new carb

Well, first of all, if a carb worked good in the past, it won't "de-tune" itself.
So don't start to turn all the screws for 'the heck of it'.

There are only a few possible causes:
- dirt in the tank / tubes / carb or bad fuel --> Clean everything and fit a fuel filter
- worn gaskets --> replace them with new ones
(to check if the gaskets are worn: spray something like deodorant over the manifold while the engine is running, and check if the rpm changes)
- broken carb, cracks,... --> like James said, now is a good time to replace it with another.
(Rebuild an OT carb, or replace with a Shengway 18mm: cheap and good with the petrol cock fitted.)
- Loose parts --> Check and put back in place correctly

Based on the symptoms, you should also check ignition and cam timing, valve clearance.

To check the fuel mixture, you should look at the spark plugs color.
White --> mixture is too lean (lack of fuel, or too much air)
Coffee with cream --> perfect
Black --> mixture is too rich (too much fuel, or not enough air)

Thanks everbody for your help my boss has a c90 so i put his carb on 2day and it idles fantastic you can hardly hear it run, BUT and its a big BUT when driving it plucks its nuts off! ive had a word with motorbike shop and he said its jetted up different on c90! so ive order a copy carb i will let you know, thanks again to everbody regards lee

If it's got a brass float in it,they go porous and cause all sorts of problems. Changed mine and all is well

yeah! it has got brass float but looks ok, ive replaced carb i will let you all now shortly when its stop raining, but on idle its fantastic you cant hear it running. but had to do some swapping of bits. regards lee

As the float becomes porous,it absorbs petrol. To check,remove float and drop in jamjar full of petrol and leave for a bit. Take it out and shake it. You'll hear it sloshing around if it has become porous. Replaced mine with a plastic 1.