I have a 1991 Mercrusier V6 4.3lx i/o. I have always had run on issues when turning the engine off, even on 91 octane. I was advised to check the timing. Timing was at 0 degrees. I adjusted it to the factory setting of 8 degrees BTDC, also put in oem spark plugs gapped at .035. The idle went up pretty high when i advanced the timing, so i adjusted that to about 750 rpm out of water. I tightened the ditributor bolt and re-checked the timing for accuracy. Went to the lake and the boat ran significantly better, but was still idling high. I planned on adjusting it after a family swim. Run on was greatly reduced at shut down. 40 minutes later went to start the boat and the motor turned over maybe 3 times and then a load knock sound and lock up. No other sounds just stops cranking. No armature sounds from the starter. I did a complete visual of everything in and out of the boat, everything seems normal. Over the next hour i tried it 5 more times with the exact same result. Motor turns over about 3 times then the load knock and then it just stops cranking. I've owned this boat for 14 years and do all my own maintenance on it. All fluids are fresh and full, new water impeller, new battery. Anybody have any ideas of where I should start???