I'm thinking that engine probably has a compression release for easier starting. At very low RPM (starting) it would open the exhaust valve just slightly on the compression stroke so the engine turns over easier at startup. I've run into this before on a Generator. Assuming it's an overhead valve (OHV) motor, you would look for the (probably silver) valve cover. It should be easy to remove and I was able to reuse the gasket as there's not a lot of oil pressure there. With the valve cover off, you'd see the rocker arms and where they engage the valves. With the spark plug out, turn the engine over slowly with a wrench. You should see the rocker arm for just the exhaust valve move slightly on the compression stroke. If the arm is moving, but not actually engaging / moving the valve then the arm probably needs adjustment. You'd need to look up the valve specs and set it with a feeler gauge, but it's really pretty easy to do and you should be able to adjust it right at the rocker arm.
I had a generator that used to be really hard to pull at startup and this was the problem.. Unfortunately I broke a couple pull starts before I worked that one out
Good luck!!!