Like Bill says, chances are good that you have a grommet issue, especially if this engine has seen salt water use. Adding insult to injury, this 15hp may have two pesky grommets. Later model 15hps like yours have an “open inner exhaust tube”, along with a short water tube exiting the powerhead….
That being said, I am assuming that you cleared out the overboard indicator hose and plastic restrictor nipple, oftentimes these are plugged…Surely don’t want you to pull this thing all apart needlessly.
And yes, like Bill says, that spring belongs there, allowing the thermostat to lift off its seat at higher RPMs, so more water flows through the powerhead, bypassing the thermostat. Be sure to order the new rubber seal, and be sure the SS cap is in place over the new seal. Carefully clean out the thermostat pocket so the thermostat won’t be stuck and can move freely off its seat, once water pressure can overcome the spring pressure.
In spite of some unforgiveable issues, these are great engines. The powerhead and gearcase are very robust. The cheesy plastic throttle gears are a pain, and the cooling system needs regular maintenance when used in salt water. Be sure that little water drain in the bottom of the gearcase is thoroughly cleaned out while you have the gearcase off, you may want to enlarge that drain hole a little bit as well…
