how it dies could be a clue. is it like the engine was shut off , just dies suddenly with no firing , If it's fuel it often will stumble before it dies or you may see things like it almost catching during cranking but firing sometimes. That might indicate it has a spark.
I'm not sure where you are but heat is a possible contributing factor some areas are hot now. coil getting hot and not working consistently perhaps? any signs that the carb is overfilling and spilling fuel out it's vent?
distributor cap or wires arcing? , check for any carbon tracks in the cap. look at the rotor contacts in the cap are they all green and dirty or clean?
if it has points are they pitted or is it an electronic ignition? on mine the thing completely died during storage and that was a problem with the "module" on the side of the distributor.. basically a transistor or amplifier for the spark. It has a heat sink to draw heat to the body of the distributor. I don't know if they always fail suddenly or if they are prone to being intermittent.
check your grounding , a loose or dirty battery cable or a bad ground from body to engine, check the ground on the engine has good contact with the vehicle. you can just try putting a jumper cable from battery ground to body ground and to engine ground as a quick test.
if you turn your lights on then you might see if the electrical system is being drained or interrupted , perhaps. you can check the voltage at the battery , then check it when it is running. It should be higher when running. take your lead off the negative post and put that on the ground and the voltage should be the same indicating the body is grounded.
a weak coil or bad plug wires could cause an intermittent issue.. easy to swap out. you might be able to unhook the coil and check the resistance through and see what that is.. maybe we can compare to another vehicle.
the fuel pump should be running , it could be weak or getting stuck or filter plugged. you might be able to check fuel delivery to the carb is consistent and that it isn't starving for fuel. don't start a fire. gas burns really really well.. had a friend who drained gas to a bucket and tripped on his trouble light and that was a horror story so be respectful of the fuel but it should run with a bottle gravity fed as a test.. or just pump some into a container but be careful let the engine cool first.
you can pull the plugs one at a time and note their color. see if they look tan and normal or if they look black and carbon-ed up or white indicating too lean.
If one is different from the rest that means something , they should be alike.
if it absolutely wont fire try a squirt of quick start. If it wont fire when cranking but it runs a bit when you spray in just a little then that might sway towards a fuel issue. you dont need much , just a test.
pull the dipstick if you see any white it can indicate a head gasket issue and then you may see water of vapors from the tailpipe or the water bottle running low. check the water make sure it isn't getting oily.
sounds dumb but sometimes people make the mistake of ading dieesel to a gas car, not often but it happens.
others will chime in and some have more experience, but there is a few thought just to get you started..
first thing I'd do clean the battery cables. easy and it's maintenence you shold do anyway. hope you are that lucky because that's easy.
