Are you using full choke as soon as you start cranking? These old engines are pretty cold-natured. If the fuel level in the carburetor is right and the ignition is well-tuned, it should start in the first 5 seconds or so of cranking with the choke pulled out all the way. If the engine won't crank for 30 seconds at normal cranking speed without a boost, you need a new battery, or the cable connections and grounds are bad.
Jerry



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