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About 75mx125

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  1. The smoke should be tinged blue. 32:1 is correct for the RM80.
  2. Is it running very rich? If so, it will be blowing lots of blue smoke and can cause the bike to bog out like you say. On the same bike, I found an identical issue to be the reeds. I have a video, listen and tell me if my bike acts similar to yours.
  3. It could be. Make sure that exhaust seals up.
  4. Try testing the compression. The huge piston in the 500 is going to make it seem like it has more compression than it does. Just because the old owner said he rebuilt it doesn't mean he did. The bike is showing more than a few symptoms of low compression. Compression test should read 150+ PSI. If it is any lower, open up the top end.
  5. There is supposed to be 17 numbers in the VIN, but you only gave 16. Find that other number on the plate mounted on the frame and we can tell you the year.
  6. That flywheel weight would probably help more than it does on a 250. These 85cc engines have so little rotating weight. The heavier flywheel will make it much easier for her to control the power smoothly.
  7. That's a two stroke.
  8. It should be resulting in a dirty piston and dark, wet plug if it is actually running rich. The big bore kits often require entirely different jetting specs, so yours may be running lean. By the sound of it, it may actually be running on the lean side. There should be a small amount of visible smoke while idling. The piston top should look like this: Your silencer packing should also feel a little tacky but not wet.
  9. If excess fuel is coming in, excess oil should too, as they are in a set ratio.
  10. If you want more power, reliably, get your cylinder ported. A good porting job doesn't run that much money, and it can totally transform the bike. My favorite has been Eric Gorr. He works for pretty cheap, has a good turnaround time, and does a beautiful job. Only $125 for an 85cc. http://www.goforwardmotion.com/2Strokes.php Also, fresh tires that suit the track are key.
  11. It sounds like the clutch probably blew itself apart. The KTM 50's have always had reliability issues when it comes to their clutch. If the oil isn't changed every ride, the clutch shoes wear prematurely and often shatter. The damage is always bad. Take off the clutch cover and verify whether or not the bike is toast.
  12. Both 50's use the centrifugal clutch.
  13. Rich mixtures carry lots of fresh oil to the cylinder head, where it gets burned and leaves carbon residue. Lean mixtures don't allow oil to burn on the head because all of the fuel combusts entirely.
  14. The YZ has it's ups and downs. I've spent a lot of time riding one that was essentially bone stock. What's great: Awesome top end power Great ergonomics for small and large riders Good brakes Solid transmission Feels light. Extremely stable and confidence-inspiring Fun What's bad: Power nonexistent until mid-high revs (no powervalve) Bad clutch (mushy, fades fast, especially bad with no low end power) Vague front end feel Slow turning and doesn't like to hold an inside line The YZ isn't the best choice for a bike to use at a motocross track. The combination of its' lazy handling and unreasonably peaky power make it a chore to keep up with the other, more manageable bikes. However, the huge amount of power and high-speed stability make a great desert bike.
  15. Will you post a video? How are your tolerances between cylinder and piston? Did you check for play in the rod and wrist pin? Any marks on the piston?