ok recently i installed a edelbrock performer intake manifold. and every since then my transmission shifts weird. under normal driving it shifts from 1st to 2nd fine but it shifts about 500-700 RPMs late from 2nd to 3rd. but then when im at WOT it shifts from first to second really early like at 20 MPH when it normally shifts at 40 MPH, but then it shifts from 2nd to 3rd like it used to. could putting the intake manifold on cause it to be getting too much vacuum becuase it gets better flow now?? we put a little valve on it to where you can open and close the vacuum however much you wanted but that did absolutely nothing.

 

Sounds like a vacuum leak to me. Check all vacuum connections and the base of the carb for leaks. Check your intake gasket as well and make sure it did not slip out around a intake port. Also, be sure the trans is connected to manifold vacuum...the TH400's have an adjustment screw on the modulator valve you can try adjusting it.

I do not believe simply changing the intake would cause a significant change in your vacuum pressure. It did not on mine...16" before and after...

 

The lack of vacuum would cause a late shift. If you screw the adjustment of your modulator valve (located inside of where the vac line connects) all the way in, it will restrict the vacuum and cause the trans to shift only when the line pressure gets high enough to override and it should hammer 2nd (like a "safety shift")

Screw the adjustment way out, and it will allow the vacuum to maximize to the modulator and the trans will shift as early as possible.

What happens when you shift the car manually? Can you force it to act correctly?

I really can't see that the intake change would cause a problem with early shifting.

You might check your governer gear. Located behind a cover held on with 4 bolts on the side of the trans. The gear on the end of the governor is plastic, and it is possible that the gear is "apple cored" limiting the output shafts ability to make it rotate at the correct rpms.

Just my 2 cents. Hope it helps.
good luck - bk

 

What is your manifold vacuum and did you ever check it before the manifold change?

What gasket did you use and how many pounds of torque on installation
Did you use silicone on the front and rear rathar than gaskets

Is the intake leaking any oil anywhere, and last
How long after the install of the intake did you wait before you started the engine?

 

Agree with the change in vacuum cause. On my 76 the light and transmission vac lines are tee'd into the same source (not stock but works ok for now...) and I have a slightly leaking right headlight accuator. I notice my car has a sublte shifting difference when the lights are up vs down due to the slight difference in vacuum at the tee.

Make sure the vac line to the transmission didn't come loose when you were disconnecting/reconnecting it.

 

lack of vacuum will not cause an early shift.

that is how a modulator valve works, when your vac drops, like during exceleration, then the trans is held into gear until vac increases enough to move the valve behind the vacuum modulator, then it lets it shift.

if you disconnect your modulator, your car will not want to shift. it will, eventually, if you give it enough rpms and force the trans to override the modulating circuit.

the intake could have changed and caused the later 2-3 shift you are noticing (less vac). but, at WOT, there is no vac going to, or required, at the trans.

but, your problem with the way early 1-2 shift at WOT is most likely internal trans probs.

your governor valve is taking a reading on output speed (like mph) and tells the trans when to shift based on ground speed. (this is done with centrifically controlled weights/springs in the governor)

try shifting your trans manually at WOT. pull it into low, floor it, and shift into 2nd when you are ready for it. (the trans might 1-2 shift on its own, even if you hold it into 1st too long.... safety shift)

this will tell you if it is able to do what it is suppose to.

i bet that if you drop the pan you will find some trash floating around causing your valve body to hold on to valves that should be moving freely.

although, it seems that about the time i think i have something figured out, it turns out i figured wrong.

let us know what you find when you get it fixed.