There seem to be many conflicting theories here,
Normally black smoke indicates incomplete or poor combustion often caused by overloading, ie Overpitched prop or rope
around it but the vibrations are normally a giveaway with that.
White smoke as different to steam, check for water flow from exhaust and feel temperature of exhaust after engine and salt water
injection point, if cold and good water flow it wont be steam.
White smoke can be due to excessive unburnt fuel and lube oil
A 75 HP Yanmar
will not have manual decompressors so that wont be the cause unless an exhaust or inlet valve is not closing, seems unlikely on a new engine.
But I did have a Yanmar that oversped due to an uncontrolled source of fuel on launch day!!!!
At 1 hr of use it had leaked a large amount of fuel into the sump, so much so that we emptied 600 ml of diesel
LO mix out of the plastic silencer box from the air inlet.
This was setting up a smokescreen (White) that a world war ! battleship would have been proud of.
Engine could only be stopped at 4000+ RPM by choking air supply.
Its interesting to note that a diesel
dies much more quickly than a human when it stops getting oxygen, the latter observation is only from watching movies.
Yanmar never had the courtesy to inform me of the problem which took over a month to resolve.
Their drawings dont show seals
between the fuel pump
pistons as in their words it would only confuse people and the pumps should only be pulled apart by Fuel Injection specialists. Agreed.
One of these seals
had to be missing to put 4+litres of fuel in the sump in 50 mins of running at very low revs.
Beware of Yanmars, their in factory Quality control is woeful from my experience, two engines two problems.
Spend time watching and feeling around your engine when running; learn your salt
and fresh cooling water flow paths and normal operating temperatures, its amazing what you can pick up before it becomes a major and expensive problem!
Learn how to kill your engine if it overfuels and overspeeds, you will not have very long to stop it before it self destructs with potential for boat damage and fire.
Got off the track a bit here, but the white smoke brought back many unpleasant memories on what should have been a day of celebration!
level closely to ensure there is no fuel going into sump.
Low engine revs and white smoke would possibly indicate one cylinder not firing due to lack of ignition temperature ie: Lack of compression.
If this is the case you will need a good diesel mechanic