I purchased this boat in 2002 and this boat has always dieseled. This mainly happens after shutting down after running a long distance. Boat has been very well maintained and always good gas. New plugs evey year and recently new wires. Timing Maybe??
I have 99 Capri with the 5.0L, and has just seems to started doing it this season. I believe still original plugs, on my list to replace those soon. Yes, letting it idle a little first does help. Will also try turning on the blower first. Wonder why just recently started? Different gas mixes?
I have a 175 with the 3.0 and noticed that whenever I run regular or a lower octane fuel the same thing happens. After cruising to a stop I can put it in nuetral and rap it off once and it does not deisel or let it idle for a bit.
The best way to reduce this is just use premium fuel or an octane booster additive.