Environment Canada's Environmental Technology Centre (ETC) has conducted tests on two-stroke outboard engines for a variety of compounds formed through the combustion process. The tests showed that two-stroke outboards produce 12 times as many of certain types of hydrocarbons as four-stroke engines, and five times as much oil and grease.
The ETC also compared exhaust emissions from a light-duty van, a 9.9-hp two-stroke outboard and a 9.9-hp four-stroke outboard.

The two-stroke also emitted 15 times more unburned hydrocarbons than the four-stroke and nearly 125 times more than the van.

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That means that even if 2 strokes only account for 15% of the craft on the water, they will contribute nearly 3/4 of the unburned fuel spilled into the river. For those who doubt the math, here it is (x = amount of fuel spilled by one 4 stroke motor).


85(x) + 15(15x) = total fuel spilled.
85x + 225x = total fuel spilled.

310x = total fuel spilled.


225x/310x = fuel spilled by 2 strokes = 73% of total fuel spilled

85x/310x = fuel spilled by 4 strokes = 27% of total fuel spilled


Get rid of the 2 strokes and you will eliminate the problem.