'83 KZ750 stalls when put in first gear?

Weirdest thing keeps happening, the bike idles fine right about 1,000RPMs but whenever I try to put it into 1st, the engine seems to starve itself of gas and I need to give the bike full throttle to even keep it alive, but it inevitably ends up dying out. Anyone know what this issue could be?

I'm running into the exact same thing on a KZ400 that I just recently put together and got started for the first time.

Some of the things that came up in other searches were:
- kick stand safety switch may not be working properly (my bike does not have one though)
- make sure timing advance is not stuck
- check to make sure clutch is not seized. Put bike into third gear, pull in the clutch and try to roll it forward/backward. If it rolls freely then the clutch should be okay. If it does not then the clutch is probably stuck and you can try to free it up or you may need to pull the clutch and disassemble, inspect, and lube it.
- you may simply need to adjust the clutch

Unfortunately none of these things worked for me so it looks like my next step is pulling the clutch basket out and seeing what is going on.

I would adjust the idle a little higher, maybe to 1100-1200 & do a basic clutch adjustment (see the owner's manual).
When turning the clutch release adjusting screw on the 750E, it's probably counterclockwise first until hard to turn, then clockwise 1/4 turn. It's sort of counter-intuitive. Also, a carburetor synchronization may help.

I would also test the petcock to make sure it's working right.
ON & RES. Engine running -->> fuel should flow. Engine off -->> spring returns diaphragm to closed position -->> no fuel flow.
PRI. bypasses the diaphragm assy -->> fuel should flow -->> engine off or engine running.

I believe I got my clutch situation worked out. I think it was just sticky from sitting for a while. It came loose (I could feel when it did) by a combination of
- rocking the bike back and forth with clutch pulled in 1st gear, then 2nd, then 3rd, etc.
- with bike started and warmed up I put it in gear, kept holding the clutch in and give it a few throttle blips
- it was better at this point but it still would not take off from a stop without dying when I released the clutch, though it was not dying so quickly now. So I set about giving it plenty of throttle and feathering the clutch to keep it running until I could get up a little speed ( I was riding in the yard so maybe 5-10 mph ) and then I was able to release the clutch. After a couple laps around the yard I felt something give and the bike then picked up speed and revved freely. After that it was no problem taking off from a stop.

Hopefully this or the other things I pointed out helps you figure out your issue.