2002 Yamaha 70 2stroke bottom plug rusty

kevin11mic

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I have a 2002 Yamaha 70 2stroke that I had to put a new head on last year due to corrosion eating through the head right at the spark plug. New head on and ever since the bottom cylinder hasn’t always acted right. Sometimes it will fire and sometimes it won’t. Pulled the spark plug out this evening and it is rusty on the inside. The other two look good. I’m guessing I’m getting water in the cylinder somehow. Any ideas? Head and head gaskets we’re both changed. Could it be on the cooling head side? Or do I need to redo the head gasket again. There was some metal loss on the power head around the bottom cylinder but it seemed like there was still enough to create a seal. Any ideas would be helpful.
 

kevin11mic

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This was the block prior to installing a brand new head
 

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kevin11mic

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I guess my question should be. What are the possible options for water to get into the cylinder?
 

Sea Rider

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Water can get into any cylinder if both case and head mating surfaces were not immaculate cleaned to perfection, can use a medium grade sandpaper glued to a glass to polish/clean both sides evenly. Water can also get into any cylinder through the exhaust covers gaskets, the motor being 18 years old probably have them already dried, hardened, not sealing right or were never re torqued to specs since new.

Both exhaust cover gaskets and cylinder head gaskets should be retorqued to factory specs soon after has been changed inmediately after say first 5 run hours...

Happy Boating
 

kevin11mic

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I plan to regasket exhaust cover and retorque the head and see if it fixes. Thanks for the input.
 

Sea Rider

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I plan to regasket exhaust cover and retorque the head and see if it fixes. Thanks for the input.

Much better to regasket both exhaust covers, clean both mating surfaces well. Did you changed both the upper and lower cylinder head gaskets or just the lower one ? Report if everything went well when all the work that needs to de done has finished.

Happy Boating
 

kevin11mic

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The engine was completely regasketed about 3 years ago and then both head Gaskets last year with new inner and outer head pieces. This engine was used in salt and put up wet by the prior owner. I got it super cheap and rebuilt it but I’ve been fighting corrosion from the start. I will report back after I make the repairs and let you know what I find.
 

kevin11mic

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I pulled the exhaust cover and the head and there was no obvious Paths of water getting into the cylinder. All the gaskets looked fine. Is there anywhere else water could be getting in? Could the block be cracked? No obvious signs of cracking but the cylinder is obviously seeing water as it’s plenty rusty.
 

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Sea Rider

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Being a used motor could previous owner had a severe overheat and the cylinder head is warped ? check it with a metal ruler, head should be flat even on the entire extension. When you torqued previous cylinder and exhaust gaskets was with hand or with a torque wrench. Assume none were re torqued back to specs, right ? Are any of the mating surfaces pitted, uneven for gskets not to sit evenly Yep. plenty water intrusion for the lower plug to be so darn rusty...

Happy Boating
 

kevin11mic

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The cylinder head is brand new. Has one season on it. I never retorqued when I installed it last year. It was torqued with a torque wrench however. This go around I will retorque everything properly After the first use. Also changing the exhaust cover gaskets and cleaning/honing All gasket surfaces extensively. This was previously done however the exhaust cover gaskets are about 5 years old and were not Yamaha brand at the time. Going back with all oem gaskets and hoping for the best.
 

Sea Rider

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Had water intrusion with exhaust and cylinder head non original gaskets, from that bad experience all are OEM now. Is not that have a fixation with retorquing gaskets, have retorqued several after first 5-10 hour use since replaced and all needed a small retorque. Heat compresses them a bit and lose the torque force previously torqued to so a retorque is mandatory. It's stated on most Service Manuals, but most boaters skips this recommendation.

Happy Boating
 

kevin11mic

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OEM gaskets will be here today. Everything is cleaned and block is honed. I believe the OEM head gasket has a sticky adhesive on it already. However what is the opinion on applying a thin film of high temp copper RTV to the exhaust cover gaskets? I was thinking this could help seal off any little pitting/ irregularities. I've done this before on my old Toyota engines that can leak with new gaskets. On those engines I've got to where I coat all gaskets with RTV before assembly. I've even successfully used in on an exhaust manifold that was slightly warped and got it to seal. I also always use copper gasket spray on my head gaskets but with the Yamaha sticky on the gasket I don't feel that is necessary on the head gasket.
 

kevin11mic

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I ended up installing everything by the book. Dry gaskets and engine oil on bolts. My surfaces were pretty darn straight and clean so I’m super confident that the exhaust cover and head gaskets aren’t letting the water in. First run on the hose and after about 5min I pulled the bottom plug and it had a faint milk tinge oil on it while the other two were black. I’m still getting water in it. It’s not enough that it hurts the running of the engine however I didn’t fix the problem. Where else can water get in? I’m assuming either a block crack or possibly the lower crank seal or powerhead seal. Is there any way to test for block cracks?
 

Sea Rider

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So all gaskets were torqued to specs. Have you made a compression test on each cylinder ? if lower crank seal is bad will find 2 stroke oil and fuel mix emulsified on the drive shaft and pan. Where is the powerhead seal located ?

Happy Boating
 

kevin11mic

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Yep everything torqued to spec. Compression is top-120 middle-120 Bottom-105. Bottom is low due to some scoring from the previous owner he overheated it and melted the bottom piston. Cylinder mic’d out other than scoring. I replaced the piston and have been running it for 6 years like that. I’m referring to the upper casing gasket between the powerhead and upper casing. I just changed the water pump and didn’t notice anything on the shaft. My water pressure doesn’t seem normal and it seems like more water than normal is coming out of the Rectangle weep hole on the front of the lower unit At the mating surface. Which makes me think something is going on with that gasket or something in the upper casing. Not sure if water could get sucked into the cylinder if the gasket failed. I’m beginning To think I’ve got all I’m going to get out of this corrosion infested powerhead. I’m leaning towards buying a new powerhead at this point. I prefer the 2stroke over a new 4 stroke and I can buy a new powerhead for about $2500. Beats a $10,000 new 4 stroke. The engine is in overall great shape minus the last owners lack of care. Maybe one last ditch effort to pull the power head and look at the upper casing. If nothing looks wrong I’ll assume corrosion/cracking causing the water intrusion and I’m that much closer to installing a new powerhead.
 

kevin11mic

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I found the hole/crack. Its on the bottom cylinder exhaust port. See Video. Based on this while the engine is running after the first start I shouldn't have water in the cylinder. Once I turn it off it can then drain into the cylinder causing my misfire at idle on the bottom cylinder/internal cylinder corrosion after stowing away for weeks/months at a time. This engine has had corrosion issues from the previous owner so I'm assuming this is what eventually caused the hole/crack. The way I see it I've got three options if i want to keep this engine.

See if JB weld would hold up and get maybe another season or two out of this crankcase. I doubt it in the exhaust chamber this close to the cylinder but what are your thoughts? cost is neglegable

Replace the crankcase only with used or new and reuse my crank and pistons. cost ~$1300-1500

Buy a whole new crankcase assembly and part out mine and maybe recover 30% (crank/rods, pistons, head, etc.) cost ~$2500. This would get me a "like new" engine considering the rest of the engine is in really good shape and looks new.

Video Link
 

Sea Rider

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Nice to read the crack was finally found. Would you need to remove the bottom cylinder to weld the crack well ? If so, why don't you install a new cylinder sleeve at a machine shop to rise compression close to the other two cylinders. Assume the cost will be way lower than any of the other options ?

Happy Boating
 
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