I have a ds 650 model year 2004, and it just backfire
it compresses normal and gets normal petrol, but when I tried to fire it up its
like something striking together

 

I am betting you have sheared off the crankshaft keyway - it's a common problem in these bikes - I have had it happen a couple of times. When it does happen the timing goes out causing the engine to backfire. You need to get the engine to TDC and lock it- insert a special bolt under the gearshifter in the crank. Then remove the side cover by the brake pedal, Pay attention how everything comes apart, it needs to go back together a certain way. Then remove the flywheel, you will need a special tool to do this. They can be bought off ebay. Inspect the crank, I am betting that the timing has jumped when the key broke.

 

Yup. Agreed. Pretty common on a lot of different make ATV's. Timing is off, thus backfiring.