Gearbox failure
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Hello,
I somehow doubt you've damaged the actual gearbox. The cogs are knarly stuff and can withstand a lot of punishment. Typically there is not any movement between the cogs, therefore it will withstand the torques applied to it.
It is more likely that the problem lies else where, dog ears, selector drum or merely the clutch basket.
First point of call though is to drain the oil and see what comes out.
Whip the clutch cover off and investigate any abnomalities behind the cover. If that looks okay then drop the engine and split the cases.
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Check the woodruff key on the crank primary drive cog
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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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@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. -
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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@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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@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.
