Sudden jump in RPM's

[asbennett]

Hi - I have a 1982 OMC 2.5L 4CYl with 400 series out drive. The boat has been running great all summer. Last two times I have been out, after running on the water for around 10 minutes and then I stop - as I take off again the boat starts to come up on plane then the RPMs jump way up like something is slipping. I throttle back and ease into it again - starts to go then slips - finally it gets up on plane and runs great. Happens about 3 times the entire day - always when I am taking off -(under load). The prop is not stripped. What else can I check? Any ideas?

 

[JB]

Re: Sudden jump in RPM's

You may be ventilating or cavitating the prop, Stringer. Cavitation sounds more likely.<br /><br />Ventilation occurs when the prop blade tips break the surface and "grab" air, creating foam in which the blades spin without creating thrust. This is the equivalent of getting your tires on ice. . . spin, but no thrust.<br /><br />Cavitation is more like wheel spinning on dry pavement. The torque of the engine overcomes the ability of the prop to move water. Some of the water gassifies creating the same situation as ventilation.<br /><br />The most common cause of cavitation is prop damage on the leading edges and tips of the blades.<br /><br />Try a new or reconditioned prop.<br /><br />Good luck.

 

[asbennett]

Re: Sudden jump in RPM's

I hope it is the prop - (easy fix). My prop is old and has a slight vibration from dings. But one question: Can the prop get in this state (cavitation / venting) when it is completly submerged?

 

[chris in va]

Re: Sudden jump in RPM's

You have a 2.5...I seriously doubt it's cavitation. My bet is on ventilation, but since it just now started happening it could be something else entirely. <br /><br />Can you just put an new prop on there and see if it fixes the problem?

 

[Boomyal]

Re: Sudden jump in RPM's

if your prop has a rubber hub it may be slipping. It just happened to me. Check out my post of a few days ago. (copy and paste if not active)<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=003867

 

[asbennett]

Re: Sudden jump in RPM's

I can try a new prop - but for a better comfort level, can someone assure me there is nothing in the upper or lower units that could slip like this? I wouldn't think it could be slipping out of gear - and I would suspect that if the gears were damaged it would do this all the time. Again, it is when I accelerate. When it slips, there is almost a small thud when it catches, then the thing runs great. Again - a prop I can deal with - internal damage is much more $$$.