Tom - That's a 1999 150 two stroke Saltwater Series II . . . 2.5 - 3 is MPG not GPH. There is NO WAY he's moving that boat at 2.5 - 3GPH except trolling at idle speeds.
Here's a couple of similar setups from the yamaha motor's archive.
http://yamahaoutboards.com/sites/def...0112-CEN-Z.pdf
http://yamahaoutboards.com/sites/def...0175-CSK-Z.pdf
http://yamahaoutboards.com/sites/def...0127-CSK-A.pdf