RPM question

[bamajoker]

1985 Mariner. 150 XR2

Bought the motor and it was turning 5800 rpm smoking and running rough.
I rebuilt the fuel pump it had a hole the size of a dime in it.

After rebuilding the fuel pump the motor is running way way better.

But running wide open I am now turning 7000-7200 rpm and it spikes up to 7500 running 55mph

the boat is a 18' Bumblebee with a 12" jackplate. 25p four blade prop.

 

[Dave1027]

Re: RPM question

Must be getting air. Jackplate jacked too high maybe? Or maybe a spun prop hub?

With that motor at that rpm with a 25 pitch prop you should be doing over 80.

 

[bamajoker]

Re: RPM question

how can i check to see if the hub is spun

 

[achris]

Re: RPM question

The best way is to take the prop to a prop shop. If you have a way of securing the prop hub and turning the blades then you can check it yourself. Or you can make a mark on the outer and a mark on the inner, take the boat out for a run and check to see if the marks still line up....

Chris....

 

[Chris1956]

Re: RPM question

Are you sure your tach is accurate?

I am surprized you can get that kind of performance out of the motor/engine/prop combo. I would think that pitch is 2-4" too tall.

 

[bamajoker]

Re: RPM question

Tach works fine, i called the local prop shop and he said it sounds like a have a bent ear on my prop, i looked at it though and i did not notice anything bent to the naked eye but maybe the slightest bend could throw it off.

also it is 24p prop not 25p after I cleaned i could read it better lol.

 

[j_martin]

Re: RPM question

Tach works fine, i called the local prop shop and he said it sounds like a have a bent ear on my prop, i looked at it though and i did not notice anything bent to the naked eye but maybe the slightest bend could throw it off.

also it is 24p prop not 25p after I cleaned i could read it better lol.

Either the prop is slipping or the tachometer is wrong. If that's a bone stock XR2, I'd bet on the tach error. You could also be way over trimmed, but I doubt you could get it that high without losing water pressure, unless you have a low water pickup on it.

That's about the right prop. should be running in the low 60's at around 6 grand.