I have a boat with a 1990 GM 305 in it. Last year I replaced my computer controlled ignition system with a basic HEI / distributor based ignition system. Everything worked great for the entire boating season last year.
This year is a different story. The first time attempting to start the boat, I got grinding noises from the starter when attempting to start the engine. The grinding was so prevalent I was unable to start the boat with any consistency. You could also see the engine clearly kicking back during attempting to start. I also had weird buzzing noises coming from my ignition coil. Long story short, I replaced the 'brain' unit in the distributor which handles the timing and that fixed my buzzing noises from my ignition coil and fixed most of the grinding noises when attempting to start the engines.
I am still getting grinding noises during starting and I usually have to attempt to start the engine multiple times before it will fire. If I remove the ignition wires, I have NO grinding what so ever. Clearly, the grinding it related to the ignition system which would also make sense for it kicking back.
Why would this happen? I have the ignition timed at 8 degrees BTDC as confirmed by my timing light. The only solution I can think of is to retard the timing (maybe something like 6 degrees BTDC) to alleviate any kick-back. This still leaves the question as to why it grinds sometimes and starts others.