Originally Posted by mangomuffins
How about this?
drag or resistance keeps the engine from turning the normal revs for a given amount of fuel supplied... friend continues to apply power (fuel) resulting in excess fuel to the cylinders which doesn't get combusted fully thus... smoke?
That's a very clear articulation of the way things happen.
I would be a bit concerned if the engine was REALLY turning anything like 4000 RPM
. I don't know of any small Yanmar
that should be running that fast. If it is, the injection pump and governer may need some serious adjusting.
If the smokeing only hapens in reverse, and the engine turns close to specification RPM when running in forward w/o smoke, then it's not really a big deal (unless your friend drives around backward a lot!) Full throttle reverse should be a relatively rare event in a normal boat's life.
As people have already ponted out, props frequently don't load the engine right in reverse even if they work well in forward.