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Yamaha SX225TXRZ

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Randel1

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I'm working on a customers Yamaha and it is getting water in #6 cyl only. I ran a compression check and it is within factory spec. It has rust and brown build up on the plug. I pulled the head and the top of the piston showed evidence of water/fuel burn with some slight pitting.I had the head machined thinking it may have been a warped head problem. The head gasket did not give any clues as to water entering thru the metal gasket seal. I put it back together with new plugs and ran the motor on hose for about an hour varying the RPM from idle to about 2500 RPM and pulled the plug and it looked normal. I let it cool down and re-torqued the head to factory spec. My customer ran the boat over the weekend and said it ran better but still seemed to be lacking power at WOT and his hole shot was suffering. The first thing i did was pull the plugs and look at the tip for burn residue. #6 again showed signs of water getting into the combustion chamber causing brown build up on the plug and eventually fowling it out. My question is: Can this motor get water into the cylinder via the exhaust system? If so is it usually just a gasket failure around the inner and/or outer exhaust cover. I have never encountered this problem before on the Yamaha outboards however this motor has had a previous rebuild and I can't rule out that as a source or cause of the problem.
 
Is this engine reaching operating temp? Did you do a leak down test on the cylinder in question? Did you inspect the cylinder head for holes? I do this with alcohol.
 
Thanks Chris
Yes the engine is reaching N.O.T. I didn't do a leak down because the compression was 128 PSI which was +/- 3 PSI of all other cyls. I did inspect the head visually for cracks or holes and requested the machine shop check it for the same they said it was good. I pulled the exhaust cover and inspected the water passage and exhaust for indications of a crack or pin hole, nothing out of the ordinary found with one exception. There are 6 round mounds with flat tops and they are a little smaller dia. than a dime. They are molded into the casting and the only one that has a hole in it is the one on #6 cylinder. I don't know if that is by design and normal or if it is a result of corrosion. I have never noticed it before but I have not chased a problem like this before. I have attached picture #2.jpg#3.jpgYamaha water jacket.jpg to help identify what I am referring to.
 
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Did the leak check with soapy water and detected no leaks. So, I have eliminated the head, head gasket, cylinder sleeve, and exhaust/water jacket as a source of water intrusion. I guess the next step is pulling the power head to check out the casing and exhaust manifold gaskets and the crankshaft seal. A leak in the manifold or casing gaskets should affect more than just #6 cylinder I would think. However a lower crank case seal would more than likely just show up on #6 so I'm laying odds that I may have a bad lower crank shaft seal however the motor doesn't seem like it sits low enough in the water to expose the lower seal to water.
Does anyone have any other suggestions before the power head comes off?
 
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