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I'm thinking it wouldn't run at all well at 20° retarded 🤔🤔🤔
Should be half error at camshaft timing (from crank markings) some history of people moving it one tooth out, which suggests it's possible without causing valve damage. Expected affect would be smooth and possibly increase in low rpm torque, but compromised higher rpm performance. Agree to inspect piston valve potential contact, borescope available ? Then set it back to correct and timed at the "T" mark on crank for running.