Need help and feels kind of a goose for asking but here goes. I got myself a VTR250 a few weeks back and decided I better tighten the chain as it had way to much slack in it, around 5-7cm. Checked the manual and it states 2-3 cm is ideal so up she goes on the rear stand and I tighten it. I take the bike of the stand check the tightness again and all seems fine. I ride the bike to work and back (30km round trip) and re-check the chain and to my disappointment it's tight as hell less than 1cm of slack. I Re adjust the chain make sure the rear wheel is tight as hell and off I go again for a ride. Re-check it a few times and it's fine but a week later it's tight as hell again. What am I doing wrong. I'm loosening the rear axle nut undoing the lock nut then adjusting the chain with the adjustment nut then tightening the lock nut and rear axle nut again. It's not rocket science but I'm stuffing it up. I suspect the chain might be crook but I was told by the previous owner he replaced the chain and front sprocket a few months before selling the bike.