Two Stroke Motocross

Two Stroke Motocross Forum => Vintage Two Strokes => Topic started by: bower100 on June 23, 2013, 04:58:45 AM

Title: Rebuild of '80 YZ250G all but done but .... BUMMER ! Bike runs bad... ignition.
Post by: bower100 on June 23, 2013, 04:58:45 AM

I need help from guys with YZ experience or at least CDI ignition knowledge. Bikes now all but done upon starting it won't rev past maybe midrange, just blubbers/ breaks-up sounding "sorta" like it s rich... but it's not. Absolutely not a carburation issue... though I did replace parts in there, floats, fuel inlet needle and seat, 370 main jet.( other stuff in there is stock sizes and look good.)
  I bought bike knowing P/O claimed ignition problem... he's right.
    Things ruled out:
   -Kill button is off ,not an issue.
 - I've tried 3 different high tension coils and it runs same. Plus, the original coil tests ok anyway.(Pri. winding= .6 ohm. Sec. winding =5.8K ohm. Plug cap and wire insulation looks good.
  -I bought a USED Ebay  CDI box. Came from a running 80 or 81 250.... so the seller claimed. No tests specs for CDI box I know of. .... IS THAT CORRECT ?
  - both coils on stator plate check out resistance ,  one is .6 ohms, other is 1450 ohms. Well within specs. They also LOOK like new.
  - Rotor feels to have strong magnetivity....any ferrus metal sticks like hell to it.

 So, is running symptom indicative to still having a bad CDI box? I HATE to buy another ebay used one but whats a guy to do ! !

  help ! !   And thanks. dave

Title: Rebuild of ''80 YZ250G all but done but .... BUMMER ! Bike runs bad... ignition.
Post by: SachsGS on June 23, 2013, 05:28:30 AM

Correct. CDI modules typically can't be checked "on the bench". Does the bike run different cold vs hot and is the sparkplug cap a resistor style (have you checked it with a meter)? How are the crank seals and no obstructions in the exhaust (found a rag more then once)?

Title: Rebuild of ''''80 YZ250G all but done but .... BUMMER ! Bike runs bad... ignition.
Post by: evo550 on June 23, 2013, 06:50:08 AM

I know this sounds left of center, but I had a CR125 do exactly as you are describing after a rebuild, got to midrange then just farted, coughed, sneezed, burped and backfired but would not rev out........turns out someone put a piston in backwards  You could also check reeds are not chipped or cracked.

Title: Rebuild of ''''''''80 YZ250G all but done but .... BUMMER ! Bike runs bad... ignition.
Post by: TMKIWI on June 23, 2013, 09:28:36 AM

Check that the high speed stator coil is not grounded.