mercruiser 350 250hp wot vs top rpm

[bdirect]

I am a bit confused about where my top RPM should be at WOT. 24ft cuddy I am running a Mercruiser 350 250hp Bravo 1. I am running a Mirage Plus 19 pitch prop. Mercruiser says i should be turning a top rpm of 4400-4800. I can not get over 3100 WOT but out of gear in the water it will rev right up there. So any suggestions?

 

[vipzach]

Re: mercruiser 350 250hp wot vs top rpm

IS this a new to you boat? How about a recent tune-up? If the boat isn't new to you, has it ever reached WOT?

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

Boat is new to me this year. I have been shaking down things and just recently did tune up. I do not know if it has ever reached wot

 

[vipzach]

Re: mercruiser 350 250hp wot vs top rpm

What exactly did you do in your tune-up?

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

Plugs, wires, rotor and cap. I have not checked timing as it was on the computer 3 weeks ago and timing was fine compression (i aplologize don't have exact numbers) was excellent but indicated missfire. Thats when i found the bad cap and coil

 

[Haut Medoc]

Re: mercruiser 350 250hp wot vs top rpm

Dumb Question: You sure you have the wires right? You should be able to get close to 4200-4600 with that prop.....Although it is alot of boat for a sbc. Check the timing/ advance again.... Broken flapper? A vacuum gauge would tell you if you have an exhaust restriction....JK

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

Had to go check to make sure. Wires are right.<br />will check timing again

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

Timing is fine

 

[Haut Medoc]

Re: mercruiser 350 250hp wot vs top rpm

Exhaust restriction? Or....It is now time to put the vacuum guage on it and see what it says..... Then a compression test.......Then a cylinder leakdown test.....If you are convinced that your firing order, plugs & wires are right then this is whereI would go next.....Then a fuel pressure check.....JK

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

could you Explain this exhaust restriction thing to me and how it would effect rpm/ If there was a restriction woudn't it effect rpm when in neautral as well? This part is very new to me except i do remmeber I had a collapsed exhaust pipe on a 72 Malibu (350 250hp 4 barrel) I am beginning to see a pattern here and yes it drove me nuts before I found it. But I am pretty sure that even in neautral I had rpm loss with that

 

[Haut Medoc]

Re: mercruiser 350 250hp wot vs top rpm

Exhaust restrictions generally only show themselves under load.......You'll get w.o.t. all day long in neutral, but when the engine has to work, it can't breathe...I would have a look for a broken flapper stuck in the Y-pipe or bell housing, drive.....On the vacuum gauge it will show itself by dropping to 0 p.s.i. and gradually rising back up.....Look in the "search' function under 'Trouble shooting with a vacuum gauge' there are some really good posts/links in there......JK

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

AHHHHHHH I see said the newbie to the wise one...makes perfect sense now. easy enough to jus take off and look at in my boat and I realy want to inspect my risers etc so i am going to take a look see at all of that

 

[Bondo]

Re: mercruiser 350 250hp wot vs top rpm

but out of gear in the water it will rev right up there. <br />If there was a restriction woudn't it effect rpm when in neautral as well?

It looks like JK has you headed the right way,.....<br />I just want to emphasize,......<br />What the motor will do in Neutral,.........<br />Has Nothing What So Ever to do with what it'll do Under Load..............<br /><br />An Engine that's about Dead,.. will turn 5000rpms All Day in Neutral,....... <br />But not be able to move it's own weight Under Load.................<br />

I have been shaking down things and just recently did tune up.

Are these Engine Things you've been shaking down,......<br />There's Alot of Restrictions that Can cause your Issues,.......<br />JK's got you Looking in the Right Places,......<br />Or,..... It Could be the Fuel Side of things,............ <br />Or,.....Without a baseline,.... It's Still possible that the Prop Is Too Big,...........

 

[Don S]

Re: mercruiser 350 250hp wot vs top rpm

I have not checked timing as it was on the computer 3 weeks ago

Is this an EFI engine? What is the year, and the serial number of the engine.

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

Not engine things realy that I have been shaking down. This engine was put in the boat last fall. Started up winterized and then I bought it this spring. We (the sevice manager at the yard i use)took it out and ran it for a couple of hours.<br />We noticed at that time we were only getting 3000 rpm WOT. I did not get my speedo and GPS on the boat until 2 weeks ago so that is when we knew there was an issue as weknew this setup should pmove the boat faster than it was gong with the prop. We have checked the prop and it is one of the 3-5 props recommeded by Mercruiser for this engine. They spec top rpm 4500-4800-- top speed around 47 so i am way off the top end .So by shaking down i have ben checking accuracy of guages etc. The tach was suspect even then but just got to this issue last week. <br />Had a serious issue with engine cutting in and out running rough. We found a bad coil,(thing would only arc after it got hot enough) bad cap.relaced those and also changed plugs and wires. (habit i almost always change those parts at the same time for sanity sake)<br />So now I am dealing with the wot vs rpm issue. <br />Now the folks I bought the boat from will do all of this for nothing but I like being able to figure these things aout and troubleshoot my own engine problems and yes fix them if I can. I find the recources here excellent as long as i supply the information needed and then follow through with what test etc need to be done. My goal is to be able to work on this engine myself. For example I now have a very clear understanding about exhaust restrictions that I did not realize before doesn't bother me as much now that I understand it. If it does turn out to be the problem I also know i can fix it with the help here.<br />Also i do not take exceptin when asked if I am sure of something. Like the wires. I have crossed wires on engines and will probaly do it again so I find those double check reminders helpful and will almost always go check again before i say it is fine.. <br /><br />It is not EFI it has the MERCRUISER diod for analysis on it. I will post year and serial number tommorow they are on the boat and I want to make sure i give accurate info.

 

[bdirect]

Re: mercruiser 350 250hp wot vs top rpm

It was a bad flapper!