Won’t stay running in gear
 

We have a 71 Torino with a 302 ci engine and a C4 transmission. Engine runs great, when in park. If we put it in gear (drive or reverse) with a foot on the brake the engine dies. We have checked the timing, checked for vacuum leaks. It has electronic ignition. If we jack up the back end and shift it into gear with out the brake applied it will run.

I am am assuming the problem is with the transmission. Is appears that when the brake is applied it locks everything up, instead of just stopping the rear portion of the drive train.

Has as anyone out there had this type of issue? Could this be the torque converter, transmission valve body or maybe the cliches? Any suggestions?

Off idle, (slight throttle application), does the engine still stall?
Power Brakes?
If so,, checked to see if its losing vacuum through the brake booster?
Have you installed a vacuum gauge on the intake and applied the brakes to see if the vacuum falls off? Or, what the idle vacuum values actually are...Cam/valve timing, carb etc can greatly affect vacuum values that may or tend to show up more within idle conditions under load such as trans application.. Possible a trans issue, but would show up 'off idle as well.
Monitoring idle vacuum may help in a direction to go..

Checked vacuum
 

So we checked the vacuum. It is right at 20 inches at idle, 17 inches when in gear (park or reverse) with brake on. It will idle for a while (maybe a min or so) then it dies. Idle is set at about 1100 rpms. In gear it is about 800 rpms until it just dies. Checked the timing it is about zero to +1. We don’t have power brakes.

A few questions if you will,
After it stalls , easy/hard to restart? If not , do you have spark available ? Can you substitute another ignition coil if available? No power to run when under load?

With the rear wheels off the ground, trans in neutral, engine off, brakes off can you turn the rear wheels and see the driveshaft turn ?
What about if the engine is running ? What we are trying to determine is, if the trans is free with no pump pressure versus not free with pump pressure.

Jack up rear end
 

So we jacked the rear end, transmission in neutral, spun the rear tires and the drive shaft moves. We started the car, left it in neutral, spun the tires and the drive shaft spins. Car is idling at about 1000 rpms, put it in drive with it jacked up and it idles at about 1000 rpms, put the brake on with it still jacked up and the engine drops to 700 rpms, but still runs. Now we lower it back onto the ground and put it in gear with the brake on and the engine dies. It starts back easy most times. We have replaced the coil, no change. Yes, no power under load.

What carb?

Carb
 

No tag, simple 2 barrel carb.
https://cimg1.ibsrv.net/gimg/www.for...d6791550a6.jpg

Sometimes we have to think out of the box

So here goes,
Has anybody been in the carburetor? If the float level is incorrect , yes with the vehicle jacked up the level could be higher and enough fuel is available until you let down to level. Sounds crazy ,but can't overlook anything.
If the carb was never updated for use of unleaded gas ,the needle may have a neoprene tip that will swell with unleaded gasoline and lower the float level.

The part of the engine dying when the brake is applied, Is the pedal linkage possibly contacting anything under the dash when the gear selector is in drive or reverse position, this could also be an unrelated curve so to speak.

What can you tell us about the electronic ignition ? Until we know it is working correctly , it is suspect.
I noticed a ballast resistor in the photo, Is this supposed to be bypassed for starting and then switched over to 6 volts for running ? Any chance of measuring what is going on and is it working correctly?

I'm sure there will be other suggestions so lets take them one at a time.

Bent/warped flex plate
 

So we have checked the carb, it seems fine, fuel bowl level is good, it is an updated carb.

Shift linkage is on the tunnel, so pushing the brake doesn’t interfere with anything.

We we have now found a bent/warped flex plate. We changed it out with a new one and are finding the same issue. Went on flat and straight to the crankshaft. Did see any issue connecting to the torque converter. Now when we turn the engine over manually the flex plate moves forward and aft, at least a quarter inch. Is it possible something in the C4 tranny has an issue, allowing the torque converter to wobble?

Did anybody have the trans out for some reason ?
After I reread your last post received my answer.

The torque converter has two indexing extensions that fit into the pump gear . Are you sure the converter is correctly inserted into the trans pump gear?