Correct prop for Sunbird 1989 Fish & Ski with 90 HP Johnson O/B

Derek Domino

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May 6, 2017
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Hi, I'm new here and a very new boater. I bought a Sunbird 1989 Fish & Ski with 90 HP Johnson O/B about a year ago and now I have a spun prop. I want to replace it and I know it's had at least a couple of different props on it because the seller gave me the two old ones. One is 13 1/4 x 17 (the spun prop) and the other seems to be 14 x 18. I also have an 14 x 17 on the boat. They are all pretty old. I'm sure there are a lot of possibilities. Reading the threads I just get confused since they are not specific to my boat and motor. Does anyone know what the original factory prop was? What should I replace it with? I'm more concerned about economy and handling. Thanks!
 

b.gagnon

Master Chief Petty Officer
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Apr 28, 2001
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What RPM were you turning At WOT...with the 17???
 

Derek Domino

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I'm going to take it out tomorrow. The RPM gauge has been intermittent, maybe I'll get lucky.
 

ondarvr

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If you can't confirm the tach works correctly try using a shop tach, or replacing the one you have. What came stock on the boat doesn't mean much, they frequently just stick a prop on there that may be close for how they think someone may use the boat, or it may be way off. You need to test it yourself in the exact way you plan to use the boat to dial in the correct prop.
 

Derek Domino

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May 6, 2017
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Okay, finally got a shop tach and tested the prop (14x17) today.
Results: WOT
with trim all down on somewhat choppy water 8600,
with trim up, on plane on choppy water 9350,
with trim up, on plane smooth water 9950.
 

ondarvr

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You need to put the tach on the correct setting for a 4 cylinder 2 stroke, it can't turn those RPMs without a great deal of modifications, and then only for a very short period of time. It should be somewhere around 5,500 RPMs
 

Derek Domino

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I was thinking that myself. Especially since the tach on the dashboard only goes to 7000 RPMs. How do I know what the proper setting should be for the shop tach?
 

ondarvr

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I'm not what sure the settings may say on that tach, but it may say something about firing on every stroke. Some 4 strokes and all 2 strokes fire every rotation of the crank, other 4 strokes fire on every other rotation.

​The numbers you listed are probably double what they should be.

​I'm assuming this is a normal automotive shop tach.
 
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Derek Domino

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May 6, 2017
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The modes in the manual read like this: 1P1R which stands for 1 spark per revolution. An auto mechanic friend said he wasn't sure what the setting should be but it sounded like I was reading twice as much also. Info on the internet seems to say that it is 2 sparks per revolution. Any thoughts?
 

Sea Rider

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Sep 20, 2008
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If it's a 2 strokes, usually runs 2 sparks per revolution, What's idle rpm at neutral ? Has low or high rpm readings ? Should test combo running parallel to water surface on calm, flat water cond as usually loaded. To achieve that OB must be trimmed at 90? when combo is on plane. Check under those cond and report back your findings.

Happy Boating
 

Maclin

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May 27, 2007
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A four cycle needs two full revolutions of the crankshaft (Per individual cylinder) to produce one power stroke, so there is one spark event for every 2 full revolutions, A two cycle engine produces a power stroke on every full revolution, so there is one spark event per full revolution.
 

ondarvr

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A four cycle needs two full revolutions of the crankshaft (Per individual cylinder) to produce one power stroke, so there is one spark event for every 2 full revolutions, A two cycle engine produces a power stroke on every full revolution, so there is one spark event per full revolution.

​Some 4 strokes also fire on the on the non power stroke, so it's also one spark per rotation. More sophisticated and modern 4 strokes fire every other stroke.
 

Sea Rider

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A two cycle engine produces a power stroke on every full revolution, so there is one spark event per full revolution.

Not quite so, some 2 strokes OB's produce 2 sparks per revolution, one spark is a wasted one...

Happy Boating
 

Maclin

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​Some 4 strokes also fire on the on the non power stroke, so it's also one spark per rotation. More sophisticated and modern 4 strokes fire every other stroke.


Thought I had deleted that one,I tried to as I hit Post and then thought ahead to these types of responses about engines in this modern era. Some engines even have sparks on the exhaust stroke for emissions cleanup type thingies (ECTT's, lol) I think. My 5.7 early 2003 Hemi Ram had that, with two spark plugs per cylinder but they did not fire at the same time, one sparked with the exhaust. If a modern engine is doing all those extra firings with the same plug then that is what the modern tach setting is for, and was why I wanted to delete my post so as as not to distractify.

Please continue without me :)
 

b.gagnon

Master Chief Petty Officer
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Apr 28, 2001
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If the boat is 17-18' and doing 33mph..... I would say you have the right prop! You do need the RPM #'s to fine tune it.
 
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