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1988 15 HP Johnson, Speed

T

The Waco Kid

Member
Hello everyone. Well, I finally put a fish finder on my 14 ft V hull aluminum boat, with a 15 HP Johnson 2 stroke on the back. The fish finder has GPS, so now I can see that at full throttle, no wind, two adults in the boat, my max speed is 6.5 mph. Should I expect to go faster? Or, is that a reasonable top speed. I know there are a lot of variables, and I am thinking I need to put some tilt on the motor (it is currently missing the tilt pin, but not for long!). My second question is, why would I need a different prop at high altitude? Here in Denver we have lakes near town at around 5300 ft above sea level. In the mountains, we have lakes in South Park that are about 8,900. That is where I registered the 6.5 mph. I have yet to test my speed at the lower altitude lakes. Any insight or advice will be greatly appreciated. Thanks

The Waco Kid
 
I have a '79 9.9HP that we use on a 14' aluminum boat and with 2 people it usually runs about 15MPH with two people. This is on a basic Starcraft V-hull that has 3 bench seats and the rest is wide open.

If you're boat is light and similar to mine you have something that isn't right if you can only hit 6.5MPH with a 15HP motor. Either you don't have the correct prop pitch or you are way down on power.
 
You should be going faster.-----What pitch prop is on your 15 HP motor.------And it sounds like it is only running on 1 cylinder.----So do some ispection for spark on both leads.
 
I am pulling the prop tonite to determine pitch. The motor came from sea level. What would be considered "normal" or "Factory" pitch and size? And, at a whopping 7 mph we are certainly at a bow up attitude, since i weigh 150 lbs more than the person in the bow. I tried the 1st hole on the tilt adjustment...does hole #2 or 3 sound more appropriate? Oh, by the way, both cylinders are firing, and the motor itself seems to be running well. Thanks
 
The Waco Kid said:
I am pulling the prop tonite to determine pitch. The motor came from sea level. What would be considered "normal" or "Factory" pitch and size? And, at a whopping 7 mph we are certainly at a bow up attitude, since i weigh 150 lbs more than the person in the bow. I tried the 1st hole on the tilt adjustment...does hole #2 or 3 sound more appropriate? Oh, by the way, both cylinders are firing, and the motor itself seems to be running well. Thanks
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Aha....A sea level engine running at high altitude is not happy. You need to confirm that your carbs have been re-jetted properly for 5,000+ feet above sea level. If you're using sea level jets it's entirely possible you're running rich because of the thinner air.

Use the Solas prop calculator available on this site. http://www.marineengine.com/parts/solas-propeller/ I used it for your engine and it recommended a 9.25" X 7, 8, or 9 pitch prop. if you have any pitch higher than that you might be lugging the engine.

Changing the trim pin position will get more of the bow out of the water and can help with reducing drag, but trimming out too much can cause other issues. I usually run with mine in the position closest to the transom. Try snugging your motor up against the transom and try that to see if you get the bow down.
 
Kevinj, I took off the prop. It is a Michigan Wheel prop 9.5" 10 pitch. Thank goodness they have an obsolete part number look up. We like to troll for trout and walleye. Sometimes we are at 8000 feet, down to lakes in Nebraska at 3000 feet. I am currently unable to troll any slower than 1.7 MPH. I am thinking shoot for the middle, with an 8 pitch prop. What do you think? As for re-jetting, how would I know if it is re-jetted for high altitude? The owners guide references a high altitude kit. I assume that is a jet kit. We might be moving to Iowa next year, so i am hesitant to tear into the carb with only 2 months left in the season.

Thanks for the help!

The Waco Kid.
 
The Waco Kid said:
Kevinj, I took off the prop. It is a Michigan Wheel prop 9.5" 10 pitch...We like to troll...sometimes we are at 8000 feet, down to lakes in Nebraska at 3000 feet. I am currently unable to troll any slower than 1.7 MPH. I am thinking shoot for the middle, with an 8 pitch prop. ...how would I know if it is re-jetted for high altitude?
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Sounds like you're over-propped. As a general rule you typically gain 200RPM @ WOT for each 1" of pitch change. So going to an 8 pitch should bump you up by 400RPM. Do you know what RPM you're getting now? A TinyTach could help to determine where you are. You should be turning 5500RPM to about 6500RPM @ WOT. It's best to know where you are with your current prop and that will tell you how far to go down on pitch.

The jets should have numbers stamped on them. You have to remove the jet to see it and will be very tough to see without a magnifying glass. I don't know which jet is the regular one and which is the high altitude. JoeReeves or one of the other "experienced" members can hopefully chime in with that info.
 
The high speed jet on the 1988 15hp is part number 326928 with the ID of .055. As such, the jet is simply marked #55.

This would be the commonly seen 15hp model... NOT... the SEL model (sail).
 
joereeves said:
The high speed jet on the 1988 15hp is part number 326928 with the ID of .055. As such, the jet is simply marked #55..
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Thanks Joe. I'm assuming the jet included with the 'High Altitude Kit' would be smaller to decrease fueling. Is that correct? If so, do you know which jet I.D. that might be?
 
kevinj said:
Thanks Joe. I'm assuming the jet included with the 'High Altitude Kit' would be smaller to decrease fueling. Is that correct? If so, do you know which jet I.D. that might be?
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Yes, the ID would be smaller to decrease the fuel flow.

Sorry, I do not know the ID number..... Hopefully another member will have that information.
 
joereeves said:
Yes, the ID would be smaller to decrease the fuel flow.

Sorry, I do not know the ID number..... Hopefully another member will have that information.
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Thanks Joe.

Looking at the parts diagram it shows a #50 orifice plug for the 9.9 models. http://www.marineengine.com/newparts/part_search.php?part_num=323708

I'd be curious if that jet might do the trick.

Hey Waco Kid, have you pulled your plugs to look at them? Are they wet with fuel and/or blackened? If so, then it's quite possible you're running rich. Just to cover bases make sure you do a spark gap test and confirm the plug leads will jump a 7/16" gap. If that is good and you have blackened spark plugs then a rich mixture is a likely place to look.

KJ
 
I have a 15 hp evinrude 1982, my boat is a fiberglas one, pretty ****ty actually, it's about 4-5 meters long and does about 20-25 knots with two people in it. Alone it does 33 knots and I'm a big fella so id say youre pretty slow
 
DrCoxie said:
I have a 15 hp evinrude 1982, my boat is a fiberglas one, pretty ****ty actually, it's about 4-5 meters long and does about 20-25 knots with two people in it. Alone it does 33 knots and I'm a big fella so id say youre pretty slow
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Unless my conversion is wrong a 1 knot = 1.151 mph. So 33 knots =37 mph I highly doubt that unless your boat is a potato chip. Or you have the fastest 15 hp anyone has ever heard of or seen.
 
flyingscott said:
Unless my conversion is wrong a 1 knot = 1.151 mph. So 33 knots =37 mph I highly doubt that unless your boat is a potato chip. Or you have the fastest 15 hp anyone has ever heard of or seen.
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I checked it twice and the app I downloaded definitely said 33 knots and I checked the accuracy against my cars tach (I felt it a bit high too) and the app showed just slightly more than the tach, so call it 30 knots but those are the numbers it spat out at me=S
 
DrCoxie said:
I checked it twice and the app I downloaded definitely said 33 knots and I checked the accuracy against my cars tach (I felt it a bit high too) and the app showed just slightly more than the tach, so call it 30 knots but those are the numbers it spat out at me=S
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What does a tach have to do with knots? I run the exact same 15 hp on a very light 14' boat 18-20 mph is all I ever got with just me. Get a gps not an app that lies to you. I also run a 16' boat that weighs #650s with a 70 hp motor that hits 38 mph.
 
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flyingscott said:
What does a tach have to do with knots? I run the exact same 15 hp on a very light 14' boat 18-20 mph is all I ever got with just me. Get a gps not an app that lies to you. I also run a 16' boat that weighs #650s with a 70 hp motor that hits 38 mph.
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It's a velocity measuring device which I used to check the accuracy of the app, assuming my tach is somewhat accurate, as I said. The phone is a gps, I'm not saying the app is right, I wouldn't be surprised but it's a fact that the app showed the same speed as my tach did to within 5 km/h. I assume that the app measures you gps location every x seconds and calculates the speed, the only thing I could think that would throw the readings on water off is a worse signal on the water and a worse approximation but I checked the app twice going back and forth and it kept showing me the exact same number.
 
DrCoxie said:
It's a velocity measuring device which I used to check the accuracy of the app, assuming my tach is somewhat accurate, as I said. The phone is a gps, I'm not saying the app is right, I wouldn't be surprised but it's a fact that the app showed the same speed as my tach did to within 5 km/h. I assume that the app measures you gps location every x seconds and calculates the speed, the only thing I could think that would throw the readings on water off is a worse signal on the water and a worse approximation but I checked the app twice going back and forth and it kept showing me the exact same number.
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Do you know the difference between a tachometer and a speedometer? You cannot use a tach to determine speed! I have never seen a 37 mph 15 hp except for some high performance racing models.
 
Sorry, speedometer lol my bad.
As I said I'm not defending the app, but that's what it told me and I don't really have a reference from other boats as this is my first boat. I guess I'll download some more apps and see if some of them spit out something more reasonable.
 
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I agree with putting Tiny tach on.I did on my 20 hp qmotor and it does help in tuning in your motor.My 14 ft Jon goes much faster with a 9.9 than your 15 for sure.
 
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