Yamaha 15hp no compression on one cylinder

[BipolarBear111]

Hi all,

I recently got ‘gifted’ 2 old 15hp yamaha outboards. One is siezed so I have dissasembled it for parts. The second one was in seemingly good condition but it wouldn’t run.

I tracked down the cause to it having no compression in the top cylinder. The other cylinder reads around 95psi (see picture)
The top cylinder just has enough compression to make a hiss when I bleed the compression tester but the needle won’t move.

I decided I would disassemble this one and see what was happening. I found a stuck Piston ring and some minor cylinder and piston scoring.
I did a quick hone of the bores and replaced the piston and rings with parts from the spare motor which had one perfect piston and rings that was exactly the same. With lots of swearing trying to fit the needle bearing on the little end bearing I got the job done and attempted to start it.

The compression is still zero psi on the top cylinder after my redneck rebuild. It still gets solid numbers on the bottom cylinder. Damn it….. What have I missed?

There is a chance I got the piston rings mixed up when I installed them but I would have expected to still see some improvement.

One thing I should mention is that I left the head intact during the disassembly process because I am a tight ass and didn’t want to buy a new gasket.

This is where I am thinking I should start looking next.

I have read the reed valves do not affect the compression. So I am ruling that out?

What else in a two stroke can cause compression issues on one cylinder?

I asked my friend who gifted me the outboards and he said the thinks it may have got hot from lack of oil in the fuel. Which would explain the scoring and stuck piston.

I didn’t see any cracks in the bore but couldn’t see too much because I left the head on and the engine is quite small.

I guess the next step is to pull the head off and inspect the gasket and the head for warping.

Please let me know if you have any ideas where the pressure could be leaking out.

You can feel the difference in compression just putting your finger over the spark plug holes, so I am confident in the compression readings.

It would be an awesome to sort this motor as cheaply as possible and put it on my little tinny here on the east coast of AUS.

Thanks in advance for your help.

 

[BipolarBear111]

So I just pulled the head off and I am super confident that it’s the problem

For one the metal ring on the head gasket around the problem cylinder is torn/ broken

Secondly the head was likely warped because I broke 4 head bolts trying to remove them :/
I was slow and gentle and tried head and penetrating fluid but they snapped regardless.

Any ideas on how to remove tge left over bolt/ stud. A couple broke short and two broke long.

I’ve lathered it in penetrating fluid and walked away for now because I am likely marking the issue worse in frustration

But Tried vice grips and heat from a tourch.

I just tried reinstalling the head gasket and stuffing cloth in the cylinder and welding a nut on the threads but my weld keeps braking and shortening the studs.. I thought the heat from the welder would do the trick, but no.

Any help would be appreciated.

 

[KJM]

Maybe use the other engine block and head? You don't say how bad that one was?

 

[BipolarBear111]

Yeah the other block is toast. The bad piston was seized so bad I had to hammer it out with a sledge.

I will have to work on getting the studs out.
I found this response on another thread.

I have done this perhaps hundreds of times too. Heat the aluminum with a carburizing flame with your acetylene welding nozzle, about a #2. If the bolt is protruding any, you tap in on it when things are hot. The key here is to heat quickly and knowing when to stop. Map gas will give you much faster heat in your small torch than propane. It is cheap too. Anyway, once you have tapped on the broken bolt, quench it with cold water.

Then weld on your piece, or nut.... whatever suits you. When welding on a nut, use a 3/8 nut, for instance, on a 5/16 bolt.....so one size larger. Nice part about using a nut is that it keeps the weld apart from the block, or head surface. Also use as high of a heat setting on your welder as possible. This time let it cool slower because if you quench it too fast, it will weaken the weld bond of the nut to the bolt. A solvent quench of say atf and mixed gas will actually work into the hot threads a bit., Then fire up your torch, but don't get it quite as hot as before, when you tapped on it. Why? Sometimes the aluminum will stick to the bolt and create even more friction. So, then, while it is toasty warm...crank it out with a small box wrench or socket on a breaker bar so you can rock it if needed.

 

[BipolarBear111]

Yeah the other block is toast. The bad piston was seized so bad I had to hammer it out with a sledge.

I will have to work on getting the studs out.