Several of my motorcycle mags got buried, unread, under piles of stuff for months.  I recently rescued and started reading the Feb 2004 issue of Rider.

Tonight I ran across an interesting exchange in the "Andy's Answers: Tech Q&A" column on pages 66 and 67.  Click HERE to read the question (about a 2002 Kat 600) and the answer - which talks about the kind of float valve seat that OUR carbs have.  I thought it might be useful info for future reference....

PS - I believe the GS500 float valve seats ARE brass and not plastic.

I donno about the 01+ ones, but the older carbs had brass ones.

Yes, I thought ours were brass, too. While a bike is running, and creating heat, and say the float needle seat shrinks, making the hole smaller, the hole is still round. The float needle is round. And tapered. And should still function no matter how small the hole. Back to the bike running: fuel is flowing past, but now it's flowing past a smaller hole, which should restrict fuel flow at higher speeds! If our bikes had plastic seats.

On my 1992, the needle presses up into a tube (I believe that it's plastic).  The tube itself is mounted up into the carb body using an o-ring to seal it in-place.  You can pull the whole tube out, complete with o-ring, to aid cleaning it. I think that the article refers to this outside o-ring, and not to the seat within the tube.  Fuel may be leaking down into the bowl around the needle tube, without ever passing through the needle valve seat orifice at all!

From the author's description, this would mimic a needle valve which is leaky... but only when the engine is hot.  In that case, it should be easy to measure: does the float height go up and out of spec when the engine is hot?  It would have to go up enough to enrich the mixture enough to foul the plugs, which I would think would be at least a few millimeters, so should be pretty obvious.

Hmmm.. Sounds suspicious to me.  Plastic will NOT shrink when heated.  But, it might just be that the metal around that tube expands more than the plastic expands.  Thereby making the gap the o-ring has to seal bigger.  Looks like a slightly larger O-ring would be the real solution to the problem.  To me anyway...