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20hp mercury problems

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Dbozeman81

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Engine running perfect at the beginning of the year. Then it started to bog under load, now it's bogging under load and idling high in neutral. So far I have gone through the carb, no issues with fuel starvation. Pulled plugs to replace them, they both looked fine. But I noticed after I pulled them the top piston had normal carbon buildup, but the bottom piston was spotless. Trying to figure the reason for this, and if it's related to the running issues.
 
Way to low double check with a different gauge or if your gauge has the o-ring in it only snug it and retry. I would suspect the gauge as they are both the same but retry anyways.
 
Just double checked the compression. Both still around 60psi. Don't have another gauge to use. I checked gauge against a known good 2 stroke engine, the compression on it was 110,112psi.
 
Remove bypass covers and inspect pistons and rings.-----And throttle position does not matter when compression testing a 2 stroke like yours.
 
OK, I am sure something is wrong with it internally. It just feels weak and idles high, I'm sure it has bad seals too. Motor never been into before. What compression should this motor have? My first time working on a merc.
 
60 PSI is so low that I would wonder if you have a hole in something or reed valves stuck open. It will not run right at 60PSI. The sad thing is that year of mercury is fabulous! I have a 9.9 hp that is the best running motor I have ever owned. It will idle all day long and has plenty of power too. I would carefully inspect the entire block for cracks. I would remove the carb and look inside that end to see if the reed valves are stuck with debris. You may be looking at a rebuild or getting a good used powerhead off ebay to get it back on the water.
 
Reeds are good, will be tearing it down this week. Both cylinders are glazed bad. Will inspect everything at that time. Motor has always ran good until this year. It once rode a mile up river with a broken lower unit and half a prop.
 
What is the ideal finish in a mercasil cylinder? What have is extremely mirrored. Pistons look great, rings may need to be replaced, no scarring on cylinder walls. Exhaust cover was leaking water into lower cylinder.
 
I have a fine bar style hone. Will it be any issue with the mercasil walls? I'm going to be replacing the Rings and gaping them properly. Also replacing all gaskets and upper and lower crank seals
 
You cannot use a standard ball hone on a Mercosil block and it is finished at 20 degrees not 45!!! You use diamond hone and want the finish to look almost polished. You will need to take to a motorcycle shop as the hone are expensive and if you need to replace piston/s make sure you get the correct ones as different rings and aluminum. It sounds like engine has been overhead as this is what kills these engines...
 
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