1989 Yamaha 40 HP 3 cylinder 2 stroke oil injected.

Symptoms

Bogging/sluggish under load
Suspect fuel/carb issue

Also

Clanking/tapping at low and mid RPM.
Almost sounds like flywheel hitting starter gear but noise comes from crankcase. Doesn't do it at idle, or at WOT. Could just be drowing out the noise though at high RPM.

Suspect piston slap/worn crank bearing or wrist bearing-whatever ya call it. If that is the case I'll probably live with the noise and run it until she spontaneously disassembles.


Bought engine this winter for $800. It was not able to be started in the circumstance of the transaction.

Did compression test with engine cold, will do it warm later though.
Top and middle were 107.
Bottom was 98.

Plugs- bottom one is a little fouley I guess.




Was going to start working my way from fuel tank to carbs to fix the bogging, but wanted to make a post first to see if anyone had any comments for me. Was also going to decarb it. Thanks guys.

Is the engine shaking pretty good during the clanking noise? I've had a Yammy with worn motor mounts do that so something to look into.

I would inspect/clean/rebuild carbs first. As for the tapping possibly being a bottom end bearing, you'll know real soon if it is.

I would have to hear it, some of those motors were pretty noisy as Bateman said, but would run a long time. That compression variance and the plugs looking the way they do would concern me though.

Pappy said:

Rule of thumb before you start tearing into things is 10% between cylinders. You are within. I would run some Engine Tuner through the engine and let it sit overnight. Add some additional oil to protect the engine as the tuner rids the carbon then run the engine on another trip and then check the numbers again.

Click to expand...

I'm going to do the "Dunks" sea foam cocktail method and see what happens.

Are you advising to mix a few ounces into my normal tank afterwards in addition to the oil injection? Maybe 100:1 in the tank?

I bought new plugs, suppose I should wait to install after all of this and I am back to normal operating conditions?

Either rod bearings or piston-to-cylinder wall clearance is loose. Those old 40's, when they're tight and running correctly, are really quiet running when they're below 800 RPM. Good motors. I had one come apart in the test tank (horse trough) a few years back-same motor-guy's complaint was a little bit of noise at no load, he couldn't hear it under a load thought. I stuck it in the tank and fired it up...after it warmed up, it started rattling horribly and went ka-boom, all in about a 2 second time period. Rod out the side of the block and needles went everywhere inside the engine. It was junk. He ordered a new crank/cylinder assembly and had me install it for him, and to my knowledge is still going today. So what caused it? After I took the old block home and finished disassembling it, I found shadows of corrosion on the cylinder walls and crank journals-indicating that it had not been taken very good care of. Probably sat for a length of time without being fogged. The rest of the outboard looked great, didn't look "new" but it wasn't abused either. I did find evidence of prior dirt dobber nests under the bottom of the cowling, and remember....most outboards don't have an air filter, so when those nests get loose from vibration, where do they go? Right down the carb throats, through the engine, and being a 2 stroke, through the ENTIRE engine. That's why I stress watching out for dirt dobber nests. Seen way too many destroyed powerheads because of those little boogers!

I bought an early 90s 40hp with that same wrist pin/slap whatever ... paid $500 for it ... and it ran fine otherwise for the 5+ years I had it. I see it running a few times a year and I bet the new owner never rebuilt it.