You seem to be familiar with dieseling not only by the title but also your description of it running backwards and sucking in water. You know the sound: turn off the key it keeps running for a second or few but a little funky and then a sort of loud sigh when it kicks backwards and if you were looking > a puff of smoke out of the carb. If this is what you have then its not the key.
In the video about timing its interesting that is exactly this type of car (and older ones too) that were most susceptible to dieseling. There are several potential causes, some mentioned above. Here is a Wiki article outlining them all (
link). Back in the day with muscle cars if one was dieseling, we always attributed it to “carbon buildup" which could only be cleared with high-speed runs of the car. I think it was more an excuse for us to take joy rides
. But in some cases, it did work. Most typically was the idle set too high.
You recently had your spark plugs out > how did they look? Dark and filthy? Or white and crusty? Here is a
link about plug evaluation. What you are trying to determine is are they (1) rich or (2) good or (3) lean. If you don’t remember, pull a couple out.
While I do not have experience with a boat engine dieseling and sucking back in water, I can only imagine that continued operation without fixing it could lead to needing to replace the motor due to hydrolock and a busted engine. Thus, for now, suggest put the boat back on the trailer if not already to allow troubleshooting and fixing while minimizing risking the engine.
In summary:
- If you have the engine running at any point report the RPM at idle as well as idle RPM on sticker if has it
- Report on ign timing.
- Report on plug condition.
- And post the engine serial no. and or the sticker that indicate what flavor of engine you have.
And if any questions about tools needed (RPM meter, Timing Light, etc.), post questions.
Also, this is more then likely an easy fix, so stick with it and you will be back in the water soon
