How is gas getting in oil and what locks down starter

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C/convertors were plugged,dug out honey comb, welded back but started running rough all sudden, almost stalling, backfireing. I replaced plugs and wires. Tryed starting and backfired once. Seemed lime a stuck vaulve but engine bacame like it was Harder for it to turn over and the as if starter was locking down.? Tryed starting again and loud backfire. I pulled new starter and a spline or tooth was broke off. Also same time all this just happening I pulled engine oil dipstick believe gas started getting in oilll. Why or what is locking down the starter ? Engine was pretty sound, used no oil runned like top.