keeps over reving

hi everyone, I am Graham from the UK, I have a WR250F 2002,when I shut off my throttle it over revs to 1500 for about 30 seconds then goes back top normal, then when I rev it again it does exactly the same have cleaned my carb out numerous times, checked throttle cables for tightness, but everything ok there. put new carb seal from carb to engine

Please help   

Well this is a WR250R forum which is a completely different bike than a WR250F.

The WR250R has fuel injection and not a carburetor.

Well ill try and offer some help. It sounds like your engine is running lean. A lean engine will typically have the rpms hang after you let go of the throttle. Is the air/fuel screw adjusted properly and did this problem occur after you changed the boot that holds the card to engine as it also sounds like an air/vacume leak

If the bike is starting hard check you valve clearances too.

I have seen tight valves cause engines to over rev at idle once they are warm but not in the exact manner in which you describe.

Double-check every rubber gasket and O-ring for deterioration or even the possibility of being missing.
The little O-ring at the fuel screw can cause this if it's gone or falling apart.

When I bought my own 2002 WR-250F in July of 2012, it had sat in the guy's garage more than anything for 10 years (it was a garage queen).
One day, he had another guy prepare the bike for sale when he decided to finally sell it since he practically never rode it.
Even though it was obvious someone had the carburetor apart for a cleaning, most of the rubber parts were deteriorated badly and needed replacing.

It sounds like you've got an air leak. Check your mounting boots for cracks or any signs it has perished (include flexing them around). Make sure you have mounted everything properly.