86 Yamaha 70hp no compression in #2 cylinder

[mcgray8]

I have an 86 Yamaha 70 (70etlj) that runs fine with the muffs, but when put in the water loses all power after 15 seconds and then stalls and can not be started again. Compression is good in cylinders 1 & 3, but there is ZERO compression in #2. We used a borescope to look into the cylinder and found that there was a lot of carbon built up on the piston head. The piston still moves with no metal-on-metal, grinding, or other bad sounding noises when the engine is turned over. Please help!!!

 

[Toddboat]

Re: 86 Yamaha 70hp no compression in #2 cylinder

It's easy enough to get rid of the carbon, just decarbonize the motor with Seafoam or other kind of decarbonizer. There is a good thread somewhere on this website on exactly how to do this. It recommends doing it several times until no more carbon burns off.
Your problem is not the carbon on the cylinder's head. I believe they all have at least some amount of carbon on them because they are in the combustion chamber. Your problem is likely in the rings. Sometimes excessive carbon makes the rings stick to the piston, leaving a gap between the piston and the cylinder.
You can quickly diagnose this by removing the exhaust cover located on the side of the engine block and looking into the cylinders. Rotate the flywheel by hand until you can see the rings on the #2 piston. If they are stuck to the cylinder as I suspect, then you can try a few aggressive applications of Seafoam to see if they come unstuck. If they do not, then you may have to disassemble the motor to access them to get them loose. They are a bit fragile and may break if you try to remove them to clean between them and the cylinder. If they are damaged in any way, you will have to replace them. Put some oil on them before putting the piston back into the cylinder to avoid metal on metal friction when starting the motor. If the rings are stuck to the cylinder, I would suggest doing everything you can (including thorough research) to unstick them without having to take the motor apart. Accessing them is a pretty major job.
Forgot to mention... the rings have a break in them somewhere. If you can't get them unstuck, try to rotate them until you find the break. This is your best access point where you have the best chance of getting under them to unstick them.

 

[mcgray8]

Re: 86 Yamaha 70hp no compression in #2 cylinder

I agree with having some carbon build up, but the other two look great. There is a significant difference with the #2 cylinder. Also, even if those rings are stuck you should at least get some compression, right? We hooked it up to a gauge, and got absolutely nothing.

 

[Toddboat]

Re: 86 Yamaha 70hp no compression in #2 cylinder

In order to get compression, you would have to have a seal between the rings and the cylinder. I've seen pistons where the rings were stuck to the cylinder to the point that they were flush with the outside of the piston (as opposed to sticking out past it like they're supposed to). In these cases, there would be a considerable gap between the rings and the cylinder walls. You would not have compression with a large gap between the piston rings and the cylinder walls. In a very healthy motor, there is a very tiny gap, and oil seals it. But in a case where the rings are stuck to the piston, the gap is too big for oil alone to do the trick.
There is another possibility... you may not be getting oil on the #2 cylinder. If you have oil injection, perhaps the oil line to the #2 cylinder is kinked or clogged somehow. You can try draining all gas from the carb bowls, fuel lines, filter, etc, and run the motor on premix (50:1) and see if it makes any difference in the compression reading. Or squirt oil into the #2 cylinder's spark plug hole and see if that gives you compression. I would do all of the above, and also check what's going on with the rings by peeking through after removing the exhaust cover.

 

[Bamaman1]

Re: 86 Yamaha 70hp no compression in #2 cylinder

Sounds like a good winter project. Chances are the engine is going to need to be disassembled. It's not too complicated of a job if you're mechanical.