What to watch out for: 1990(ish) 15hp Johnson

What are some things to watch out for on these motors? I’m going to look at what I believe is a 1990 Johnson 15 hp on Saturday. Dude says it runs good but could stand to have a tune up and the electric start doesn’t work. I know next to nothing about these motors other than that they have a reputation as being good and reliable (for an old outboard anyway).

Many thanks and I apologize if this has already been asked. I tried searching but didn’t get any hits.

They are not hard to pull start. A compression check should be done as well as an in water test. Should idle well....if all checks out...anything used is a gamble...plan on replacing water pump and spark plugs anyway.

airshot said:

They are not hard to pull start. A compression check should be done as well as an in water test. Should idle well....if all checks out...anything used is a gamble...plan on replacing water pump and spark plugs anyway.

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Im figuring on new plugs, a seafoam treatment and a starter switch at a minimum. Possibly a carb rebuild kit, water pump, and starter as well.

Compression test should come out around 100psi, right?

Since it has pull start, I ‘d focus on the rest of the motor before the electric start. It is fairly simple to diagnose between starter motor, starter solenoid, switch, or wiring. My 25 Merc had a problem this summer and it was the solenoid.

Other than owner abuse, there really isn't any issues with those motors. They're as rock solid as any motor ever was.
Due to age, I'd want to make sure its got a fresh water pump impeller and thermostat in it, two new plugs won't hurt either.
Before throwing any money at the electric start, I'd do some testing to see what's going on first.
I'd start by making sure the starter motor runs when you put 12v to the positive terminal on the side.
Parts for those are cheap and readily available as that motor went pretty much unchanged from 1974 to 1992.

For me, before putting any older motor into service I usually like to do a basic reseal on the lower unit, replace the water pump impeller and thermostat, rebuild the carburetor, replace the plugs, and on electric start, I like to open up the starter and lube the bushings and check the brushes for wear. I've had over 50 of those motors over the years, with at least 15 or so in the garage on a rack right now.
Compression wise it'll vary with cranking RPM, with the recoil starter, 90-115 psi is about the norm. but when cranking with the electric start I've seen as high as 150 psi. What you don't want to see is one with 90 and one with 150, or one with 40 and one with 100 or such. Being equal is more important than the max pressure you get with a gauge.
I keep a compression gauge in my trunk so I'm always prepared to check compression on any motor I happen to run across. Lately there's been many as it seems no one fixes anything these days. (I've trash picked more lately than I've bought)

FuzzyGrub said:

Since it has pull start, I ‘d focus on the rest of the motor before the electric start. It is fairly simple to diagnose between starter motor, starter solenoid, switch, or wiring. My 25 Merc had a problem this summer and it was the solenoid.

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The electric starter is a “eventually” thing. I’ll do some basic diag to see what it needs but it is down towards the bottom of the list of things to work on.

Seafarer said:

Other than owner abuse, there really isn't any issues with those motors. They're as rock solid as any motor ever was.
Due to age, I'd want to make sure its got a fresh water pump impeller and thermostat in it, two new plugs won't hurt either.
Before throwing any money at the electric start, I'd do some testing to see what's going on first.
I'd start by making sure the starter motor runs when you put 12v to the positive terminal on the side.
Parts for those are cheap and readily available as that motor went pretty much unchanged from 1974 to 1992.

For me, before putting any older motor into service I usually like to do a basic reseal on the lower unit, replace the water pump impeller and thermostat, rebuild the carburetor, replace the plugs, and on electric start, I like to open up the starter and lube the bushings and check the brushes for wear. I've had over 50 of those motors over the years, with at least 15 or so in the garage on a rack right now.
Compression wise it'll vary with cranking RPM, with the recoil starter, 90-115 psi is about the norm. but when cranking with the electric start I've seen as high as 150 psi. What you don't want to see is one with 90 and one with 150, or one with 40 and one with 100 or such. Being equal is more important than the max pressure you get with a gauge.
I keep a compression gauge in my trunk so I'm always prepared to check compression on any motor I happen to run across. Lately there's been many as it seems no one fixes anything these days. (I've trash picked more lately than I've bought)

Click to expand...

The guy sent me a couple videos of it running. It’s definitely going to need a carb clean at a minimum. It runs ok at idle but you can hear it start to bog when he gives it throttle.

Thank you for the info about the compression. I’ve been reading up on the motors as much as I can but I hadn’t run across a good set of numbers.

Well I bought the motor. I only got about 60lbs of compression; but, it was even across both cylinders and I was using the pull cord with a HF compression tester so *shrug*. I know it was at least partially running a couple of weeks ago.

I’m going to put a drill on it when I get home and see if I get better compression. If I do, I’ll rebuild the carb and put a new water pump, thermostat, and plugs in it. If I don’t get better compression, I guess I’m hunting down a manual for an 88 15 hp Johnson lol

I'd certainly like to see more than 60 psi in each cylinder but if both are even, its rare for a motor to drop compression in both cylinders and remain dead even.
I'd look real close at the gauge and the seals, and make sure you pull the rope enough time to max out the gauge each time.

I've got a 1985 Johnson here with rope and electric start. The former owner whom I got it from about 20 years ago told me it ran great but needed a lower unit. He had used it since new on a small duck boat with a blind. The lower unit turned out to just be some corrosion around the water intake grate, a bit of weld and a reseal and it was all good again.

The electric start didn't work, at least not all the time, it turned out to be just a starter with some dry bushings. I took the starter apart, turned the armature, replaced the brushes, and soaked the bushings over night in hot oil and reassembled it. It was hard to start and it would stall if I bailed on the throttle real fast while in the water. It also seemed to be running cold. Knowing that the lower had corroded I wanted to give it a good check out. (I had bought it for $20 off CL and it was about 20 years old then.

I checked the compression, first by just using the recoil and got 55 and 62 psi pulling it normally. Figuring it may have been sitting for a while I gave each cylinder a shot of 30wt motor oil and rechecked it, after about three tests, it was giving me 67/70 psi. The harder I pulled the higher the reading. I went ahead and pulled the head, replaced the gaskets, the thermostat, and all the intake and carb gaskets giving he carb a good cleaning with some 2+2 and compressed air. With the starter working and the carb gone through and all the gaskets replaced, (the cylinders both looked really good), I put two new plugs in it and ran it in a barrel for a bit. It didn't want to idle at first but as it ran a bit it got more stable and i was able to get a good idle adjustment.

Having run the motor for about an hour or so total, I rechecked the compression and this time using the starter, cranking it till the gauge stopped climbing, I got 115/117.
For comparison, with the rope, the most I could get out of it was 80/85 psi and that was ripping on it so hard it had to strap the barrel to a tree with two ratchet straps.

I've run that motor every season now for over 20 years, the compression is still the same, (checked it again last fall). I replaced the impeller four or five times, and the whole water pump twice. its still got its original prop, ad I replaced the ignition module back about 7 years ago because the wires corroded off it.
On my 14ft boat, with two big guys, two ammo boxes, several shot guns, two anchors, and the duck blind made of aluminum and grass, it'll get to about 18 mph on the creek with no wind. I hung my fishing motor on it once, a newer Mercury 9.9hp and it barely got to 12 mph. The 15hp Johnson is way faster. It replaced a 9.9hp Evinrude that just wasn't enough and it didn't have electric start. (My original intention was to just swap over all the electric start and 15hp carb to the 9.9hp but decided to keep both motors instead.

My 9.9hp is older, a 1979 model, and on rope only it gives me 76/77 psi on a hot compression test, if go out and check it after it sat for a year I suppose it would be far less. My gauge is an old Sears Craftsman from the early 80's or so. I have a better Snap On set but the hoses on that don't bend well due to the length of the crimp while trying to get at the lower cylinder on most motors. It reads a tad bit higher on the top cylinder. maybe four or five psi.

I'd check your compression gauge against a known good motor, something you know runs right and has good compression before assuming the compression is actually low.
I've not heard of any issues with the HF gauges but they are Chinesium so proceed with caution.

Watching that video, its hard to tell if its the audio quality or if that motor is just very noisy? It sounds like its got a bit of a rattle to it but a phone vid can be misleading sound wise. Even worse played back on a laptop with 1" speakers underneath.

oilsmoke said:

I'd certainly like to see more than 60 psi in each cylinder but if both are even, its rare for a motor to drop compression in both cylinders and remain dead even.
I'd look real close at the gauge and the seals, and make sure you pull the rope enough time to max out the gauge each time.

I've got a 1985 Johnson here with rope and electric start. The former owner whom I got it from about 20 years ago told me it ran great but needed a lower unit. He had used it since new on a small duck boat with a blind. The lower unit turned out to just be some corrosion around the water intake grate, a bit of weld and a reseal and it was all good again.

The electric start didn't work, at least not all the time, it turned out to be just a starter with some dry bushings. I took the starter apart, turned the armature, replaced the brushes, and soaked the bushings over night in hot oil and reassembled it. It was hard to start and it would stall if I bailed on the throttle real fast while in the water. It also seemed to be running cold. Knowing that the lower had corroded I wanted to give it a good check out. (I had bought it for $20 off CL and it was about 20 years old then.

I checked the compression, first by just using the recoil and got 55 and 62 psi pulling it normally. Figuring it may have been sitting for a while I gave each cylinder a shot of 30wt motor oil and rechecked it, after about three tests, it was giving me 67/70 psi. The harder I pulled the higher the reading. I went ahead and pulled the head, replaced the gaskets, the thermostat, and all the intake and carb gaskets giving he carb a good cleaning with some 2+2 and compressed air. With the starter working and the carb gone through and all the gaskets replaced, (the cylinders both looked really good), I put two new plugs in it and ran it in a barrel for a bit. It didn't want to idle at first but as it ran a bit it got more stable and i was able to get a good idle adjustment.

Having run the motor for about an hour or so total, I rechecked the compression and this time using the starter, cranking it till the gauge stopped climbing, I got 115/117.
For comparison, with the rope, the most I could get out of it was 80/85 psi and that was ripping on it so hard it had to strap the barrel to a tree with two ratchet straps.

I've run that motor every season now for over 20 years, the compression is still the same, (checked it again last fall). I replaced the impeller four or five times, and the whole water pump twice. its still got its original prop, ad I replaced the ignition module back about 7 years ago because the wires corroded off it.
On my 14ft boat, with two big guys, two ammo boxes, several shot guns, two anchors, and the duck blind made of aluminum and grass, it'll get to about 18 mph on the creek with no wind. I hung my fishing motor on it once, a newer Mercury 9.9hp and it barely got to 12 mph. The 15hp Johnson is way faster. It replaced a 9.9hp Evinrude that just wasn't enough and it didn't have electric start. (My original intention was to just swap over all the electric start and 15hp carb to the 9.9hp but decided to keep both motors instead.

My 9.9hp is older, a 1979 model, and on rope only it gives me 76/77 psi on a hot compression test, if go out and check it after it sat for a year I suppose it would be far less. My gauge is an old Sears Craftsman from the early 80's or so. I have a better Snap On set but the hoses on that don't bend well due to the length of the crimp while trying to get at the lower cylinder on most motors. It reads a tad bit higher on the top cylinder. maybe four or five psi.

I'd check your compression gauge against a known good motor, something you know runs right and has good compression before assuming the compression is actually low.
I've not heard of any issues with the HF gauges but they are Chinesium so proceed with caution.

Watching that video, its hard to tell if its the audio quality or if that motor is just very noisy? It sounds like its got a bit of a rattle to it but a phone vid can be misleading sound wise. Even worse played back on a laptop with 1" speakers underneath.

Click to expand...

I thought I heard a bit of a rattle as well but assumed it was normal for a 2-stroke outboard. I suspect a big part of the problem in the video was the gas. When I pulled the carb, it reeked of stale gas.

oilsmoke said:

I'd certainly like to see more than 60 psi in each cylinder but if both are even, its rare for a motor to drop compression in both cylinders and remain dead even.
I'd look real close at the gauge and the seals, and make sure you pull the rope enough time to max out the gauge each time.

I've got a 1985 Johnson here with rope and electric start. The former owner whom I got it from about 20 years ago told me it ran great but needed a lower unit. He had used it since new on a small duck boat with a blind. The lower unit turned out to just be some corrosion around the water intake grate, a bit of weld and a reseal and it was all good again.

The electric start didn't work, at least not all the time, it turned out to be just a starter with some dry bushings. I took the starter apart, turned the armature, replaced the brushes, and soaked the bushings over night in hot oil and reassembled it. It was hard to start and it would stall if I bailed on the throttle real fast while in the water. It also seemed to be running cold. Knowing that the lower had corroded I wanted to give it a good check out. (I had bought it for $20 off CL and it was about 20 years old then.

I checked the compression, first by just using the recoil and got 55 and 62 psi pulling it normally. Figuring it may have been sitting for a while I gave each cylinder a shot of 30wt motor oil and rechecked it, after about three tests, it was giving me 67/70 psi. The harder I pulled the higher the reading. I went ahead and pulled the head, replaced the gaskets, the thermostat, and all the intake and carb gaskets giving he carb a good cleaning with some 2+2 and compressed air. With the starter working and the carb gone through and all the gaskets replaced, (the cylinders both looked really good), I put two new plugs in it and ran it in a barrel for a bit. It didn't want to idle at first but as it ran a bit it got more stable and i was able to get a good idle adjustment.

Having run the motor for about an hour or so total, I rechecked the compression and this time using the starter, cranking it till the gauge stopped climbing, I got 115/117.
For comparison, with the rope, the most I could get out of it was 80/85 psi and that was ripping on it so hard it had to strap the barrel to a tree with two ratchet straps.

I've run that motor every season now for over 20 years, the compression is still the same, (checked it again last fall). I replaced the impeller four or five times, and the whole water pump twice. its still got its original prop, ad I replaced the ignition module back about 7 years ago because the wires corroded off it.
On my 14ft boat, with two big guys, two ammo boxes, several shot guns, two anchors, and the duck blind made of aluminum and grass, it'll get to about 18 mph on the creek with no wind. I hung my fishing motor on it once, a newer Mercury 9.9hp and it barely got to 12 mph. The 15hp Johnson is way faster. It replaced a 9.9hp Evinrude that just wasn't enough and it didn't have electric start. (My original intention was to just swap over all the electric start and 15hp carb to the 9.9hp but decided to keep both motors instead.

My 9.9hp is older, a 1979 model, and on rope only it gives me 76/77 psi on a hot compression test, if go out and check it after it sat for a year I suppose it would be far less. My gauge is an old Sears Craftsman from the early 80's or so. I have a better Snap On set but the hoses on that don't bend well due to the length of the crimp while trying to get at the lower cylinder on most motors. It reads a tad bit higher on the top cylinder. maybe four or five psi.

I'd check your compression gauge against a known good motor, something you know runs right and has good compression before assuming the compression is actually low.
I've not heard of any issues with the HF gauges but they are Chinesium so proceed with caution.

Watching that video, it’s hard to tell if its the audio quality or if that motor is just very noisy? It sounds like it’s got a bit of a rattle to it but a phone vid can be misleading sound wise. Even worse played back on a laptop with 1" speakers underneath.

Click to expand...

I thought I heard a bit of a rattle as well but assumed it was normal for a 2-stroke outboard. I suspect that a big part of the problem in the video was bad gas. When I pulled the carb, it reeked of stale gas.

I don't think that is a 1990 with the angled lower cowling. Sounds like it has gunked up carbs. I hope you didn't pay much for that one, but you should be able to get it running well with a little elbow grease. Here is what I would prepare for:

• New impeller/water pump
• Thorough carb cleaning and install a new kit if needed, gaskets at the minimum.
• Replace the spark plug with a Champion OEM suggested plug
• Replace the fuel lines in the motor if needed. (90% of the ones that age need it)
• Check all wiring for cracked/missing insulation, including the primary leads from the power pack to the coils.

I hope it serves you well

thill said:

I don't think that is a 1990 with the angled lower cowling. Sounds like it has gunked up carbs. I hope you didn't pay much for that one, but you should be able to get it running well with a little elbow grease. Here is what I would prepare for:

• New impeller/water pump
• Thorough carb cleaning and install a new kit if needed, gaskets at the minimum.
• Replace the spark plug with a Champion OEM suggested plug
• Replace the fuel lines in the motor if needed. (90% of the ones that age need it)
• Check all wiring for cracked/missing insulation, including the primary leads from the power pack to the coils.

I hope it serves you well

Click to expand...

It’s an 88. I payed $500 for it and an older evinrude parts motor. Took the carb off yesterday and the gaskets still look ok from the outside but it absolutely reeked of bad gas. I’m debating whether to just give it a good soak in B-12 or totally rebuild it.

I was already planning on doing the water pump impeller and plugs. Fuel line and wires appear to be ok.

I order all of the parts from marineparts. The prices are comparable to Amazon and I know that I’m getting OEM parts.

Edit: I just figured out that I was supposed to do the compression test when the motor was warmed up. I did it cold which I think would account for the low compression.

2nd edit: its marineengine, idk where I pulled marineparts from…

Most likely the gauge. Also, carb should be wide open. I do three revolutions, but not sure if that is a "standard".

Often, once you add fuel and oil and run it a bit, like in the pic, compression will go up.

Let it turn over for 4 seconds before stopping. That gives you what you need to know. 3 revolutions may not be enough to pump up the tester to full pressure with a small-displacement motor. It's not a car engine.

Often they get better and better after doing a decarb treatment. I forgot to mention to run a can of SeaFoam through the first 2 tanks. Half the bottle in the first tank, and half in the second. Other decarb products work too, but I really like Seafoam.

Bantou said:

Took the carb off yesterday and the gaskets still look ok from the outside but it absolutely reeked of bad gas. I’m debating whether to just give it a good soak in B-12 or totally rebuild it.

I was already planning on doing the water pump impeller and plugs. Fuel line and wires appear to be ok.

Click to expand...

You have the carb off... Rebuild it!!! You never know what kind of gunk is loaded up in there.

Feel the fuel lines, especially the one that goes under the powerhead to the fuel pump. If they are hard as plastic, replace them. If soft and rubbery, then you are okay.

$500 is okay for a running motor. Hopefully, it serves you well after a little maintenance.

thill said:

You have the carb off... Rebuild it!!! You never know what kind of gunk is loaded up in there.

Feel the fuel lines, especially the one that goes under the powerhead to the fuel pump. If they are hard as plastic, replace them. If soft and rubbery, then you are okay.

$500 is okay for a running motor. Hopefully, it serves you well after a little maintenance.

Click to expand...

Fuel line was surprisingly still very pliable and not hard or crumbly. Based on how the carb looked once I got it off, I suspect it had a rebuild shortly before getting taken out of service. I ordered a rebuild kit anyway. I’m going to have to take it most of the way apart to clean it. Figured I might as well rebuild it while I’m in there.

I’m looking into how to replace the bottom end seals now. I might do that while I’m doing the water pump if it isn’t going to be a nightmare.