You might be able to find it, there was a Mercruiser TSB on this problem, can't recall what they advised to get it to stop, but I do know I never had this problem with my '88 OMC 4.3 with the Quadrajet carb in all the years I owned it. Might be unique to the carb Merc was using at the time. In any case you do want to solve that problem because it can pull water back up the exhaust which is never a good thing. I'm wondering if the needle valve in the carb is not holding so fuel is still dribbling into the venturis, and the combustion chamber is hot enough that it will run even without spark. Might try colder heat range spark plugs, getting the idle as low as you can without stalling. I set mine at about 600 rpm in the water. Also if you have carbon build up that can be a factor the carbon glows hot enough to auto ignite, perhaps using a top engine cleaner might help. Higher than normal compression test results would be a hint to that. We had a 1970 Ford 302 that did that, due to an auto choke that never opened up, (previous owner never fixed it), I fixed that but had to use a whole can of GM top engine cleaner to blast out the carbon. It was like chunks coming out the tail pipe. After that, it didn't diesel anymore! and would run on regular without pinging.
