1974 Grand Prix 400 performance issues

Oliverd642

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Hey all, wondering if I can get any advice.

I have a 74 GP with a 400 motor, the car pulls really well from a stop but feels under power at speed. It does not feel like a 220bhp vehicle. I’ve recently changed the cap, rotor and plug wires.

It does not hesitate, it just picks up speed really slow say from 30mph +

Sometimes it will kickdown hard and fly like a rocket other times it will kickdown and slowly get up to speed.

*new air filter
*new ignition cables, rotor & cap.
*64,000 original miles
*original Rochester qjet carb
*true duel exhaust, no cats
*running 98 octane European fuel
*newly serviced trans, engine and diff oil

Small exhaust manifold leak, small oil leak from engine.

Runs really well otherwise
 

melsg5

Staff member
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First we have to determine which transmission you have. It could be Th400 or a Th350. How many cables do you see attached to the throttle linkage of the carburetor?
If you only see the gas pedal throttle cable you have a Th400.
If you see a second cable you have a Th350.
The Th400 uses a switch attached to the gas pedal to control downshifting along with a vacuum line from the engine connected to a modulator on the transmission case.
The Th350 uses the second cable to control downshifting along with the vacuum line going to the modulator.
So which transmission do you have?
 

Oliverd642

New member
28
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1
First we have to determine which transmission you have. It could be Th400 or a Th350. How many cables do you see attached to the throttle linkage of the carburetor?
If you only see the gas pedal throttle cable you have a Th400.
If you see a second cable you have a Th350.
The Th400 uses a switch attached to the gas pedal to control downshifting along with a vacuum line from the engine connected to a modulator on the transmission case.
The Th350 uses the second cable to control downshifting along with the vacuum line going to the modulator.
So which transmission do you have?
Forgot to add that part! I have a th400 trans. Where exactly is the vaccum line?
 

melsg5

Staff member
SUSTAINING MEMBER
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You should see it running on the passenger side of the engine down to the transmission.
Also make sure the wire on the gas pedal switch is connected at the switch and at the solenoid on the drivers side of the transmission case.
______________________________
 

Oliverd642

New member
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1
You should see it running on the passenger side of the engine down to the transmission.
Also make sure the wire on the gas pedal switch is connected at the switch and at the solenoid on the drivers side of the transmission case.
Okey thanks I will check today after work
 
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