Low RPM at WOT

[Boating_Al]

I have 21 Sea fox with a Johnson 135…the maximum rpm at WOT are 4500 (32 MPH) and the specs read 5300-6000. Is my prop correct? The prop on the boat is the OMC 391202 14.25 x 21, the specs in the manual have it at 14.25 x 19…could the difference in pitch be the problem.<br /><br />Thanks

 

[seahorse5]

Re: Low RPM at WOT

That boat runs best with a 17" prop. If you are not getting 5500-6000 with one, then the motor should be checked out.

 

[Boating_Al]

Re: Low RPM at WOT

Thanks SeaHourse,<br /><br />Would a 17" prop be too big...I was thinking that the pitch was too much

 

[phatmanmike]

Re: Low RPM at WOT

the prop you have now is too big. 21p is too much.<br /><br />if you move down in pitch to a 17p expect to gain anywheres from 500-800 rpms... which would put you @5000-5300 which is much closer. also , check the your butterflies are open all the way. sometimes its just a matter of adjusting the cables

 

[Boating_Al]

Re: Low RPM at WOT

Thanks phatmanmike <br /><br />I did check the butterflies, they are fully opened at WOT. The other thing maybe the fuel, the boat/motor actually only has 25 hour, bought it with 19 hours two weeks ago, the fuel is about a year old and it contains 2 cycle oil, the previous owner was still doubling on the oil…maybe this is causing the low-rpm. I’m putting in a new gas-filter and burning all this old fuel… <br /><br />Thanks again…

 

[Boating_Al]

Re: Low RPM at WOT

I’m looking around for the 17p prop that I need; what the manual recommends is 14.25, but most that I see are 15” or higher (Ebay). Would a 15” prop work better/worse/same. Would the pitch have to change?<br /><br /><br />Lots of questions…thanks

 

[Mark42]

Re: Low RPM at WOT

Seafox,<br /><br />I don't think the double oil ratio will affect the RPM's to any degree that you will notice. At least it didn't when I ran that way with my much smaller 40hp earlier this summer. But old gas is not good. It will make for harder starting, as most of the butane, etc has evaporated. Ask if the owner put in any gas stabilizer over the winter. It will help prevent it from going "bad". <br /><br />Mark