Not hitting RPM

[W.D Cain]

I have a 1996 Four Winns Volvo Penta SX w/5.8w FSI. My red line shoulb be 5000rpm, and last boating season it would hit 4800 , 70 mph. This season it will only hit 4000 rpm. Starts 1st time , idles great, no miss ,but isnt reving as fast as last year.
Tried new gas, new stock prop 14:25 x 23SS,,, no change! Help!!!!
Dont really know where I am going but thinking high pressure fuel pump ,checked low pressure pump.

 

[QC]

Re: Not hitting RPM

Where you getting your speed numbers? How fast for 4000? Do you know your gear ratio? Why play with Fuel pump stuff without a full tune-up first? What was old prop that got you to 4800? We don't talk redline with boats, we talk WOT RPM range. 4800 sounds good, 4000 doesn't. You do not have to hit 5000 to make full power . . .

 

[W.D Cain]

Re: Not hitting RPM

Thanks for your reply... Dont remember the gear ratio off the top fo my head but it is a slightly modified U 19. @ 4000 rpm it hit 64 mph w/wind at my stern.

 

[QC]

Re: Not hitting RPM

Wind at stern can sometimes be slower. Air is thinner than water How are you getting your speed? GPS? Pitot?

 

[QC]

Re: Not hitting RPM

Both props were/are 23s? SX Drive? Cool boat. I was not aware of the U-19. Like this?:

 

[W.D Cain]

Re: Not hitting RPM

Speed registered via VDO and a Lorance GPS! The props are identical and the boat is tuned.

 

[QC]

Re: Not hitting RPM

boat is tuned.

I have been told that I require too much clarity . . . With that said, the engine has had a full tune-up? Fuel filter changed?

Did you personally observe the 70 MPH and 4800 RPM your ownself? Also GPS? No other changes? Additional weight? Wet winter = waterlogged foam? Bottom condition good?

Back to my clarity statement . . . everything else exactly the same except for RPM and speed? Same altitude, same trim settings, same people, same ammount of beer, same amount of fuel etc. etc. etc.?

 

[W.D Cain]

Re: Not hitting RPM

I am the person that runs the boat, it seems to me that you are more hung up on the speed than my problem.
A stock boat like mine does 65-68 from the factory, no I dont know how they tested theres. Mine has some minor modifications,,,, I picked 2 mph.
The fack is that the boat doesnt run like it did last year!!!!!!

 

[QC]

Re: Not hitting RPM

you are more hung up on the speed than my problem.

Are you incapable of reading? Almost every single one of my questions has had to do with clarifying ambiguous comments of yours. When did I bring up another boat except to show a picture of what I think you are trying to tell me you have and saying "Nice boat"? When you first said you have a U-19, I thought prop, that's how clear you are being . . .

Since everything is OK, and everything is the same, then I am sure you don't have a problem. I'd try swapping out your crankshaft. I'm done

 

[QC]

Re: Not hitting RPM

Because I can't let well enough alone, and because there are others following this thread that may also be wondering where I was headed. The "fastest" SX drive ratio I can find is 1.43:1. With a 23" pitch prop and a very generous 8% slip, your combo should be going 67.3 MPH at 4800 and only 56.1 at 4000, sooooooooooooooo, your speed numbers made no sense to me.

 

[boatguya1]

Re: Not hitting RPM

Is there fresh fuel in the boat? First thing I'd look at is the throttle cable, at full throttle is it against the stop on the motor end? Carbed or injected? If injected I'd go with a fuel pressure test and injector flow balance test. If carbed I'd go through it. These things of course are after making sure all the tune up stuff is good. 4800rpm divided by 8 cyl's is 600rpm add a couple hundred rpm loss for the rest of the motor draging a dead cyl along with it and you've got 4000rpm. I wouldn't be surprised if you've got a cyl producing no or almost no power due to lack of spark or fuel. So while going through the tune up stuff look for a plug that looks different from the rest.

Is that the 4-winns clone of the Donzi? You have to remember that most people will loose track of other things when the boat owner claims to be going 70mph in a mid 90's small block 4-winns.

James

 

[W.D Cain]

Re: Not hitting RPM

Thank you for your responce. Yes I do have fresh fuel, and the plugs look good.
I will look at the fuel pressure next.
Thanks for your ideas.
Dan

 

[W.D Cain]

Re: Not hitting RPM

I took the moderators recomdation and proceeded to check the crank, very helpful.
I found a vacume leak at the Idle speed selenoid and it seems to have fixed my problem.
For the record..... my outdrive is a 1996 Volvo Penta SX with a ratio of 1.51:1.

 

[QC]

Re: Not hitting RPM

Well, I for one am happy to hear the crank is good . . .

Please understand that your 1.51 ratio makes your 64 MPH at 4000 RPM and 70 MPH at 4800 even more impossible; they both result in negative "slip" i.e. you went farther than the propeller can push you . . . When someone gives us data and asks us to comment on possible issues that the data supposedly supports, it is really difficult when the data is flawed. It is very possible that you have found the reason that your boat would not reach the RPM and speeds it had before. It is never my intention to tell you that your boat isn't broken if it is. But there is absolutely no way for any of us to give accurate assistance if the information given only confirms that the information is bad . . .