3.0L Mercruiser Diesling

[bayliner911950CL]

91 Bayliner with 3.0L Mercruiser. Took it out after having it in the shop and it ran perfect, no diesling at all. The carb is freshly rebuilt, brand new plugs/wires, fresh oil change plus a drive upper rebuild. Took it out the second time and it has started diesling all of a sudden. I think that the idle is set a little high. Boat mechanic is going with me to the river so that we can possibly diagnose and adjust the idle, but I just thought that I would see what some of you guys thought as well. I'm seriously thinking about selling the starter boat and getting a newer boat too. Thanks in advance for the advice!

 

[Maclin]

When ready to shut the engine down it needs to idle for a minute or so to get the temps down. Sometimes this idling period before shutdown can eliminate the dieseling if it is not due to high idle.

 

[Bulbash]

Diseling is caused by the detonation, there is a few reasons why it may happen. Most likely your combustion chamber temperature is higher than normal because of fuel mixture is too lean, can be caused by vacum leak around the intake manifold. Another issue could be incorrect sparkplugs, issue with cooling system, not enough water flow, bad thermostat... Diseling can be caused by using a very high pitch prop making engine work very hard...