So i decided to rebuild my bike a 2001 Yz 125 before the season started fro me. So i tore the bike down new chain sprocket change all fluid fork seals ect. Well i aslo had a piston ring and im pretty mechanical but i didnt know you had to file the ring until after my problem. So i went riding the bike ran awsome, But i only got about 6 hours out of the motor. We went the last time on friday and i dumped the bike in some pretty deep water and hte bike was flooded and wouldnt start so i dint think anything of it and i loaded up and came home. Dried the bike out and wont start. So i did a compression test and got about 75 -100 psi. So I have weak compression but the motor turns. with no problems. I noticed on friday twords the end of day on friday the bike had no power at all i couldnt spin the back tire and it sounded really weird so i dont know where to go from here. any ideas? How much damage do you think is done? Thanks Phil
Big Trouble!
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rmc_olderthandirt
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- Apr 18, 2006
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I had to re-read your post a few times and I am not sure that I understand.
Are you saying that before you dumped the bike into the water it was already starting to lose power and run bad?
You may have siezed the engine, quite possibly due to a too tight end gap on the rings. As the engine heats up the ring will grow in size. If the cylinder grows with it then things are okay, but normally that doesn't happen. A sustained, hard run can quickly heat the piston while the cylinder takes a lot longer to soak up the heat. If the ring gap isn't sufficient to allow for the thermal expansion then it gets too tight and siezes against the cylinder.
I would recommend pulling the cylinder head off and inspecting the cylinder. If the cylinder looks okay then I would recommend a new head gasket and making sure it is torqued down properly.
Rod
Are you saying that before you dumped the bike into the water it was already starting to lose power and run bad?
You may have siezed the engine, quite possibly due to a too tight end gap on the rings. As the engine heats up the ring will grow in size. If the cylinder grows with it then things are okay, but normally that doesn't happen. A sustained, hard run can quickly heat the piston while the cylinder takes a lot longer to soak up the heat. If the ring gap isn't sufficient to allow for the thermal expansion then it gets too tight and siezes against the cylinder.
I would recommend pulling the cylinder head off and inspecting the cylinder. If the cylinder looks okay then I would recommend a new head gasket and making sure it is torqued down properly.
Rod
rmc_olderthandirt
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- Apr 18, 2006
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If the ring gap was too small then here is what is likely to happen:
When you first start up it would be great. In fact, the small end gap gives you an even better seal, best possible performance.
As the piston/ring heats up, however the thermal expansion causes things to grow. The ring gets bigger, and the end gap decreases. As long as there is still some end gap things are good.
Then you get to a stretch where running the engine for a sustained period at high RPM and WOT. Now the piston heats up a lot and the ring grows to the point that the end gap closes down to zero. Any further expansion and the ring is being forced into the cylinder wall.
This added force against the cylinder wall creates a lot of friction, which only heats the ring more, adding more pressure, making more heat, causing more expansion, more friction, and the cycle continues.
The increased friction means less power, which you may have noticed as you were riding. The increased heat is also doing bad things to the piston and possibly cylinder and head. If the heat builds up to the point that things start to melt then it is possible, likely even that the piston jams into the cylinder and siezes to the point that it won't rotate anymore.
Your's may not have made it to that point. The piston may still be intact without any holes or significant distortion. The ring may not have fared so well.
I would guess that the partial seize has scored the cylinder wall lightly and made the ring such that it doesn't seal any more. I will modify what I said earlier, you should remove the cylinder and replace the ring. When you replace the ring you will also be able to inspect the piston and make sure that the groove that the ring rides in hasn't collapsed.
Rod
When you first start up it would be great. In fact, the small end gap gives you an even better seal, best possible performance.
As the piston/ring heats up, however the thermal expansion causes things to grow. The ring gets bigger, and the end gap decreases. As long as there is still some end gap things are good.
Then you get to a stretch where running the engine for a sustained period at high RPM and WOT. Now the piston heats up a lot and the ring grows to the point that the end gap closes down to zero. Any further expansion and the ring is being forced into the cylinder wall.
This added force against the cylinder wall creates a lot of friction, which only heats the ring more, adding more pressure, making more heat, causing more expansion, more friction, and the cycle continues.
The increased friction means less power, which you may have noticed as you were riding. The increased heat is also doing bad things to the piston and possibly cylinder and head. If the heat builds up to the point that things start to melt then it is possible, likely even that the piston jams into the cylinder and siezes to the point that it won't rotate anymore.
Your's may not have made it to that point. The piston may still be intact without any holes or significant distortion. The ring may not have fared so well.
I would guess that the partial seize has scored the cylinder wall lightly and made the ring such that it doesn't seal any more. I will modify what I said earlier, you should remove the cylinder and replace the ring. When you replace the ring you will also be able to inspect the piston and make sure that the groove that the ring rides in hasn't collapsed.
Rod
- Thread starter
- #6
so i have an update i put in my old ring it had 150 psi and the biek started and ran but seemed a little weak. So i took it out today and well it blew up again now i have near no pressure in cyliner. I need to change my filter clean my carb. But do you think a bore would fix my problem. I would like to get this finally fixed so i can actually go ride with my buddies. Tnaks in advance.
DannyMoto07
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- Apr 12, 2007
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I wouldn't BORE it out any larger... but I would hone the cylinder smooth to remove any damage this caused. Then install brand new rings in the cylinder first and check your gap... I don't know what they are on your bike but they should be less than .010"
Then reinstall your piston, use PLENTY of 2-stroke oil on it so that you don't scar the inside of the cylinder walls.
Now... if/when you get it started... DON'T RIDE IT FLAT OUT. Do not open your throttle more than 3/4 the way until about 10 to 12 hours of riding. Go easy on it... The rings need time to seat properly and the motor needs to get used to the new piston, heating and re-heating the metals to break it in slowly.
After a good 12 hours or so... go ahead and goose it up a bit... but don't thrash on the thing for a while. Breaking in a new piston is a process... if you skip a step you'll mess things up.
Dan
Then reinstall your piston, use PLENTY of 2-stroke oil on it so that you don't scar the inside of the cylinder walls.
Now... if/when you get it started... DON'T RIDE IT FLAT OUT. Do not open your throttle more than 3/4 the way until about 10 to 12 hours of riding. Go easy on it... The rings need time to seat properly and the motor needs to get used to the new piston, heating and re-heating the metals to break it in slowly.
After a good 12 hours or so... go ahead and goose it up a bit... but don't thrash on the thing for a while. Breaking in a new piston is a process... if you skip a step you'll mess things up.
Dan
DannyMoto07
Member
- Apr 12, 2007
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Well, anytime I've ever done a rebuild on any motor, 2 or 4 stroke they always reccomend a 300 mile (12 or so hour) break in period of 3/4 throttle driving with little to no idle time and good long trips to get everything warmed up evenly. I've never seized a new motor in my life and don't plan on it in the future.. lol.
Dan
Dan
DannyMoto07
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- Apr 12, 2007
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If you've damaged the stock piston or the weisco piston, you'll need a new one. There's no way to actually refurbish a broken piston. What you could do is get a mild-agrogate buffing wheel and polish up the piston, take just the carbon off... then inspect it, and if it's clean.. no cracks, no big burn spots... then go ahead and re-install it with the new rings you've already gapped.
Good luck.
Dan
Good luck.
Dan
