Merc 5.0LX rpm questions

[pneeb]

We have a 92 210 CCR Crownline with 5.0LX, 4bbl carb. Can't figure out why we can't get the recommended rpms for the motor at WOT. I know rpm has to do with prop, but think there may be another issue at hand. The boat runs perfect, smooth, always starts, couldn't ask for better. Our concern is that even at WOT the rpms only hit 4000. Should be 4200-4600 at WOT. The boat will hit the correct range if we put the alum 15x15 prop on. We purchased a rapture ss 14.25x15 prop, and it won't get there fully trimed out. The best we have ever seen is 4100. We know that being under in the rpms isn't good for the motor either. Flame arrester is clean. Boat maintained as should be. Have checked all the basic stuff and just need some expert input on where to look next.<br />Thanks,<br />Allen

 

[hondon]

Re: Merc 5.0LX rpm questions

You got sold the wrong prop . Generally when you switch from alum. to ss.you can deny yourself an extra inch or so of pitch to account for less slip and the cupping usually involved with ss props.

 

[pneeb]

Re: Merc 5.0LX rpm questions

3/4" is going to make that much difference?

 

[Fouled Plug]

Re: Merc 5.0LX rpm questions

That 3/4" is the diameter, not the blade pitch. Subtracting an inch of pitch should bump up WOT RPM's 150-200.

 

[pneeb]

Re: Merc 5.0LX rpm questions

so you're saying going to a 14.25x14 prop?

 

[Fouled Plug]

Re: Merc 5.0LX rpm questions

That theoretically would put you at or near your optimum RPM's at WOT. Is there a dealer in your area with loaner or test props? It's easier to select a prop by the seat of your pants as opposed to a catalog.

 

[pneeb]

Re: Merc 5.0LX rpm questions

there are dealers, but none with test props. the only one i know of just has alum merc props and very few of those. they're supposed to be a merc test prop place. the recommendation of the size of prop came from crownline and from mercury. we did do some research prior to purchasing the prop we did buy. as i mentioned the boat came with a 15x15 alum prop and both recommended staying with the same size, but going to ss. when we bought the boat the original owner had a 14.25x19 high 5 on it. and obviously he was way under propped with that. we had a turbo 14.25x17 from our old boat we tried and still didn't get anymore rpms, actually it stayed exactly the same rpms as the other two props were, so we put the alum one on to see what it would do and it hit where it needed to be. (although the alum prop blades are apparently not balanced as they should be anymore. they look ok to the eye though because we get alot of vibration with the alum prop) but others with the same boat and same motor set up as our boat can turn anywhere from 17-21 pitch props, so we were thinking that there may be something else that we needed to look at.<br /><br />thanks for your help.<br /><br />patty