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Discussion Starter · #1 ·
New to the group. I have recently bought my son a 2009 50sx. He rode it once in the front yard the day I brought it home which was Wednesday then it was put up till Saturday morning. It would not start before I loaded it in the trailer to take him riding. Gas is good, carb and jets are clean, put a new plug in it, it does have spark, tested stator and coil. When I did a compression test on Sunday it tested multiple times at around 65-70. The previous owner said it had a fresh top end and the bottom was solid. Top end had about 5-7 hours. This is what the jug and piston look like. Does this cylinder look useable? Any thoughts? Thanks for any advice.




 

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Not sure what compression should be but that seems low to me. Cylinder has definitely seen better days. From the pictures it looks like a lot of scoring on the walls I would try and track down a new cylinder for it.
 

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Welcome to the forum. The piston looks re-usable, but with new ring fitted. The cylinder needs a new coating of Nikasil and would be all good.
Looks like some one has tried to revive the cylinder by giving it a "once over" with sandpaper... What has caused the damage? (the diagonal scraping). A few options:
  • No oil in he fuel (use 40:1 to be safe!)
  • Over heating of cylinder and piston (low or no water in radiator).
  • Racing engine with out air filter or no oil on the air filter. Or air filter not fitted properly.
And I have more bad news for you sadly... If the damage was caused by any of the three mentioned above your crank might also be damaged (conrod bearing). You can fit the new piston and barrel to test. But if it was my bike I would check the crank or rebuild it just to be sure...

You mention you have spark. So your stator and the coil are working.

Best of luck!
 

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Discussion Starter · #7 ·
Thank you very much for the helpful info!!!! I do have spark! I’m hoping the compression or ring was bad enough to keep it from starting. Crazy thing is it started and ran every time I went to look at it “twice “ and it ran the day I brought it home to my boy. He rode it around the front yard 3-4 times with no issues. That was on a Wednesday. Our first time taking it to the track was Saturday. I loaded the bikes up one by one running them each before putting them in the trailer and as normal mine and my daughters were gold and his would not start. Friday night I did an oil change on his new one and his filter was clean and oiled properly. I did start his bike after the oil change and it fired right up. After that I did take his throttle tube off to put some lockons on and realized they were not the right size so I put his all back together. I did not try to start it after that till Saturday morning. Even the old owner said he has no clue. I tried everything I knew of. Took bowl off carb all jets were clean. Float working. I did not remove carb. This is my first 50 and extremely disappointing that my son saved up most of the money himself and can’t ride it. I have a brand new ktm cylinder and full top end kit that should be here on Tuesday. Hopefully that is the fix. I did even give it a small shot of carb cleaner in the air boot and kicked it but it didn’t even try to start.
 

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I expect that when you have the cylinder and new piston fitted she will start and run. The "sandpaper" fix that the past owner has done has damaged the Nikasil coating, but given it enough of a "wake-up" to increase the compression for a short while. Once the piston rings "polished" the bore all that was left was the scoring of the coating.
 

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Discussion Starter · #9 ·
Fingers crossed. Another question you may be able to help with. The front brake is very stiff and the pull is very short. I did take the cap off and there looked to be some dirt or muddy looking substance in the bottom. Should I just bleed the brakes or try the reverse bleeding technique.
 

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Could be a few issues:
  • Seized piston in master
  • Seized pistons in caliper
  • Old brake fluid. The brake fluid will with time get water in it from temperature changes and "sweating" master and caliper.
 
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