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

Soft hoses are good. Pull the bowl and take a look. Takes seconds, and will tell you a lot. Typically, the bottom of the bowl gunks up first. Maybe you should just clean the bowl and the uptake tubes, put it back together and see what that gets you. If noticeable improvement, great, you can rebuild later.

It never hurts to clean the carbs, especially the small idle orifices, like behind the idle screw and also the 3 little pin holes at the top of the throat. You may have to pull the Welsch plug on top of the carb to clean them properly, if it doesn't have the black top plate The kit will come with a new plug to replace it with.

Hard to say from a video, but it almost sounds like it's only running on one cylinder. Before you work on it, get it running, and pull the top spark plug wire and put it back, then repeat with the bottom plug. There should be a dramatic change in sound with either. If it runs with one pulled but instantly dies when the other, that narrows down which cylinder is doing all the work. Focus on the one that doesn't change things much.

Start with spark. Look at the plugs and see if they differ. A plug that isn't working may look super clean or badly carboned up. A good plug will look wet and oily right after running. Often, the orange primary wires going to the coils has corrosion inside the boots or on the coil terminal itself. Clean those up as needed. Twirl a properly sized drill bit in there with your fingers, and then spray out with WD40. Sometimes, that will smooth things out.

Any milkiness, gray color or water in the LU fluid? Is the shift rod hard to move up and down?

I was thinking it was missing a cylinder as well

As far as the compression, gauges are all over the place, some do better on small engines other on car engines.
I was having trouble with getting consistent readings from my older MAC tools gauge set on small engines, often it read so low I knew it wasn't right.
I bought a few gauges and finally ended up with the Snap On set made for motorcycles and a short hose, 'small engine' gauge i got from one of the parts suppliers online.

When checking compression you should crank the motor until the gauge stops gaining pressure, the longer the hose and the larger the gauge the longer it'll take.
For a small engine, the best gauge is one with the fewest valves, connections and the shortest hose.
You want see how much pressure the rings can make, not what they make in one or three strokes at low rpm.

Since the starter spins the motor over faster than the rope, it will always give better readings.
Rings are also designed to be forced outward against the cylinder making them seal tighter and it takes compression to do that. Often just testing with the recoil doesn't do that very well, or consistently.

I never use the 'cork' or rubber type gauges, they're not accurate and too hard to hold in place.

I'd add some oil to the cylinders, not much, just enough to make sure the rings are wet and see what you get then. If it jumps up a lot, it could be worn rings, but having some run time on it and being a warm motor will give you the most accurate results.

thill said:

It never hurts to clean the carbs, especially the small idle orifices, like behind the idle screw and also the 3 little pin holes at the top of the throat. You may have to pull the Welsch plug on top of the carb to clean them properly

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As cheap and easy as it is to rebuild these carbs, I'm just going to rebuild it since I have to take it most of the way apart to clean it anyway.

thill said:

Before you work on it, get it running, and pull the top spark plug wire and put it back, then repeat with the bottom plug.

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Thank you for the tip. I'll do that as soon as I get the fuel tank and line. Could bad gas have been causing the rattle? I know it can cause knock in a 4 stroke motor but my knowledge of two stroke is very limited.

thill said:

Any milkiness, gray color or water in the LU fluid? Is the shift rod hard to move up and down?

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Shift rod moved easily. I haven't checked the fluid in the LU yet. I just found out where the fluid check hole was last night while looking into changing the seals. Knowing what I know now, I would have checked before buying it, but it's a little late for that at this point.

hoarder said:

When checking compression you should crank the motor until the gauge stops gaining pressure,

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The gauge stopped moving at around 60lbs after the third stroke. I pulled it a couple more times after the gauge stopped to confirm that, that was all I was getting. I haven't had time to play with it much since I picked it up on Saturday. At this point, I'm really hoping I didn't wind up with a massive project. My ol' lady already isn't happy about me having to do the basic "new to me" stuff on top of having a project boat, on top of all of the other various projects I have going. I'll be in the dog house for sure if I wind up having to rebuild the motor lol.

That's not a fuel knock type of rattle, that sounds like a wrist pin or piston sort of rattle. I had a 50hp that sounded like that a few years ago, the guy said it ran like that for years. We got about 2 miles from the dock and it never got any worse, we fished all day and it was fine, but he showed up at my house two weeks later saying his starter was bad, when I looked it it I found a seized motor. He said he was at the ramp, had launched the boat , parked the truck and went back to start the boat up and it wouldn't crank. When I pulled the plugs, I fished a rod bolt out of the plug hole with a magnet and most of the piston skirt was in the exhaust cover area. My guess was that the piston skirt broke from excess clearance, the piston caught the port and it got worse from there. The bottom rod was in 8 pieces, the crank was dented by each roller bearing, the block was cracked and torn up form the carnage.
He said it ran fine for 11 years with a slight rattle. Not any more. Now in comparison, that motor had good compression, he said he checked it the week before it blew up and it had 145/146 psi. When I was with him, it ran strong, and the noise was barely noticeable. But a noise is never good. The time to fix it is BEFORE it turns from a 'noise' to loud bang or the sound of a rocks in a metal can rolling down the hill.

Any way to check for that without having to totally strip the motor?

Thank you for the reminder pappy. I have a horrible track record when it comes to buying used. Boatyard’s post had me half convinced that I had done it again.

Pappy said:

Check the fluid and if it is not milky do not re seal it.

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I pulled fluid from the drain hole and it was a pretty shade of black with no signs of the forbidden chocolate milkshake or off smells (not that gear oil smells all that good to start with). Feels good to check that one off the list.

80w90 Marine grade oil to refill it with after I change the impeller right?

If no metal shavings on the magnet, you could just close it back up and move on to more important stuff for now

I'm in a holding pattern right now until my parts arrive. I ordered a carb rebuild kit, impeller, and new plugs from Marineengine.com. ETA is this Saturday. Fuel tank and line are coming next Tuesday. I went with one of the Vevor 3g tank/line combo's. I've had good luck with other stuff from Vevor. We'll see if that trend continues or not.

Parts came in today. I'm going to try to get the carb rebuilt and re-installed this evening. The heat index is currently 105F and my ol' lady nixed the idea of rebuilding it in the house for some strange reason lol. So, it's going to have to wait until it cools off this evening or first thing in the morning.

Bantou said:

Parts came in today. I'm going to try to get the carb rebuilt and re-installed this evening. The heat index is currently 105F and my ol' lady nixed the idea of rebuilding it in the house for some strange reason lol. So, it's going to have to wait until it cools off this evening or first thing in the morning.

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"idea of rebuilding it in the house"

Seems perfectly reasonable to me.

Might be a few days before I get back to the carb. I accidentally shanked myself in the hand with a screwdriver. The fuel bowl seems to have glued itself to the carb body. In the process of attempting to pry it loose, the screwdriver slipped and found a new home between my thumb and index finger... Thankfully, it missed everything important and I'm current on my tetanus shot.

Ouch! Get well soon.

Bantou said:

Might be a few days before I get back to the carb. I accidentally shanked myself in the hand with a screwdriver. The fuel bowl seems to have glued itself to the carb body. In the process of attempting to pry it loose, the screwdriver slipped and found a new home between my thumb and index finger... Thankfully, it missed everything important and I'm current on my tetanus shot.

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Been there, done that. That's what hands are for it seems. I have scars from hog bites, punctures as you've mentioned, knife cuts, abrasions, and a circular saw injury. Add a bunch of arthritis and you'll wonder what the purpose of those grub hooks are.
Get well soon.

I worked for years as a mechanic in a dealership, you get used to stuff like that after a while.
I can;t count how many times I got cut or stabbed by something and just wiped it off with a grease rag and taped it up with electrical tape and went back to what I was doing.
You haven't lived till you get gas in an open cut or have hot oil run down your arm to your armpit or partially severed something and from experience knew whether or not it'll grow back or leave a mark.

I'll trade you gas in a cut for Brake Clean... That stuff has a knack for finding every bump, scrape, and nick you've ever had on your hands.

Hand is mostly better already. Hurts like hell if I hit it wrong and I don't have full grip strength back yet; but, it's good enough that I'm going to take another whack (literally this time) at the carb this evening.

I'm going to try gently tapping on the side of the fuel bowl to see if I can get it to dislodge that way. At $50 a pop, I don't want to break it. I have to get it off somehow though and prying on it obviously isn't the best idea

Stand Up said:

Add a bunch of arthritis and you'll wonder what the purpose of those grub hooks are.

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Based on personal experience and a lot of 2nd hand knowledge, I'd say their primary purpose is to make blood sacrifices to the mechanic gods. I can't seem to fix anything motorized without it taking its pound of flesh.

Got the fuel bowl off and it’s in worse shape than I thought. The idle tube had completely disintegrated and the nozzle well is cracked where the idle tube hooks up.

Marinerngines has the idle tube but shows the nozzle well to be discontinued…

I have an older Evinrude parts motor. Hopefully the nozzle well on it is in better shape.