Motorcycle Repair: YSR50 will not start, air screw, rich mixture

Question
Chris,

I'm helping my neighbour with a 90 YSR50 that will not run properly. Won't kickstart at all but will push start(eventually) and will not idle - Plug is always wet. You need to rev the crap out of it to keep it going. Things I tried:

Checked float level needle and jets and air screw. All stock, within spec and not worn.
Replaced plug with known good plug.
Checked for spark - grounded it and looks good.
Replaced carb with known good carb - no luck
Checked exhaust for blockage - OK
Checked reed valve - OK
Checked compression - OK
Replaced coil with known good coil - no luck

Seems to me that for some its not getting a good or any spark spark when the plug is installed in the cylinder. Any suggestions? Could it be the magneto or CDI. Spark looks nice and strong when I ground it. I have taken the carb, plug and coil out of the mix and have no clue where to go next. I will be checking the wring over today but I doubt I will find anything.

I am beyond frustrated.

Help!

Thanks

Rick  

Answer
Hi Rick.
 Since the plug is wet, that is indicative of a rich mixture.  Tis means that either the mixture is set wrong or there is something else in the system causing the fuel mixture to change.

 When you changed out the carb, was there any change at all, or was it the just the same as before?

 Have you checked the air filter?  A dirty air filter will cause the bike to run rich, since it will restrict the flow of air to the carb.  Because the engine will want to pull the same vacuum all the time, then it has to pull from somewhere.  The fuel.

Let me know what you find.
Good luck.
FALCON