Two things come to mind: points need gaping and/or ACR not releasing at low rpm.
1st thing, try setting your points to .20 inches at maximum opening. The engine sounds like the spark is firing the mixture way before TDC. That may be what is stopping the piston in its tracks. Pull the plug wire the next and see if it turns over OK without stopping it. If it turns over OK, then it may be your Automatic Compression Release not opening the exhaust valve a little until the engine gets to about 650 rpm or so. This would mean your exhaust valve is set to close without the ACR being able to keep it open until engine speed gets to 650 rpm.
When the engine reaches 650 rpm, the ACR cuts off and the valves close normally. It sounds OK when it runs.
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John
Proud Owner of a Model 100 and a Model 124.
A homemade cart, 2 x 42" mower decks, a 38" deck,
a 42" front NF blade, and a lawn sweeper!
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