15 H.P. Evinrude, speeds up and slows down at full throttle

When I am headed up rapids and sometimes on flat water the engine speeds up and slows down erraticlly. Whats up with that?

Re: 15 H.P. Evinrude, speeds up and slows down at full throttle

Sounds like what I call surging, Rifflerunner.<br /><br />It is usually caused by restricted fuel flow, but there are other suspects.<br /><br />Your prop could be slipping on a bad hub, cavitating or even ventilating.<br /><br />Start with the prop. Check for a spun hub or damaged blades. Check that the anti-ventilation plate is even with the keel or only slightly above.<br /><br />Then check that you don't have a restriction in your fuel supply like a partially blocked pickup in the tank or a pinched line.<br /><br />Finally, it is possible that there is debris in the carb float bowl that intermittently restricts flow to the jets.<br /><br />Let us know what you find, and good luck.

Re: 15 H.P. Evinrude, speeds up and slows down at full throttle

Thanks JB. Got a new prop, will go through the fuel system from start to finish, but what is "ventilating" and where exactly is an "anti-ventlation plate? Thanks again JB, I will report my findings.

Re: 15 H.P. Evinrude, speeds up and slows down at full throttle

The AV plate, often erroneously called the "cavitation" plate, is the horizontal plate above the prop, runner.<br /><br />Ventilation and cavitation do about the same thing but have different causes. <br /><br />Ventilation occurs when the prop is running in foam and gets little or no traction. The air in the foam can be grabbed by the prop if the blades break the surface of the water (engine mounted too high or trimmed out too far), or if you are running in foamy water (like rapids).<br /><br />Cavitation occurs when the prop blades generate such low pressure on back surfaces that water vaporizes (too much torque for the surface area of the prop), or (more common) damaged blade edges create water vapor. The prop trying to run in the water vapor acts just like ventilation.<br /><br />Think of ventilation as driving on a wet or icy surface in your car and cavitation as doing a smoky burnout on dry pavement. Either way, the tires spin rather than grip.<br /><br />Good luck.

Re: 15 H.P. Evinrude, speeds up and slows down at full throttle

... and cavitation can damage your prop by eating away at the metal. <br /><br />Ventilation doesn't do that. It is caused by air entering the prop area, either by sucking it in from above (hence the AV plate - it helps minimise this effect), or by leaks from the through-hub exhaust. One problem with ventilation is that it can cause the motor to over-rev, and this can produce bad cavitation.<br /><br />If the trailing surface of your prop gets rough and pitted - that's usually the result of cavitation. Low water pressure in this area boils the water, releasing bubbles, which in turn release energy as they collapse. The released energy corrodes your prop.<br /><br />All good, clean fun!