GZ250 power increase.

[Quimrider]

My experiences with the K&N filter if anyone else wants to try that:
130 or 132.5 pilot jet (can't decide which is better.) (120 factory jet)
needle shim eliminates flat spot.
pilot screw set at 1 1/4 turn out. (1 5/8 turn out from factory)

Caveat: Each bike is different, climate variables and modifications affect carburettion. My results are presented as a starting point for someone wishing to do the same.

My bike has always had a flat spot approx 5-6k rpm. Modifications include, 4 extra 3/8" holes in exhaust from a previous owner, and 16t front sprocket. Prior to doing anything else I would hit a brick wall at 70mph and if there was any wind 65mph was difficult to maintain. I purchased a K&N SU-5589 air filter and installed it. Took it for a spin knowing it would be bad. Wow! it was so lean it would misfire like crazy even with the supposed flow restricter. I first tried a 140 jet. It ran smooth, flat spot was gone, but the exhaust smelled awfully rich. It also sounded/felt slightly different. Very subtle and hard to distinguish over the now very audible intake noise. The best adjective I can think of is a slight "chug." I next tried a 130 jet. Flat spot was back, slightly worse than it was with the original jet and air filter. My needle valve has no adjustable slots so I did the "needle shim" and simply moved the washer to the other side of the e-clip. This seemed to be the ticket. constant pull thru entire rev range. The acceleration is ever so slightly weaker than with the 140 jet so I think it may be a tad on the lean side. I may try the 132.5 jet. While testing with the 140 jet I was able to hit 83mph with a slight tail/cross wind yesterday. While trying to maintain 70mph it only dropped to 68mph a couple of times. While my top speed hasn't increased much, It's noticeably stronger in the 65-70mph range which is what I was looking for. It's hard to draw any conclusions from my speeds since it's been quite windy here in northern Ohio the past couple of weeks. Also I wasn't able to get the idle screw to go lean. It was about 1 5/8 turns from factory. I backed it out to 1 3/4 turns and started the bike (already warmed up engine) and screwed it in at 1/4 turn increments. At 1 turn out it would idle rough and stall. I settled on 1 1/4 turn out as this seems to idle fine. I'm open to suggestions on this as the idle still seem rich.