I try to rejet a 06 gs500f with a K&N drop in filter. What I tried: 20/62.5/137.5 ; air mixture 2.75

Till 5.5k rpm the bike runs much better then previously but between 5.5k and 6.5k rpm there is a flat spot.

I do not know what to try: (is it lean or rich ?, I am confused)
1. smaller main jet - 135, 132.5 or 130? (if it is rich)
2. washer under needle (if it is lean)
3. does any affect to this problem if I set the air mixture screw? (I guess no)

Thank you for any help in advance  

Since you only changed the air filter and from your description it sounds like it's probably a little too rich on the mid-mains.  I have a drop-in filter as well and it doesn't affect the intake a whole lot, especially when running the stock exhaust.  As a first step, install the rubber restrictor ring that came with your filter.  If that gets rid of the flat spot, I would then take the ring back out and close the needles one notch at a time (so they come out of the hole later) to see if that does the trick.  You can drop a size on the mid-mains if closing the needles causes leanness elsewhere.  The good news is it sounds like you're close.  Your air mixture setting only matters at idle.  Best of luck to you.

bill14224, thanks for the tip to try to play with the restrictor.

Please correct me if I am wrong: if the bike is rich and I put back the restrictor it will be just richer. The only case when it can help to eliminate the flat spot is a lean situation (hard to believe this is the case:)).

What I do not know has any affect of the main jet to the 5.5k-6.5k rpm? My tip is the main is too big. Maybe I will change back the main to the original (130) and go for a test ride (play with restrictor).

Quote from: Aron on April 18, 2011, 12:39:34 AM
bill14224, thanks for the tip to try to play with the restrictor.

Please correct me if I am wrong: if the bike is rich and I put back the restrictor it will be just richer. The only case when it can help to eliminate the flat spot is a lean situation (hard to believe this is the case:)).

What I do not know has any affect of the main jet to the 5.5k-6.5k rpm? My tip is the main is too big. Maybe I will change back the main to the original (130) and go for a test ride (play with restrictor).

After doing a re-jet on my 2002, i have a flatspot from 7500 onwards, due to my main being too large, i'm going to drop it back down to a 122.5 and see how that goes.

Oh and i should mention, in Australia, the standard main jet is 115 and i've jumped to a 142.5 without realizing that we don't have the same standard setup as america 

Finally I have changed the main jet from 137.5 to the original 130. I went for a test drive at the night. The flat spot disappeared. The bike runs much better. So my idea as the problem was the too big main jet seemed to me correct. The actual setup: K&N drop in, stock exhaust, 20/62.5/130. I am thinking to try out 132.5 or 135 main jet.