When I was a kid back in the early 80’s I was given a Torpedo .29S. I took the engine apart and put it back together wrong so the exhaust and bypass ports were reversed. Tried in vain for hours to get it to run, it would fire on a prime for a second or two and thats it. When I took it apart again and realized my problem, upon reassembly the correct way it ran like a top.

WHat happens is the exhaust port opens first and will try and push back into the bypass and there is no way the fresh fuel charge will overcome the exhaust pressure to enter the cylinder, even after the reversed bypass opens and some exhaust manages to escape the cylinder. As I said for it will fire a few time and quit. Probably many engines like this were taken apart and put back incorrectly together - and then blame the dud of an engine, lol.