Well, there was no need to remove the pilot screw, as it isn't so affected by dirt. The washer is there for a proper spring support; the pilot screw is attached to the carburetor body. Maybe the diagram doesn't correspond exactly to your GZ250 version, so don't worry about it.

I suppose that engine-carb and carb-air filter nozzles are fine, as it could be air leaks there too.

About lean and rich mixtures, sometimes is hard to distinguish. A rich mixture is felt like the bike is accelerating but sometimes it jerks a bit like the engine is being drowned in fuel... On the other hand, a lean mixture is more like a lack of power: smooth if it's not too lean, and heavy (like running out of fuel) when it's so lean (this case could be missmatched with a rich mixture). I'm now able to feel the difference, but in some cases the symptoms are very similar.
Do you get jerks all around the rpm range or only at low-mid rpm? To rule out the possibility of a rich mixture anywhere, clean the air filter, but sure you have a lean mixture at idle.

You should clean the pilot jet, with compressed air or a carb cleaning product. No gaskets need to be replaced, unless it's worn out or broken.
Following my how-to, in the step 5:
http://img.photobucket.com/albums/v2...ker/carb6B.jpg

I had your same idle problem when I put a smaller pilot jet -thinking it was bigger-. You'll may not compensate it properly with a pilot screw "enrichment".

My fault. I meant to say "idle mixture knob".


http://www.postimage.org/Pq48wgCA.jpg

I've had to play with this several times.

Many thanks to everyone for all of the help with this but I've decided it's a bit beyond my comfort zone and have booked the bike into a Suzuki dealer to get it sorted along with a full service and an apraisl of the overall condition of the motor. Will post the result if it's interesting.

Conclusion - The bike is running well now. I decided to put it in to a dealer to sort. Just before I took it there, I decided to give the clamp on the rubber carb mount to the motor on more turn. To my amazement, the unstable idle was no more.
The second thing was the lurching at low speed. This was cured by replacing the unevenly worn (very worn) chain. The sprokets were also relaced at the same time.
All of the problems with the bike were resolved by carb cleaning, carb mounting boot clamp tightening, chain/sprocket replacement and a service.
Many thanks again for all of the input. Lyndon.