Question

[TD_Maker]

I have a 2003 Johnson 90 (Bombardier) V-4 mounted to a 17 ft McKee Craft CC Boat. 20 Gallon gas tank mounted under console. Motor runs great, idles great....no issues at all.

However,

To get the motor to run in the 5500-5800 RPM range, I can only turn a 14 x 13 pitch prop. Everyone says I should be turning a 14 x 17 minimum, so there must be something wrong with my motor. I did a compression test and it is very close to 125lbs in every cylinder. Is there something wrong?

Oh, I forgot to mention. Motor is mounted in the third hole up, and the cavitation plate is about 1/2" above the bottom of the hull.

 

[steelespike]

Re: Question

NADA says your boat is 1500 lbs.
If you look up the Etec 90hp performance reports (basically same motor same gear ratio) glass boats about 1500 lbs actually many are 15" props. But there are also others up to 19" pitch
When you checked compression did you note the condition of the plugs any change from normal brownish tan could be suspicious.
And of course they should all appear the same.
Many folks don't realize they are missing a cylinder except for the lack of power.
What is the wot rpm and gps speed with a lightly loaded 15" or 17" prop?Your tach could be off.
Check that the timing is set and advancing as it should.
How is the overall performance with a 15" or 17" prop?

 

[jestor68]

Re: Question

Isn't your WOT rpm range 4500-5500 rpm?

The hp graph didn't show up for some reason. But it showed your hp peaking at 5000 rpm.

 

[craze1cars]

Re: Question

Agreed. Why are you targeting 5500 to 5800? All resources I find say your motor's operatign range is 4500 to 5500. So going to a higher pitch might benefit your rig as you appear to currently be underpropped.

 

[TD_Maker]

Re: Question

My factory manual says WOT should be 5250-5750. Like I stated earlier, with a 13/1/4 x 17 pitch I am turning 47-4800 RPMs.

 

[steelespike]

Re: Question

What is the speed with the 17 @ 4800 rpm.How is the hole shot?water sports? Ability with a load?
The rpm without speed is somewhat meaningless because there is no way to know if the tach is accurate.

 

[Artwerke]

Re: Question

Got any grass growing on the bottom?

 

[TD_Maker]

Re: Question

The tach is a new Systems Check tach purchased earlier this year. The pole setting and pulse are set accordingly. I am getting 35 MPH (Gps) at 4700 WOT Hole shot with cupped 17 is a little sluggish. The 13 pitch gets the motor reving and on top of the water quickly, but I have to watch RPM's with just me in the boat. I am thinking 13 1/4 x 15 would be about right.

 

[steelespike]

Re: Question

A 15" prop in a similar design seems to be a reasonable number.