The M40D2 motor being the detuned version of the M50D2 motor has restricted butterflies along time avance, carbs doesn't open widely as the 50 model. OTOH if wanting to convert it to a 50 won't be a spot on neat conversion as the 50 has different parts that will be too costly to buy for a correct factory conversion. Try to adjust the upper carb to match the settings of middle and lower carbs so all 3 carbs delivers the same amount of fuel into all 3 cylinders.
Was offered moons back a new out of the box M40D2 for 2 grand, planed buying it and passing it straight to a M50D2, assumed for peanuts cost, oh boy I was wrong so decided to pass on this one..
Happy Boating
