Run On Issues

[Sorceress]

Less then a one year old Bayliner 205 Sport with a 4.3 Mercruiser engine that refuses to shut off after running awhile. Been to the dealer 3 times and they always come back saying they can't duplicate the problem. They played with the timing, lowered the idle speed and checked the timing to no avail. The lastest is that they tell me to let the motor idle 5 minutes before I shut it off. This is not acceptible because we mainly use the boat for tubing and wake boarding. I purposely bought a new boat to avoid these types of problems! What is my next recourse with the dealer to get this fixed. I have been told by other people that this motor is too small for the boat and because it works so hard and gets so hot this proplem is inherent with the boat motor combination.

[Tahiti350]

Letting it idle for a couple of minutes just makes good sense. It allows cooler water and oil to circulate while the temp in the metal equalizes and starts to cool. It also gives the exhaust valves and combustion chambers a chance to stabilize/cool before all the heat is taken away. You should at least idle in to where ever you are tying up, that should help.
To answer the second question, It sounds like the boat is underpowered for the way you intend to use it. The extra load WILL cause excess heat, and may cause localized hot spots in the engine that could dause the dieseling you are experiencing.
On a good note, it sounds like you have a good dealer who is actually trying to help you. Good on them.