I can assume that if you were to pull the spark plug that the engine pulls smoothly and easily? Set the lash .003 on the intake and .005 on the exhaust. The gap on the ignition module will also have an effect on the actual ignition timing. Start at .008 and bring it out from there should you have to. While you are there, see if the the upper crankshaft has any looseness which could prevent a stable gap. Another thing that can create this condition is that a slowly leaking carburetor or extra oil that also ended up in the cylinder will increase the compression ratio or create a partial hydraulic lock.

If tomplum's comments do not work then it is in the cam easy start spring. If this is not working it will be hard to pull and start.

The word I meant to say was the compression release spring.