Gearbox failure
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So i started opening her up. Everything looks fine gearbox wise this far. I'll split the case asap but I have to wait for a flywheel puller or ask my local Yamaha dealer to pull it for me.
Pulled the Athena 170 cilinder and head, both are already damaged. No clue why. Bottom ring is completly stuck, also explains why it had very good compression (full weight to kick it over) after the first testrun +- 10min it was 10 times easier to kick over.
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So i started opening her up. Everything looks fine gearbox wise this far. I'll split the case asap but I have to wait for a flywheel puller or ask my local Yamaha dealer to pull it for me.
Pulled the Athena 170 cilinder and head, both are already damaged. No clue why. Bottom ring is completly stuck, also explains why it had very good compression (full weight to kick it over) after the first testrun +- 10min it was 10 times easier to kick over.
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@Calum
This looks like an overheating issue? But shouldn't the damaged be at the exhaust side@llldennis I was going to say overheating as it happens. Could have been inadequate lubrication
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@llldennis I was going to say overheating as it happens. Could have been inadequate lubrication
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If it was only one side I'd say bent rod, what was the little end bearing like? And what does the plating look like near the transfer ports?
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@dtr-nsr
My guess would be doing a tad under 100mph on a new rebuild that has only just had the jetting altered
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@spookdog that's not a 4 point heat seizure from running lean though. The side of piston above the gudgeon pin see's very little load. Only time I've seen damage to the side of a piston similar, was when a gudgeon pin clip had come loose.
@dtr-nsr
I don’t know, I’m not an expert. But it looks like an ‘aggressive’ heat seizure, whether mixture or lube, to me...
Then again I’ve only ever seen other people’s work. I had a near seizure once but caught it in time when the engine ‘note’ changed. So it never smeared the piston. -
@dtr-nsr
I don’t know, I’m not an expert. But it looks like an ‘aggressive’ heat seizure, whether mixture or lube, to me...
Then again I’ve only ever seen other people’s work. I had a near seizure once but caught it in time when the engine ‘note’ changed. So it never smeared the piston.Pictures of a dt piston that heat seized due to lean mixture, from the base gasket blowing. You can see the classic 4 point seize and a little detonation damage to the piston crown nearest the exhaust port. Believe it or not, pistons aren't perfectly round, And are normally narrower along the line of the gudgeon pin, so it would be very unlikely to get a heat seizure at that point on the piston.
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@lllDennis Two-stroke piston seizures are either thermal-related or lubrication-failure-related. That one appears to me to be lubrication in nature. Could be overly rich jetting that allowed liquid fuel to wash oil off of the piston skirt as the fuel comes through the transfer ports, causing a lubrication failure.
If you need a gearbox have a couple of complete low mileage boxes can do you cheap. -
@lllDennis Two-stroke piston seizures are either thermal-related or lubrication-failure-related. That one appears to me to be lubrication in nature. Could be overly rich jetting that allowed liquid fuel to wash oil off of the piston skirt as the fuel comes through the transfer ports, causing a lubrication failure.
If you need a gearbox have a couple of complete low mileage boxes can do you cheap.Hey guys thanks for the replies. It's probably the harder break in tha caused the damage. I only had 300km's on it and probably forced it with the top speed run.
I also opened the gearbox and seem to have stripped a gear. Lots of heat was Involved.
I included some pictures below. I've never seen damage like this before. Is this normal?
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@lllDennis Two-stroke piston seizures are either thermal-related or lubrication-failure-related. That one appears to me to be lubrication in nature. Could be overly rich jetting that allowed liquid fuel to wash oil off of the piston skirt as the fuel comes through the transfer ports, causing a lubrication failure.
If you need a gearbox have a couple of complete low mileage boxes can do you cheap.@theportingmaster The jetting was extremely rich. Apparently there are 2 sorts of mikuni jet 'sizes'. In my carb there was a 250 mikuni jet. So i ordered aftermarket 250-300 for the Athena kit to test with. But the aftermarket 180 size is the same as original 250.
It was overlay rich while tuning the carb until i figured this out.
If you got a excellent low milage gearbox for me send me a message with some details. Really appreciate it
I looked at new original parts but it's a little expensive for my student wallet
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@theportingmaster The jetting was extremely rich. Apparently there are 2 sorts of mikuni jet 'sizes'. In my carb there was a 250 mikuni jet. So i ordered aftermarket 250-300 for the Athena kit to test with. But the aftermarket 180 size is the same as original 250.
It was overlay rich while tuning the carb until i figured this out.
If you got a excellent low milage gearbox for me send me a message with some details. Really appreciate it
I looked at new original parts but it's a little expensive for my student wallet@llldennis dayum, that's not a stripped gear, that's completely melted
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@lllDennis Best jetting for Athena 170 is 300 main jet on the
Mikuni tm28 but with the powerjet blocked off. Just cut the
rubber tube in half from the float chamber to the powerjet then block the 2 halves of the rubber tube with a couple of 5mm threaded bolts or similar. -
@llldennis dayum, that's not a stripped gear, that's completely melted
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@calum Any recommendations on a crankshaft? While everything is open I want to replace it anyway (Has some small grooves where the bearing is). Best with original or aftermarket?
@llldennis Didn't know they do aftermarket cranks?
Always Originate, Never Pirate!
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@llldennis Didn't know they do aftermarket cranks?
