That’s working correct. It’s a built in compression release feature of these motors, it’s called an ACR. There’s a mechanism on the cam that does just what you’re saying at slow cranking rpms.
Did you tune the carb? Does pulling the choke help any? Also good to static time these engines, start simple however sticking intake valve will exhibit this symptom as well
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(2) Original, 100, 102, 124, 73, 800, #1 and #2 cart, brinly plows, disk, IH184, IH244, 1948 F Cub
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