? I think clockwise increases RPM shiftpoint.


To my knowledge, yes.


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## armyadarkness (Dec 7, 2020)

Boomstick said:


> I ordered a new modulator. I don't know the history of this car. If it does it with the new one, I'll look into adjusting the governor. It doesn't make sense it only does it at WOT and only from second to third...


Keep in mind, my original 67 modulator lasted 55 years, and my replacement lasted 2 hours. 

So, merely buying a new one may not be a magic arrow. After you do install it, remain skeptical until the fog clears.


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## Boomstick (Sep 13, 2021)

armyadarkness said:


> Keep in mind, my original 67 modulator lasted 55 years, and my replacement lasted 2 hours.
> 
> So, merely buying a new one may not be a magic arrow. After you do install it, remain skeptical until the fog clears.


10-4

They sure don't make anything like they used to. I'm on my 4th mechanical fuel pump since I bought the car a year ago...


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## armyadarkness (Dec 7, 2020)

Boomstick said:


> 10-4
> 
> They sure don't make anything like they used to. I'm on my 4th mechanical fuel pump since I bought the car a year ago...


Wow... but guys go nuts when we tell them to buy an Edelbrock FP for $200


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## Boomstick (Sep 13, 2021)

armyadarkness said:


> Wow... but guys go nuts when we tell them to buy an Edelbrock FP for $200


Thats my next step... This one is supposed to be an OEM/NOS I picked up at a swap meet, if it starts leaking i'm going with eddy...


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## Jim K (Nov 17, 2020)

Years ago, I had a very similar problem, although my application is stick shift manual. What I found in my instance is the "curve" of the gas pedal arm (that curve has a pivot point that mounts to the floor board) had actually straightened out just a tad through the years, thereby giving the top of the arm (that the throttle line attaches to) less travel thereby not allowing a full opening of the carb. it might not be your case but if the trans is shifting normally while the throttle cable is connected to the outer hole, it might be you don't have enough travel in the cable linkage caused by not enough curve in the arm that pulls it.


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## redhotgto66 (Sep 11, 2021)

on my 65 i had a very similar issue, not with shifting but with that gas pedal, i too was not open full at floored.
i got to looking at the pedal and took it out of the car, stuck in a vise and re bent it to where it opens all the way when on the floor.


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## Boomstick (Sep 13, 2021)

So I put the new modulator in today and nothing changed. I farted with the adjustment for hours and only made it worse... I don't understand what's going on with this car.


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## Baaad65 (Aug 29, 2019)

You said you don't know the history I assume that means the tranny, do you have a good trans shop nearby that knows these transmissions? I don't know crap about autos but have you changed the fluid and filter lately ? It's puzzling that it acts good under normal acceleration, guessing it's got nothing to do with the converting would it? Just throwing ideas out hoping something clicks, I know it has to be maddening...hate to mention it but did you Google the problem? If all else fails convert it to a TKX, Army will be glad to help 😉


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## Greek64GTO (Dec 13, 2015)

ponchonlefty said:


> my guess would be the vacuum signal to the modulator changed.


It sounds like you have good vacuum in normal running or idle. 
What is your vacuum at W-O-T?

When we dialed in TH400's in the tranny shop, we would hook up a vacuum gauge with a rubber hose that would make its way to the driver's hand. Go W-O-T and watch the gauge. You should have 0 vacuum at W-O-T. Normal driving should be 16-20 for your normal shifts...........sounds like you are achieving.

From there you could play with the adjustment.


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## Boomstick (Sep 13, 2021)

vacuum at wot causing the trans to shift??? too small of a carb? 600cfm summit carb, eddy rpm intake on a 350... maybe I'll toss the 750 summit carb on and see if it still does it...


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## armyadarkness (Dec 7, 2020)

Sorry. I just dont know enough about autos, to help.


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## ponchonlefty (8 mo ago)

Boomstick said:


> vacuum at wot causing the trans to shift??? too small of a carb? 600cfm summit carb, eddy rpm intake on a 350... maybe I'll toss the 750 summit carb on and see if it still does it...


you can try that.or maybe try moving the vacuum hose to a different port on the carb and see if it changes.


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