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I think this from Leeroy's Ramblings http://www.leeroysramblings.com/OMC_9.5.htm explains what you are finding
"There is also a neutral safety lockout that prevents the motor's twist grip throttle from being advanced past the "START" position. This is simply a rod connected to the internal shifting linkage that abuts against a flathead screw from the twist grip linkage. This screw can be adjusted. However you can shift into either forward or reverse, then twist to a higher setting to start it."