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

[Derek Domino]

Here are my setting options. Hopefully y'all can tell me which one I should be using!

1P1R=1 spark per revolution
2P1R=2 sparks per revolution
3P1R=3 sparks per revolution
4P1R=4 sparks per revolution
6P1R=6 sparks per revolution
3P2R=3 sparks per 2 revolutions
5P2R=5 sparks per 2 revolutions
1P2R=1 spark per 2 revolutions

Since I'm not in the water right now I tested it on idle with the following settings and these are the results. (Drum roll, please.)

1P1R=1 spark per revolution 1870-1920 RPM
2P1R=2 sparks per revolution 1380-1450 RPM
1P2R=1 spark per 2 revolutions 5530-5730 RPM

I could have tested more but I don't trust running it too long out of the water even with the flushing muff. These seem like the likely choices. I'm guessing it's 2P1R since 1P1R gave me the high numbers when I was on the water. 1P2R is even higher, so that leaves 2P1R. Are those numbers close to where I should be at idle? I will test again on the water this weekend.

 

[ondarvr]

How is it connected, looped around one individual sparkplug wire?

​Idle should be around 1000 on the hose, 750 or so in the water, it's in the water that counts.

​

 

[Derek Domino]

Wrapped around one sparkplug wire 6 or 7 times per the manual.

 

[Sea Rider]

Way high, is it a needle or digital tach, which brand/model is it. When OB is on water and once OB is fully warmed up, check if idle sounds somewhat high. Would incline for the 2P1R=2 sparks per revolution 1380-1450 RPM, probably OB needs a slight idle adjustment ?

Happy Boating

 

[Derek Domino]

It's digital. Link is attached.

 

[Sea Rider]

Nice to have multiple spark plug firing sequences. Your 90 OB is a 3-4 cylinder one ? Digital tachs behave bit erratical when OB is at idle, it's the nature of the tach, but reads quite accurate on faster rpm up to wot.

I've installed many OB's that came with idle rpm factory adjusted way high, was a matter of adjusting correct idle rpm once sitting on water. Check at wot what's max rpm shown on display...

Happy Boating

 

[Derek Domino]

It is a 4 cylinder. It seems to idle better (and much quieter!) in the water.

 

[Sea Rider]

It is a 4 cylinder. It seems to idle better (and much quieter!) in the water.

Yes, idle rpm whould be adjusted with combo on water. After a run at speed re adjust tach if in needd to lesen idle rpm to factory specs.

Happy Boating

 

[Derek Domino]

I was finally able to get out today and here are the results.

Warmed up and idling: 840-860
WOT: 4610-4620

This is with the 2P1R=2 sparks per revolution setting and the 14x17 prop.

 

[Sea Rider]

So finally idled well at 840/860 rpm. At 4610/4620 wot rpm as usually loaded. Is OB revving inside at least middle to max wot rpm range factory stated ? Seems bit low, revving at the lugging side which is bad. ? If so, a prop maximization is urgently needed...

Happy Boating

 

[Derek Domino]

The idle sounds fine at 840, but the manual says tune up specs should be (Idle speed in gear) 500-600 rpm. I just noticed that today. I though idle would mean in neutral...? I can check that next time.

Manual also shows 5300 rpm (min test rpm). Everything I've found online shows the range to be 4500 to 5500 rpm for that engine.

I will say that we had two more people in the boat today (6) including me. Usual load is four. Maximum capacity is seven.

 

[Sea Rider]

Some OB brands specify idle at neutral and idle geared.. If usual load will be 4 up, can maximize a les pitch prop for OB tu run at least middle 5K range, but full 5500 wot rpm is prefered. Combo will achieve better hole shot. Just throttle bit less if like running wot when with less boaters on board.

The bummer now is : which less pitch will OB need for that to happen, let the experts chime in for a theoretical tech answer. Will OB achieve + 200 wott rpm gain going one less pitch as per Rule of Thumb ?

Happy Boating

 

[Derek Domino]

Yes, thanks for the advice. It seems like a lower idle speed would make my maximum at WOT even less. Does that make sense?

 

[Sea Rider]

Idle speed at neutral is just carb ralenti. Adjust idle speed so that when geared from neutral there's a soft gear engagement sound and OB doesn't die, if dies adjust a little bit more CW nut, screw, whatever, Idle speed works with carb's slow speed circuit. When carb passes that setting works with fixed jets, nothing to adjust.

OB is over pitched, poor wot rpm, probably will need to drop to a 3 blade 13 pitch prop from current 17. Your actual prop is a 3-4 blade one ?

Happy Boating

 

[Derek Domino]

Prop is 3 blades.

 

[Sea Rider]

DD,

Having all the tech parameters to go after, which is your next move ?

Happy Boating

 

[Derek Domino]

Based on this from the iboats "Propeller Help Guide": "Changing the boat propeller pitch will increase or decrease the RPM's. Adjust the pitch so the RPM's fall within the recommended range. A 2" pitch increase (21" to 23" for example) acts like shifting a car gears up. It typically results in a 300-400 RPM decrease."

It seems like 13x13 might be closer to what I need. "Sea Rider" you suggested 13x13, based on the info above that sounds right. That would shift 600 to 800 RPM's.

I would like to go out one more time and tweek the idle just to see if that makes a difference.

Thanks to all for the help. This has been very useful.

 

[Sea Rider]

If 5500 rpm is max wot rpm and achieved just 4610 rpm, there's 890 lost wot rpm, that's if liking having top hole shot with slight less top speed. If "Rule of Thumb" applies, will need to go 5 less pitches to a 12 pitch to gain + 900 wot revs. But that's a theoretical rpm gain. Have seen + 500, 700 wot rpm gains with just one less in pitch on light combos. HP/load acccounts for less rpm gain too.

This is not rocket science. So based on what stated, would not dare 100% asure that OB will gain extra + 800 lost wot revs going to a lower 13 pitch, there are other tech facts to have into consideration such as : hull shape, drag coefficient, flotability, trim, OB height, weight. As don't want to give wrong advise, it's entirely up tp you if going for a 15 or 13 pitch prop. If in the uncertainty, flip the coin.

In real boating world it's a matter of investing some cash testing different pitch props, even modifying them at a prop shop till the correct pitch is dialed to work spot on on your current application, then boat forever happily..

Happy Boating

 

[Derek Domino]

I have kinda made this rocket science I guess. LOL. So the coin flip says:

Solas Amita 3 Blade Aluminum 13.75" dia. X 13" pitch SR

There is no 13x12 for my motor that I can find. It seems like this will definitely be an improvement from where I am now and it will be new, not the beat up prop I'm using now. I'll update once I've got it wet.

 

[Sea Rider]

OK, let's see if the "pitch rule" applies well on your OB will know how much has gained per 2 less pitch ? On larger OB's won't find 1 pitch prop sizes, will need a prop shop repitch.

Will guarantee that with a 13 pitch the impovement will be oustanding..

Happy Boating