Motorcycle Repair: 1998 yz 250 not revving out, mike ford, mid power

Question
I recently purchased a used 1998 yz 250 from a friend who was unable to figure out the bogging problem.The compression was checked by two different people and ok'ed. I had the power valve cleaned and I checked the silencer packing.The previous owner replaced all the jets with new stock ones,as well as new stock reeds.The bike has a pro circuit pipe which he was running before the problem happened.I've got the clip in the second position and tried every setting but the first position with no avail.Am using the stock 172 main jet.Bill's Pipes has listings for most models as to their reccomended carb settings listed on their website..They suggest a 170 main jet.What do you think?After half throttle,it just bogs out.It starts first kick when cold with the choke open.Has plenty of low to mid power in all 5 gears.A new plug gets black and slightly wet and eventually you have to change the plug.It's ovious its running rich.At what point would you reccomend ceasing messing the fuel and reconsider a new piston?Will a smaller main jet bring back the top end?For 5 bucks it would be worth a try.I also changed out the igniton coil as well.
                              Thanks
                              Mike Ford

Answer
Mike,

Without going any further, I would try lowering the main.  But also - is it a "bog" or a hesitation/starvation? If it's the latter, you may be too lean.

Try this before changing the main.  Put a new plug in it, start it and ride 1st gear to mid throttle avoiding the bog point, but let her rev nicely at mid for a few seconds.  Then, Pull in the clutch and hit the kill switch fast! at these revs.  See what color the plug is.

Let me know how you make out.

Anthony
'Ride Today - Live Again'