Interesting point. Thanks.

From the table I produced earlier all the needles ever used by the later GS500's are "5DHxxx" which means they should all have the identical tapers and dimensions. Can anyone confirm that these needles are all interchangeable, besides the known difference of extra clip grooves?

The fact the problem got only slightly better with a dropped needle does make me wonder though.. A mm drop in needle should remove any needle wear from the equation.

However the fact the problem went away so completely with lower fuel level amazed me. I'll explore this (free) avenue before looking at needles. Should be able to report back by the end of the week.

I folks,

No had much time to play but did a bit of work before putting bike away for winter - I can definitively close the chapter on this 6k RPM dead spot. It's gone now.

As a quick recap, removing air filter also removed the flat spot so I know I was too rich. However, I did not know if it was due to the needle or main jet stages of the carb. Dropping fuel level also fixed the problem (almost), but again, this affects all stages of carb so I did not know if it was needle or main...

I can confirm that at 6k RPM with WOT, the main jet is already in operation. I don't think all the time as the slide is bouncing due to intake variations, but the slide is definitely hitting the top and bringing the main jet into play at that point.

I had found that going from +1 main (#130) down to standard (#127.5) made little difference. At the same time going to -1 (#125) made quite a big difference.  This made me wonder about my #127.5 that came on the bike. I don't have pin gauges to measure jets but with a bit of jiggling it seemed to me as though the 127.5's were the same size as my 130's. So I purchased a new set of 127.5's and voila, the problem is gone. I believe now that someone had drilled the old jets... (actually, a previous post where someone also found drilled jets in their carbs, got me thinking in this direction).

So the bottom line is that if you have a 6k RPM flat spot, try smaller on the main jets, that flat spot is actually 8 stoking where "Instead of the bike firing every 2nd revolution (as a 4 stroke would) it fires every 4th revolution (or eight strokes). It does this because the mixture is so rich that it needs the extra revolution to get enough air in to allow combustion." I believe that this is happening when the slide hits top as it vibrates up and down and too much gas hits the engine from the main jet in spurts.

As an aside I also was told that the leaning of bikes to meet emissions standards was *usually* done only up to 1/2 throttle as this is generally where bikes are ridden.  The emissions tests did not cover WOT scenarios;  so it was very rare for bikes to be jetted lean at full throttle from the factory. The lean jetting was around the early needle/needle jet stage to pass these tests.
I don't know if this is true but my experience bears this out. The number of folks that have rejetted  to main +1 and who have found the 6k RPM flat spot makes me think this might be more common than we think.

Cheers!