My port engine was running rough and conking out a few weeks ago. Had the hardest time figuring out why -- plug wires good, changed the fuel filter, checked all the routine items (Marine Power (GM) 454, electronic fuel injection). In looking everything over I found the relatively small casing ground wire from the alternator to a post on the engine block had fallen off. Ring connector, the ring just broke in half, maybe screwed on too tight. Of course I fabricated a new one, one gauge heavier wire just for kicks, marine-grade connectors both ends, properly crimped. I didn't expect it to make any difference though -- thought it was just an incidental problem. Heck some alternators don't have separate or additional ground wires from the casing at all and the whole thing is bolted to the bracket and block anyway, how could that little wire possibly make a difference? But the wire flopping around bugged me so I replaced it just for general purposes. To my surprise, made a huge difference -- my rough-running problem vanished. One of the guys on the dock said that EFI engines are very fussy about voltage flow.
Anyway, don't know what you have but there's my story on doing a proper job with a grounding wire. I guess you could do an extension, and not to be fussy, but why not just do it right? Relatively minor expense and task. My two cents anyway.
