Choke it to see if it helps keep it going a little longer. If it does then you know where the problem is coming from.
I helped a guy fix his one time and it would spark on the plug when you first started cranking it then it would quit. After changing points and condenser and cap and rotor, we found that the dist was losing ground after it cranked over a couple times. Cleaned the lockdown bar and dist flange and cured it.