‘07 Yahama 115 Four Stroke HIGH rpms - help

[Dmhawv]

i have a Bennington pontoon boat rigged for fishing. Have run the same setup for years. Recently it would surge and bog down. I assumed fuel issue and have run cleaner through the engine and new gas and have 99% resolves that issue but at 1/2 throttle the rpm’s will rocket to 6,000 and disappear off the gauge if I increase the throttle/speed. I thought the prop was spinning so purchased a duplicate new one, no change. I don’t know what to check next. Most posts I have found discuss low rpm issues. Suggestions ?

 

[99yam40]

Is the motor actually revving up, or is the gauge just showing it is?
if the motor is speeding up and not having a spun prop, then there must be some cavitaltion going on.
prop bite being lost due to air or something else causing it

 

[Dmhawv]

Motor sounds normal for increasing the throttle. Doesn’t sound like it is surging or excessively revving. The rpms just swing up too high. Sometimes the engine adjusts and brings the rpms down while keeping the speed the same but last time out I had to manually pull back on the throttle to reduce them.