Looking at the parts diagram, all the parts you mentioned have the same part number for both models, with the exception of the reed valve assembly...it seems like the 15 hp model has reed stoppers that are higher. would the wrong reed valve assembly cause incomplete combustion? Also, when I ran a compression test on it before, it gave me like 3 psi, so I assumed something wasn't right with my gauge...would too low a compression for the extra gas and air cause incomplete combustion? how would you make an engine spin faster? i see the two different models have a different part number for the cylinder assembly. Is this where the ports are and could be the issue. I had a marine mechanic and the guy that sold it to me tell me it should be no issue using a 15 hp carb. I don't even see 9.9 carbs for sale. Just trying to get a clue why i getting a ton of oil in the water.