stihl ms 250 chain won't stop spinning when the engine is at idle, is this dangerous?

Yes. This indicates an incorrectly set idle speed. The chain must not rotate at idle.

The cause is a carburetor idle adjustment that is set too high. This needs to be corrected immediately for safety.

Fix:

1. Ensure the saw is on a level surface and the chain brake is not engaged.
2. Start the engine and let it warm up.
3. With the throttle fully released, observe the chain. If it is moving, the idle speed is too high.

The carburetor adjustment requires a special tool. Do not attempt to adjust it without the correct screwdriver. This is not a user-serviceable adjustment on this model. You must take it to a STIHL servicing dealer to have the idle speed set correctly.

If that didn't work: A persistently high idle could also be caused by an air leak or a faulty throttle trigger mechanism. These also require inspection by a specialist dealer.

Heads up: This is a critical safety issue. Do not use the saw until the idle is adjusted so the chain is stationary. A rotating chain at idle can cause loss of control and injury.