Ignition

Your question is very vague m8... When you let go of the key does it sputter and die? ...does it not hold idle? ...are you able to get the RPM's up at all before it dies? Do you have spark? Does the TDC mark in the distributor coincide with TDC on the flywheel or has it slipped time? Are you getting fuel to the carb? Are your plugs, wires, and coil in good order? I mean what you say above makes no sense... if it 'starts' then it should hold idle for a brief moment at least... It sounds more like your trying to say it cranks but doesn't fire, but that's not how your worded it?

bad key switch,or loose wire on starter.

it runs, and sparks fine, I can actually "rev" the engine with the key. If I release pressure it dies

it fires and runs, and will idle as long as I am holding the key turned to the start position. Everything is fine as long as I have pressure on the key. I am thinking ignition, but it sounds complicated.

replace ignition switch and take it from there... im willing to bet thats you problem

replace the ignition switch.