90 hp Johnson 2 stroke RPM

Again thanks for all the help. Hopefully its not a water logged hull. I think I’m on to something on the fuel side. The primer bulb never really seemed to get as hard as on the other boats I’ve had. I didn’t really think much of it since it did prime up but I ran across a thread on a site that sounded alot like my issue and turned out to be a fuel pump. Through some research I found I could replace the VRO pump with a standard pump off a 1996 88 special (part # 0438556) if I had the spot on my block. I did have the spot on the block and with the new pump on the bulb gets hard and stays hard. Hopefully I can get on the water this weekend and see if it makes any difference.

Made a run to the lake this afternoon to see if the fuel pump made any difference. With the wife and I in the boat we ran 38 mph just shy of 5000. It seemed to cruise well about 3300 and 21 mph. I’m guessing we were 150 pounds or so lighter than last time.

I’ve been pondering over motor height because my anti ventilation plate seems to high when the boat seems to be trimmed correctly. What effect does a pocket at the bottom of the transom have on where the motor should be mounted?

Motor looks too low to me. It looks to be level with the bottom of the hull in those pictures. Should be a couple of inches above esp. with that setback pocket.

“Wailord”
1979 17’ Montauk
90 Johnson

Wilderness Ride 115

Based on the reading I’ve been doing I was thinking the same. When I tossed a streight edge on it, it was sitting 3/4" above the bottom. I beleieve each hole moves about 3/4", would you start with one or go two? Thanks

If you go to the Yamaha website performance bulletin and click on archived info, you will see the scout 172 with a two stroke Yamaha 90 ( C90 TLRZ). Their test calls for a 13 x 17 prop ( 2:1 gear ratio ) #2 hole,4000 rpm = 28.95 , 5000 rpm =37.55 , 5400 rpm = 39.45 mph. I would think the Johnson is close to the same.
Hope this helps.

The engine looks too low to me.

Capt. Larry Teuton
Swamp Worshiper

I would start with one hole up and see how it does under the same conditions. You shouldn’t have any problem getting a 19 pitch to work with that boat and that motor. I wouldn’t put the 19 on a 2 stroke Yamaha 90 though, it doesn’t have the torque to get on plane and pull tubes as the V4 does with a 19 pitch.

Now if you absolutely want to hit the 5500 WOT and pull skiers while having a family on board with a full tank of gas and not worry about planing, then move the motor up and get a 17 pitch prop. IMO.

“Wailord”
1979 17’ Montauk
90 Johnson

Wilderness Ride 115

BEFORE MOVING THE MOTOR, TEST RUN IT AND LOOK AT TEH CAVITATION PLATE
IT SHOULD BE AT TEH SURFACE OF THE WATER.
NO NEED TO MOVE IT UNTIL YOU VERIFY THAT ITS NOT CORRECT

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Jeez, why the all caps Chris. You make it seem like he is going to lose his motor or something if he raises it one hole up and tests it. Whats the harm?

“Wailord”
1979 17’ Montauk
90 Johnson

Wilderness Ride 115

Thanks for all the input. Beaufort that info will give me good information to compare to.

The way the cavitation plate looked while running is what got me looking into motor height. I wish I would have taken a picture while I was running. To get to 38 and 5000 I had to trim the motor way up, it was a little squirrely and wanted to drive left. The picture below is not mine but it looked a lot like this. The front of the plate still under water but the back was out of the water a inch or two. Seems like if I moved the motor up I could get the same result without having to trim so far up and the cavitaion plate would run flatter. If I trimmed it down from this point the boat felt better and drove better but lost RPM and speed. Hopefully that all make sense.

I’m not too worried about how it is for fishing as long as I can get from point a to b. I’m looking into this for when I take the whole family tubing. Thanks again.

Geronimo, lighten up just a bit.
our dealerships main management software program uses all caps, and sometimes I don’t remember to change it, and I don’t always have the time to go back and retype my advice.
most of the time I am doing something else like being on hold while I am responding to questions

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if the front of the cavitation plate is below the surface you need to raise the motor

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Moved the motor up a hole. Below is a photo of the motor trimmed out running WOT. There was not any performance gains but I think the motor looks positioned better now.

STILL LOOKS LOW TO ME

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yep, low.

“Wailord”
1979 17’ Montauk
90 Johnson

Wilderness Ride 115

Brief related aside…
Question for the motor height gurus. On my 17’ Henry O (1995 Johnson 88 spl), I tried raising the motor to get the anti-ventilation plate pretty close to the surface when running. Went up two holes. It did give me more rpms and speed. But, at 2 holes up, she would break lose in a turn very easily. So, went back down one hole - ending up one hole higher than original. It fixed the breaking lose and she handles nicely now. But, the anti-ventilation plate is definitely buried. I was not able to get the motor as high as I have frequently seen advised. I’m not dissatisfied with the way the boat runs now. But, am still curious.

When the motor is raised to show the AV plate, do most hulls have a problem with the prop breaking free in turns?
To keep the prop from breaking free, I had to trim all the way down and slow down to a very slow plane.

Or, is this condition of having to run the motor a little lower specific to the Henry O hull? This is conceivable to me. The inside chines on this hull are huge and carry air straight back to the transom. Thus, they may be feeding air to the front of the prop.


17’ Henry O Hornet w/ Johnson 88 spl
26’ Palmer Scott project hull
14’ Bentz-Craft w/ Yamaha 25

It is really specific to the hull design and more a matter of trial and error than any hard and fast rules. I raise them until they begin to ventilate in turns, then lower them a little bit until it quits. Adjustable jack plates help a lot with getting it just right.

Capt. Larry Teuton
Swamp Worshiper

prop will also make a big difference

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Sorry for the slow update. Moving the engine up another hole helped and switching to a 17 pitch prop helped even more.

I decided there had to be something else going on and decided to drill a few test holes and see if the foam was wet. Sure enough every hole I drilled was soaked. Cut the floor out and pretty much all the foam below floor level is soaked. Above the floor is dry. I think I may try and fix it, going to price out materials and go from there. Thanks for all the help.