1996 Johnson 90 RPM Rises Rapidly from fast Idle

[jr1]

1996 Johnson J90ELED. Starts perfect, cold idle rpm is perfect, warm idle rpm is perfect, idle rpm in water and gear perfect. When in water and throttle advanced to idle at 1500 rpm, not in gear, runs perfect. When advancing to 1800 or 2000 rpm, in water, not in gear, it runs well about a minute then rpm rises rapidly to over 3000 rpm - at that point there is also a rattle-like noise, more so at carb side than plug side, otherwise engine seems smooth. Squeezing fuel bulb or oil bulb have no effect. Restart motor and same behavior occurs. Have no engine warning light/horn issues. Water pump giving strong telltale flow. Was running great in lake a day before.

 

[racerone]

The engine is NOT designed to run 2000 RPM in neutral for extended time.-----Why are you doing this ??

 

[jr1]

Thanks. Yes, I got that. Hence only ran a minute. I was trying to replicate a rattle heard while running on plane. Test tank not deep enough to run more than idle in gear or water splashes out. Heading to nearby lake for further in water testing.

 

[racerone]

Check the gear oil.-----Is it clean?----Any shiny small metal flakes?-----Any metal chunks in there?

 

[jr1]

Gear oil is good, have clean magnet tips. Not sure if it is related to my noise, but will be doing a VRO oil flow test. Getting good oil smoke in exhaust, but at only idle speeds I can't see enough oil tint in the fuel filter area.