1992 Johnson 40HP - No thrust, rough idle and sounds

[Vile]

Hello!

I have a 1992 Johsnon 40 HP outboard that has been rebuilt in 2012 due to a blown engine. I've been riding this baby with little to no issues outside a dead starter since then for about 4 years straight. The previous blowup was due to the oil mixer failing, so now I mix all my oil.

The motor has been running good all last summer. I get the boat out the dock after sitting cold for the day, head out on the lake and within 10 minutes after planing, the motor starts to lose power and then stalls. This has happened before and it was due to dirty battery terminals. I go to crank it up, it cranks up and starts idling super rough and the engine sounds deeper. I go to accelerate and there is not much power. Due to prior experience, I turn it off and troll back to dock so I don't screw anything up.

So, this is a 2 stroke and the engine fires up. It's running rough and I have not tested the thrust lately. The sparkplugs have not been checked and there is a lot of oil/crud on the outside of the engine. I noticed the gas is leaking a little going into the engine and it's got me worried that maybe it's blown again.

Known Issues

• Starter died, replaced.
• New starter not engaging flywheel (spring stuck. I smack it with a hammer and starts up baby!)
• Thrust sometimes drops full speed (think this is due to the fuel issue mentioned above)
• Idling super rough now (shaking right to left)
• When accelerating, little power, but engine sounds like it's hitting the RPM's
• Deeper sound, this is a tiny engine, but sounds like a little monster now

Any ideas? What should I test first after pressure? I'm not a mechanic, but I am a computer engineer. I've heard from people that with 2 strokes, if it turns over, it's not as bad as I may think. I know with my previous blown engine, it turned over and then I tried to drive it and it blew. It wouldn't turn over after that. It just cranked.

Thanks in advance!

 

[flyingscott]

Do a compression test than a cylinder drop test. Carefully inspect the block

 

[Vile]

Do a compression test than a cylinder drop test. Carefully inspect the block

Thanks for the response!

I've never done a drop test. Do you have any good links or videos to share so I can learn how to do it right? I've seen some on cars where you are measuring the spark. How's it different for boat motors? What am I looking out for? In the case it's not sparking, what's that mean?

 

[flyingscott]

Get a plastic pliers start the motor and pull off the spark plug wires off one at a time. When the plug wire is pulled off there should be a difference in how it runs if not that cylinder is not working. To measure spark get a spark tester set it to 7/16" the spark should jump that gap. Do the compression test first and inspect the block.

 

[Vile]

Thanks.

I will try that in a few hours when I make it to dock.

I can't for the life me understand why it would do this. Had no prior symptoms and ran great without any overheats or anything all summer. I'm hoping it's just a bad plug or something. But, I sent a request for the local boat mechanic at the local marine to stop by to help me diagnose the issue too. I'm pretty sure I can at least pick up a compression tester and try the spark test. I may get a spark plug tester too if they are cheap.

 

[Vile]

Did not know this, but Autozone had compression testers for $40 bucks that you can rent for 90 days. When you return it, they give you 100% of your money back.

I snagged a ignition tester to measure the spark plugs/gap too.

 

[Vile]

No Title

Some good news I think. The top cylinder was 100 PSI and the bottom was 90 PSI. Before my rebuild with a blown motor, I had 120 PSI top and 30 PSI bottom. I tested this by first doing the top with the bottom still in place. Then I put the top back in and took out the bottom. I took two readings and these are the highest of the both.

The top plug had a lot of gas/oil with a dark color. The bottom was a bit dry with a bit of rust and lots of gunk on it. I'm no mechanic, but I would assume all that gunk is causing the issue or at least contributing to something? If so, I have some carb cleaner spray, some 4D 40 and some Seafoam for the carb.

I have not got the boat in the water yet. Its in a lift above the water and I have no power to the lift yet. So, the engine has not been turned over since last summer. Anything on the plugs is from the test and from last year.

Cranking it does sound like some misfiring is happening. I could be wrong, but still sounds a bit deep and a bit stuterry. Will do the drop test when I get power.

See screenshots for spark plug condition.

 

[Vile]

Replaced the bottom plug, still waiting for power though.

 

[flyingscott]

That is not great compression get a different tester and retry

 

[Vile]

That is not great compression get a different tester and retry

I only have one tester here at the lake. I saw a video of someone showing how to properly test compression on a outboard. Said I should try to shoot for 7 to 10 revolutions to get a better reading. I was only doing about 3 or 4. I'll try to do longer and see what happens.

If the compression is still around 100 or lower, I know it's not blown, but not healthy. I read a similar post on these forums from a 40 HP Johnston that had 100 and 75. The posters suggested a decarb with seafoam. I have a new can, so I'll toss some in the gas, crank it up and then pull the plugs to spray some in there, let it dry a bit and then hit the carbs. I'll take it out on the water after that to see if I can get a good cleaning and then recheck the compression when I'm done.

There is a lot of gunk everywhere and on the bottom plugs. I'm hoping a cleaning will help. If not, I'll take it to the mechanic and see if it's something like the rings/gasket/vale or something.

 

[Vile]

I forgot to mention that I will still do the drop test too. I've been waiting here at the dock all day for power to the lift and no luck

Maybe tomorrow will be better. I'm eager to get it in the water and try some things out to see if there is any clue to what's going on.

I'm assuming I want to shoot for around 120 PSI for a engine this size right?

 

[flyingscott]

Yes you want to be at least at a 100 PSi compression and within 10% of each other. From what you describe it sounds like a burn down I am hoping for the best Let us know I really hope I am wrong.

 

[Vile]

Yes you want to be at least at a 100 PSi compression and within 10% of each other. From what you describe it sounds like a burn down I am hoping for the best Let us know I really hope I am wrong.

What's a burn down? :joyous:

 

[Vile]

Oh, I guess you mean a burnt piston or similar. Yeah, I'm hoping it's not that. I read that those normally show much lower PSI if they are causing damage. I guess I won't know unless I look and see what's up or see how it does in the water after the cleaning.

If it is, hopefully it's nothing damaged to the walls.

 

[flyingscott]

Yes a burn down is when one or both pistons get damaged requiring engine teardown

 

[Vile]

Thanks for the clarification.

Sorry for all the questions. I'm learning a lot here. Just looked up some prior posts on that topic and found a few with 1990 40 HP motors that had similar PSI and issues. Seems they were tied to damaged head gaskets or piston rings.

I know I've been good on my mixtures. But I'm starting to get the feeling that ignoring the carb is likely why I'm in this mess. Hopefully the damage is not that extensive due to my prior experience.

 

[Vile]

Got it in the water. Got it idling then compression tested it again with 10 revolutions each. Got about 110 psi in both top and bottom. Top had almost 120.

The engine has an obvious deeper sound and a clear knocking sound. I'll post a video later.

The new plug was drenched in semi black oil/gas.

My father in law who owns a mechanic shop said if you have good compression then it's not a lose ring. May be a rod or bearing issue or just not firing a plug.

My guess is that bottom plug.

 

[oldboat1]

Did you replace the impeller -- pumping lot's of water and running warm (not hot)? Engine sound you describe could be resonance from exhaust -- amt. of exhaust water in the leg(?) Make sure it's pumping.

 

[Vile]

Here is the video of engine.

https://www.facebook.com/video.php?v=10153695700709075

The engine is clearly knocking and shaking enough to turn the engine from all the way left to the right!

Engine starts up decent enough and I when I did it last, I took the top spark plug wire out. I notice a small change in the engine sound. Like kind of losing a tid bit of power, but in general sounded the same. Plugged it back in, took the bottom one out. On the pull, I could head the spark jumping clearly on the bottom. Then it stalled when fully out. Did it again, started with bottom, again I can hear the spark jumping, and it stalled like the previous.

I wonder if that means the top is not firing?

 

[Vile]

Did you replace the impeller -- pumping lot's of water and running warm (not hot)? Engine sound you describe could be resonance from exhaust -- amt. of exhaust water in the leg(?) Make sure it's pumping.

Nope. I did not replace anything on this engine. Been running great for the past 4 years after a rebuild. Got new piston,ring set and so forth in their during that rebuild.

Battery about dead. Just got the carb visible. Nothing gunking near the carb throat. Going to clean it a bit and let it dry and will try to see if I can notice any change of sound when pulling spark plugs again.