5.7L not hitting potential?

[jwill1919]

I have a 5.7L Merc in my 23' 95 Larson and it's only getting about 4000 rpm's. The weight of the boat is 3500 to 3700 and i've been told a by a couple of different people that I should be getting anywhere from 4300 to 4700 rpm's. What should I do, or what should I check to see if there is a problem. I just put a new 4 blade prop on it and it's running good. I'm sure it needs a tune up but could that be the only problem or could there be something else??

 

[jeff13601]

Re: 5.7L not hitting potential?

There has been many good posts regarding what prop to use in the last couple weeks. Along with the blades you have to look at the pitch. If you look back in the posts you will learn a lot

 

[jwill1919]

Re: 5.7L not hitting potential?

I don't know if it matters much, but I consulted a prop shop and also a Mercruiser dealer when I got my new prop, and they told me to go with an 18 pitch 4 blade. So that indeed is what I did. My engine still turns the same Rpms as it did with a 3 blade, however, the performance is much better as is the handling and quicken to plane. But I think this is more of an Engine question as opposed to a prop question, because it should "hypothetically" turn an 18 pitch the same amount of times as a 16 or a 20, right? But as we both know, the performance will be totally different depending on the prop? If I'm wrong, please let me know, I'm trying to learn as much as I can instead of being ignorant when it comes to mechanical issues of a boat. Thanks

 

[Don S]

Re: 5.7L not hitting potential?

You do the prop test with an engine that is well tuned and the carb and timing and advance timing is all dead on, and the boat has a clean bottom.<br />After that, if you start loosing rpm's you know something is wrong.

 

[Fishermark]

Re: 5.7L not hitting potential?

it should "hypothetically" turn an 18 pitch the same amount of times as a 16 or a 20, right?

Actually, no. It's just like a car going uphill. The steeper the hill, the slower the car goes. For every inch of pitch you lose (or gain as the case may be) a couple hundred RPM. So an engine that maxed out at 3600 RPM with an 18" pitch would theoretically max out at 4000 RPM with a 16" pitch.

 

[QC]

Re: 5.7L not hitting potential?

That engine was only rated 230 bhp in '95 I believe. Don S is all over this as is Fishermark. First the boat and engine need to be perfect. Then you need to seek out the correct WOT RPM for that year model and rating and go from there. Think about it, if changing the pitch didn't result in an RPM change then why not run a 30 inch pitch and go 100? Check out the WOT and Propeller threads in the Engine FAQ section. Good info there.<br /><br />Also, that sounds really light for a 23 footer. Is that from a spec sheet or a scale? Sounds like a hull weight to me. Good luck. This can get expensive . . .

 

[Mahoney]

Re: 5.7L not hitting potential?

first things first. Check the plugs and make sure all of them look like they have been running. I had the same symptoms, engine sounded and ran fine, although a defective plug kept one cylider from firing. All I could get was 4000 RPM, changed plug, and viola 4600 RPM

 

[jwill1919]

Re: 5.7L not hitting potential?

I definitely need to check all the plugs and give it a tune up, but I'm trying to learn as much about this engine as possible. Thanks Mahoney and friends.

 

[jwill1919]

Re: 5.7L not hitting potential?

Quietcat, I got the weight from the NADA book value. it actually says 3450 net weight, it is a bowrider.

 

[Mahoney]

Re: 5.7L not hitting potential?

I think you will find as you learn more about marine performance, that the biggest bang for your buck performance wise is going to be tuning the hull with tabs of some sort, distributing wieght, and dialing in the right prop and trim combination for your specific boat. The gains made in those areas on the water definitely trump and 5 - 10 horse upgrades you might sqeak out of a marine engine.

 

[QC]

Re: 5.7L not hitting potential?

Justin,<br /><br />I would bet you are over 4000 lbs. probably close to 4500 with gear and people etc.<br /><br />Mahoney's post is right on IMHO.