WOT & best cruise rpm?

[sippi]

I have a good friend of mine that has a 28' bayliner I think it is a 1978 year model with a volvo penta 5.7 motor. The thing was running very bad so we did a tuneup, rebuilt the carb,changed the plugs pionts condenser all that stuff, and changed the oil. when we put it back in the water it fires up and runs really well but! @WOT it gets about 3500 rpms, is this right for a 350 in a boat that size or do think the prop is to big?

 

[sippi]

Re: WOT & best cruise rpm?

Oh and I forgot is thier a rule of thumb RPM for efficiency or is it pretty much different for each motor&boat combo???

 

[Haut Medoc]

Re: WOT & best cruise rpm?

Welcome to iboats.......
That is a lot of boat for a 350......
You should be @ 42-4600 @wot......
Either your engine is tired or over-propped....
Since you say you did a tune-up, did you check compression, set dwell & timing?.......​

 

[paulie0735]

Re: WOT & best cruise rpm?

Is the tach wired corectly?
Did you just lose 1500 rpm or is this how its always been?
If everything else checks out than it would seem the prop is at least 4" in pitch to big. What size is the prop?

 

[QC]

Re: WOT & best cruise rpm?

How fast was she running? MPH? GPS if possible. I am with paulie, need to check the tach. I don't think a 28 footer would plane if the true WOT RPM was only 3500 . . . Check for a little switch on the backside of the tach for 4, 6 or 8 cyl

 

[sippi]

Re: WOT & best cruise rpm?

I found out that he did change the prop right before he pulled out of the water to do the tune up, so the prop may be to big.Also when we fired it up on the trailer the motor was buring just enough oil to whear you could barely see it. So I am thinking it is a combo of a tired motor and the prop being to big.
Thanks guys I appreciate your coments and suggestions.