Prop Pitch

[16Pilot]

I can not get my RPM's past 3700. I have a 21 pitch 14 1/4 on there. Weight was not an issue because it was just myself and one other person on the boat. Would it be worth looking at going to a 19 pitch. Or could it be this power loss issue everyone is talking about due to defected wiring harness? Sorry to souns sound stupid but I am kinda confused with this. 4.3 OMC 1990

 

[tommays]

Re: Prop Pitch

16 <br /><br />it depends did the motor ever reach the correct WOT if it did you have something different now or the motor needs some type of service<br /><br />if its new to you than it may not be the correct prop for your use<br /><br />you can read about props here<br /><br /> http://www.mercurymarine.com/everything_you_need_to_know_about_propellers5 <br /><br />tommays

 

[collawash]

Re: Prop Pitch

Make sure you've checked any and evrything possible first before breaking out more $$$ for props you may not need. ANY small wiring, mechanical or electrical problem could result in the engine not being physically able to reach WOT rpms. <br /><br />For example: I speak from experience...have you checked the spark plugs? I thought my prob was pitch too, until I found 3 of the 8 plugs had hairline cracks in the insulators and arc'd all over the damn place. Replaced them and PRESTO, an instant increase of 500 rpms. Good luck.

 

[collawash]

Re: Prop Pitch

By the way, sounds like your 4.3 on a 17' boat should be able to turn a 21" prop. Have you checked the prop itself? If the prop is not balanced or the there's a non-visible small bend in one blade can have dramatic reductions in overall RPMs. Even a seemingly insignificant difference of 1/4 of an ounce can create an unbalanced prop turning at 2000 rpms in the water. Like I said, check everything else first as prop shops love to sell props. <br /><br />Curious, did you just buy the boat? Like tommays asked, have you ever reached OMC recommended rpms before? A friend of mine has a 1992 20 foot runabout with a carb'd 4.3. He turns a 21" prop at 4600rpms.

 

[Don S]

Re: Prop Pitch

There is way more than meets the eye to your problem 16. Just went back and read some previous posts.<br />This one makes me wonder what other problems you may have and how the engine runs.<br />Have you checked the timing, advance timeing, rechecked the compression, done a vacuum check (never know when an intake manifold gasket could leak). I really don't think the prop has anything to do with your low rpm's,<br />Is this the normal engine and drive combination? Has the drive been replaced and if so, was the gear ratio the same?

 

[16Pilot]

Re: Prop Pitch

I will check those out Don, Thanks. The prop that is on now is pretty chewed up. The idiot that had it before I rebuilt the entire engine backed the boat into his garage door and took a dime size chunk out of it. The timing is perfect compression is great, however, I am not sure about the intake gasket? How would I check that? Oh yeah, it is the normal engine and drive, I did build the engine up pretty beefy just to be safe. I put really nice rods, pistons and rings in. Original cam, lifters. The coil is brand new and the point and condensor is new. Plugs, wires and cap as well. I did not however change the fuel filter because it was so late in the season and a mechanic told me just to wait until spring when I actually get it out. I broke it in very carefully. It only has maybe 4 hours on it.<br /><br />P.S. Thanks again Don, I think it is great what you do for everyone here! Keep it up

 

[Don S]

Re: Prop Pitch

The prop that is on now is pretty chewed up

I wonder where the problem with low rpm could possibly be <br /><br />

I did not however change the fuel filter

For the price of a fuel filter you are taking the chance of running water and crud into a rebuilt engine <br /><br />You might want to take care of both those items and check your WOT rpms again.

 

[16Pilot]

Re: Prop Pitch

Thanks