Bike has irratic idle issues

[Gartellhello]

A friend of mine bought a 2006 GZ250 with 20K on it. I have had the carburetor off put in new needle and seat, diaphragm and gasket. Cleaned what little dirt there was inside float bowl area, removed low speed and high speed jets, blew out the orifices. Lubed the choke cable and lever.
With choke off and idle stop screw adjusted properly when you open and close the throttle the bike would then start to run at a very high idle speed. After several minutes it will gradually begin to slow down just slightly. Readjust the idle stop screw and it would be ok for a short time before it would start to run at a high idle speed again. Then readjust to slow it down and then it would kill as it was too slow. Each time this would happen after opening and closing the throttle. The throttle cables appear to be moving properly and in correct adjustment. I then replaced the diaphragm in the petcock thinking that was perhaps causing the erratic idle situation. IT did not correct the problem. Basically it is the worst designed piece of garbage I have ever seen in all my years of engine service. I have put jet kits in 600cc and 1000cc four carburetor Yamaha's that I own. I have rebuilt Holley and Carter automotive carburetors in years past. I have rebuilt outboard motors, lawn mowers, chain saws you name it. Never have I run into a useless piece of garbage as the one on this motorcycle. Kudos to Suzuki for using someones wet dream of a carburetor being fed fuel by a petcock with a diaphragm in it.