1988 Johnson 90hp WOT@4600 RPM

beavers82

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Jan 22, 2010
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26
Carbs are rebuilt recently, compression 120-125 in all 4 cylinders. Why can't I get it over 4600 WOT RPM's? It pushes my Arima 17ft. Sea Chaser at about 33 MPH which is actually just great with me, but shouldn't it be topping out around 5200-5400? Thinking maybe a different prop. Any ideas?
 

JB

Honorary Moderator Emeritus
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Mar 25, 2001
Messages
45,907
Re: 1988 Johnson 90hp WOT@4600 RPM

Yes. You should be turning more than that at WOT. My guess is you are over propped so I will move this to the Prop Forum.
 

steelespike

Supreme Mariner
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Apr 26, 2002
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19,069
Re: 1988 Johnson 90hp WOT@4600 RPM

Get us the prop size usually on the barrel or under the prop nut.
 

beavers82

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Jan 22, 2010
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Re: 1988 Johnson 90hp WOT@4600 RPM

13.25 X 17. I have no idea what this means. I have an extra prop that is 13.75 X 15. What is the difference?
 

trendsetter240

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Re: 1988 Johnson 90hp WOT@4600 RPM

13.25 X 17. I have no idea what this means. I have an extra prop that is 13.75 X 15. What is the difference?

One is a 17" pitch prop and your extra prop is a 15" pitch. Try putting your extra prop on and see the results. Chances are you will see your RPM increase from what you have now.

If the two props are identical except for the amount of pitch then you should see about 5100 - 5200 RPM at WOT with the 15" pitch prop.
 

dieselcat

Petty Officer 1st Class
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Feb 1, 2009
Messages
351
Re: 1988 Johnson 90hp WOT@4600 RPM

Had the same problem as you are describing with my 88 spl,tried everything,rebuilt carbs,all new fuel lines,fuel filter,diff prop,ect........By the way i have a stainless 13.25X19p an a landau pro 17.it's an aluminum bass/crappie boat.Anyway it ended up being the timing.took it to the shop got a link and sync done and now i get right about 5100 RPM,thinking of going to a 17p so i can get to 5500 rpm.

Hope this helps?
Shane.
 

steelespike

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Apr 26, 2002
Messages
19,069
Re: 1988 Johnson 90hp WOT@4600 RPM

First number is the diameter which simply follows the design and is not directly considered in the selection.Second is the pitch which represents the distance the prop moves forward in one revolution if water was a solid.
Lower pitch lends more power and rpm higher pitch less power more speed
kind of like a low or high gear in a car.The idea is to have a prop that allows the motor to operate within its rpm range.
All that being said I think if your boat is about 1200 lbs dry(no motor)the 17 should work for you.The 15 would be the extreme normally used for sking or heavy loads.
You may want to check a few things Is the motor in excellent tune?Throttle opening all the way?Bottom clean?Prop in excellent condition?
Is the motor at the right height(antivent,cavitation,plate just above the prop) about even with the bottom of the boat?
 

reelnative

Petty Officer 3rd Class
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Mar 28, 2010
Messages
92
Re: 1988 Johnson 90hp WOT@4600 RPM

there are also other things to look at on your prop, first off you are in the proper operating rpm range for your motor but in the lower of the range, your prop could have too much cup in it, that is the lip on the edge of the blades more cup will make your prop act like it has more pitch then it really has, the mph that your going sounds good and is in the range of a 17 pitch prop, one good way to find out about your cupping is to take your prop to a prop shop and have them tell you how much cupping you have in your prop, if it has been double cupped the shop can adjust that for you and you will gain rpm and speed, the second prop you have sounds almost like a pusher prop with 13.75 dia and 15 pitch it sounds like a prop that is made for slower speeds but can push a larger load ( pontoon boat prop)
 
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