RPM's

[JROD8768]

I have a 40HP 4 cylinder Mariner Magnum. I previously had a 40 Mariner 3 cylinder. I am having trouble getting the right RPM's. I first would only get 5000 RPM's, then I repitched my prop and only get 5100. When I trim up the motor<br />to where it seems to run correctly I only get 5100. When I try to trim up more it cavatates.<br />I had the prop cupped which helped the cavatation<br />somewhat but I only get the 5200 max RPM's<br />I have a larger prop which was used on my old motor and it seems to run good,trims up well, decent speed but only turns 5000??<br />Should I go back and repitch the prop.<br />I have this motor on a weld-craft 16x52ft flat<br />I am open to suggestions<br />Thanks<br />Jimmy

 

[Kiekhaeferscurse]

Re: RPM's

Hi Jr,You are in the operating range for the Mag so dont panic, you havent mentioned what pitch prop you have here, pitch being the one of the determing factors in engine rpm. As a general rule each inch of pitch is approx 150 to 200 rpm. I.m not much of a fan of prop shops that repitch props if you can start out with a known good prop and work form there. eg: if you are running a 15 p prop at 5100 rpm, try a 13 at the same trim angle see if you can get 5400 out of her, keep your testing perimeters the same. Trim angle, boat wieght, fuel grade even weather conditions make a big difference on that good old motor. Hope this helps, best of luck

 

[JROD8768]

Re: RPM's

Thanks for the response. I think it is a 13 pitch I am running at 5200. I also have a 15 that turns 5000. I will get someone to pitch down<br />the 13 or I will ask them to check to see what it really is.<br />Will it be ok to run the 15 pitch while they are correcting my other prop?

 

[Kiekhaeferscurse]

Re: RPM's

Your still in recomended rpm range so with good fuel & a clean boat you should be okay for a bit with that 15.