350 Mercruiser 260 4100 RPM max

[danmcmanus]

While trying to find the best pitch for my boat, a 26' Larson 250 Delta Cruiser, I realized that the boat only runs at 4000-4100 RPMs at WOT. I have always run it at 2700 to 3500 and never at FOT. I noticed that Mercury suggests this engine should run at 4400 to 4800.

I plugged the boat info into Mercury's prop selection tool and it indicated that I should be running at 42 MPH @ 4400!

Is that realistic and/or is there any way to boost mt RPM's?

 

[Bondo]

Re: 350 Mercruiser 260 RPM

Re: 350 Mercruiser 260 RPM

Ayuh,............
When was the Last Time the Driveline was Serviced,+ Tuned Up,..??..??....
And,....
It's Wide Open Throttle............. What's FOT stand for,..??...

What size Prop are you turning,..??.......
Have you tried a different Pitched Prop,..??....

 

[danmcmanus]

Re: 350 Mercruiser 260 RPM

Re: 350 Mercruiser 260 RPM

Thanks Bond-0
Tuned up end of last season. I see/hear FOT (full open throttle) and WOT (wide open throttle) in different places; same difference to me. I was using a 15 x 17 vortex prop and someone at the Perley's Marina suggested i go to lower pitch, i.e. 15 x 15, or stainless 14.5 x 15. I tried a 15 1/4 x 15 and gained 5 mph but the RPM didn't change much at 4100. I had expected to gain 400 rpm by lowering the pitch but I didn't. Now I am baffled. Boat weighs about 4500 lbs, not including 75 gallons of gas, 20 gallons of h20and passengers..I reached 32 mph @ 4100 rpm.

Think changing plugs, wires etc will change that?

 

[Don S]

Re: 350 Mercruiser 260 RPM

Re: 350 Mercruiser 260 RPM

Has the boat been sitting in the water for a while? If the bottom is dirty (even a layer of green) it will kill your WOT rpm (not sure what FOT is)

 

[Si Rey]

Re: 350 Mercruiser 260 4100 RPM max

Have you checked the accuracy of the tach? I found that mine was reading low after comparing it to another tach. On the back of my tach is a calibration knob which I adjusted accordingly. There are some timing lights wth built in tachs which is really handy.
You also might want to check for the possibility of a fuel pressure problem. I found my fuel pump was leaking internally at the diaphram not allowing my engine to receive adequite fuel under high loads i.eE. w.o.t. Also check fuel filter, a dirty one will restrict fuel flow to the engine.

 

[danmcmanus]

Re: 350 Mercruiser 260 4100 RPM max

Bottom's clean, just had it out for some work. Could be the tach but its new and that would account for he inadequate speed at WOT. Water/fuel separator filter is new and replaced the carb Fuel filter. Only thing left is possible leak/loss of pressure; I've been thinking about a rebuild. I'll try tha. In the meantime it still runs well.

Thanks

 

[Bondo]

Re: 350 Mercruiser 260 4100 RPM max

Then I guess a drop in Prop Pitch might be in order......