Runs good in park, but when put in gear why does it cut off?

How long since a tune-up? Have you checked the timing? How many miles on engine? How is your cylinder compression?
There isn't much load on engine in park or neutral. When you put it in gear, there is a load on the engine. With carbed engine 6--8 psi fuel pressure. Crub idle in drive, 575 rpms. With a high mileage engine, that idle speed may not work. Is there an idle screw on the carb, where you can up the idle to keep it from stalling, in gear? Of course if you do that, when you put it in park or neutral the idle is going to go high, I know it's a pain. If your engine is in poor shape, your probably going to have a tough time, getting it to run correctly. Just my opinion.

Sound very much like you have a fuel blockage problem, try changing filters and go from there, hope this helps

Fuel pump or filter. 98% sure its fuel related.

Don't know cars - but it might not be idling fast enough to make the gear change. Idle reset needed maybe?

Cuts out when throttled in park or in gear if gear and cuts out cam can be blamed. Gas motor put a quart of tranny fluid in gas tank and run it it will clean valves

possiblle bad torque converor or transmission

first make sure battery isn't dead because if he cut it off in gear then the switch couldn't cut the power off and it'll run the battery down and there is a electrical solenoid that allows the shifter to move if theres no voltage than the shifter want go into park and the ignition switch want turn off.

This is the de-tent or kick down cable. Most commonly known as the down shift cable. This tells the transmission when the car needs it to down shift. If the cable does not do its job then the car stays in a higher gear and dies when put into gear. Hope this helps. Good luck.

I helped another 3 series owner and he had a hesitation off idle, the hose was cracked, seems fairly common on high mile cars.

Have your Catalytic Converter checked for free flow, (if it is stopped up with calcification from unspent fuel, then the car will have trouble moving at all). A tell tale sign of oncoming Catalytic Converter problems is the overdrive balking or wanting to hesitate, when the transmission is in overdrive the vehicle is running at its lowest r.p.m. and so like at idle it will stumble or hesitate, or cut the engine altogeter, for the motor has to be able to exhale, to exhaust its combustion gases, in order to run properly, stopped up catalytic converters are a big problem with this car.