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Discussion Starter · #1 ·
Was on the river the other day and it seemed my engine was underpowered compared to usual. Continued to motor up and my motor started to knock somewhat loud. I changed the spark plugs today not expecting it would fix anything (it needed it anyway). When I pulled the plugs I noticed one had a tan colored burn on it. Unfortunately that did not resolve the issue.

Anyone have any idea what it could be? My suspicion is that there is some mechanical failure in the crank assembly but I'm not sure. If someone experienced this before maybe they some insight.

Tight lines,
J
 

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Smoke increase/decrease? Change in fuel, oil or exhaust smell? Was maintenance recently done on the engine? Need more information. 2 stroke or 4 stroke?
 

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A mechanical failure in the rotating parts like the crank would quickly lead to something catastrophic like it seizing up or not running at all.

Engine knock isn’t going to feel like you are down on power. What you described sounds more like a cylinder cutting out. Diagnosing a failing coil can be very if when cold you get strong spark and you only get intermittent cutout when things get hot.

There are fuel system issues that will make it feel like it’s losing power too.
 

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Discussion Starter · #6 ·
Engine is a two stroke (2000 Mercury EL). The knock is consistent whether hot or cold and the motor fires with no issue. Exhaust smoke is normal. The lower unit was recently rebuilt and the knock occurs in neutral or in gear. Seems to be louder at lower RPMs but that could be subjective as there is much more exhaust/engine noise at higher RPMs. I did recently use a can of 2-stroke oil that I bought at an up north gas station and have no clue how old it is. However, my tank and backup gas can are filled with the mix so I haven't been able to try something else but that was what I was going to try next.
 

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Test the compression. Also, bad fuel will cause detonation that will sound like a knock. Detonation will destroy the a piston over time.
 

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Tested compression both cylinders identical. Changed the gas to fresh with new 2 cycle additive and she runs great...
Thanks for the advice everyone!
That's great news! At least it was a cheap and easy fix. I got bad gas from a station in Newberry this winter. My snowmobile made it about 20 miles and then the motor detonated so bad it blew the top rings off both pistons. Still not sure why it didn't go into limp mode.
 

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I have seen a number of 2 stroke engines knock because of carbon buildup on the head/ or top of piston acting like a glow plug, causing premature ignition of the fuel. A complete decarbonization with Seafoam cured them.
 
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