Low compression

I did a compression test today on my 2000 Yamaha 50hp two stroke and the top and bottom cylinders read 120 but the center reads 80.

The engine runs great and idles great. This engine has the best idle I have ever experienced as its low and quiet. No vibration or odd sounds. I can get up on plane fast and engine pushes the boat fine from top end speed to low end speeds. I just changed the plugs and ran the engine on the muffs for 20 minutes. The plugs on the top and bottom were clean but the middle plug was fouled and black looking already. I don’t know much about the engines history but I just completed taking the lower unit completely apart, replaced all rings on the gear case, changed shaft seals and everything looked good.

Do I have a ring problem on the middle cylinder due to corrosion or carbon? I am going to do a ring free shock treatment this weekend to see if it helps before pulling the head. Then I will pull the head, clean the cylinders and change all the rings and gaskets. If I dont see any red flag damage what else should I do?

 

also where can I get a good torque wrench kit?

 

Thanks,

I am in the process of decarbonizing the engine with a shock treatment of ring free and have been seeing improvements in my idle range. I am also 99% sure the head gasket is bad and plan to replace it this week. When doing that I will check the rings and cylinder walls.

 

Compared to all the other gaskets the head gasket looks the worst. It has the tell tale signs of failure and is greenish brown in areas and the factory paint on the cylinder head where it touches the gasket has light surface corrosion.

I am pretty impressed with the ring free. I have been starting the engine and letting it idle high for 5-10min then shut it off for 30 minutes to an hour and come back and restart. Lots of smoke.

I can do this without a shop manual but I would like to have one for reference. Does anyone know where I can find one?

 

Just sprayed deep creep into the carbs and stalled the engine to soak overnight. I made such a large cloud of smoke a neighbor called me.

 

well I removed the head cover today and found my problem. I have a chip in the piston head and deep scoring in the cylinder walls. I am going to get a price to bore and install a larger piston but will probably repower. I am going to look at new hondas this weekend.

 

Thanks but its no big deal. I had plans to repower the boat anyways. The only upsetting thing is I cant go out and buy a new 2 stroke. How are honda outboards? I have never owned one.

 

Do you know if my engine can be resleeved or does it have to be bored smooth. I don't want to install a larger cylinder.

 

It's pretty bad. Look at the marks on the head cover.


Even if I take the head to a machine shop to bore the cylinder and reassemble new parts it's not worth it. A new piston, rings and head cover plus gaskets im putting too much into it. Plus there is that missing piece of piston that I will have to verify that is still not in the block. The head gasket was also toast and looks like water made it past it.

 

Ya but for 3k more I can get a new F50.


Engine was broken down and everything else is in good shape.

Crankshaft and bearings


Reeds



Bad piston



No pieces of metal were found in the engine and everything was pretty clean. It appears the damage was a result of the cylinder running lean. 3 new pistons were ordered from wiseco and the block should be back by Friday.

 

Going to pass on the plastic reeds.

The crank case and everything else will be smoothed out by hand with emery cloth as well as washed to remove deposits.