I have a 1999 100 hp Yamaha 4-stroke with a jet lower unit that I run on a 1990 18 ft Hewescraft Searunner. The jet has a stainless steel,4 bladed impeller. I bought the boat used this spring with less than 140 hours on the motor to run on the local rivers for Salmon fishing. Normal operating rpm range has been to 5600 max, with normal cruise at 5200, which is good for 30 mph downstream and 22 upstream with maximum load measured by GPS. Motor is almost not enough for boat, have to be above 4000 rpm to stay on plane.<br /><br />Last trip coming back upriver, halfway on 30 mile run, somebody saw something on a sandbar, so I shut down and drifted back for a look, turned out to be a Bald Eagle. When I went to power back up I couldn't get enough power to get back on plane, so I shut down and turned it off, thinking maybe the jet sucked up something. Started backup again and this time it came on plane but wouldn't go about 5200 rpm. I could give it more throttle but it wouldn't run any faster and sounded like it was loading up. So we ran on up the river at 5200. During the trip I noticed that our speed had dropped off from 22 - 23 mph at 5200 rpm to 19 mph. We made it back, loaded up and went home. The next day when I was looking it over I found milkly white motor oil on the dipstick. When I drained the crankcase about a cup and half of pure water came out first and then the milky oil.<br /><br />As a test, I refilled the crankcase with clean oil, hooked up the garden hose to the flush fitting and started the motor up and let it run for 5 minutes. When I checked the oil it was worse than before and when I drained the crankcase there was even more pure water in it.<br /><br />Any ideas? 
The mechanics I talked to have worked on a lot on 4-stroke Yamaha's and they said that they had never seen that problem before. <br /><br />Thanks for any help you can offer. Its terrible to be broke down when the Salmon are running.
The mechanics I talked to have worked on a lot on 4-stroke Yamaha's and they said that they had never seen that problem before. <br /><br />Thanks for any help you can offer. Its terrible to be broke down when the Salmon are running.