xr200r 1982 rear suspencion spring upgrade

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I am looking to upgrade the rear suspencion spring of my Honda xr 200 r. I am 200 pounds and my riding includes an old abandoned motocross track. as my skill and speed has increased my rear suspencion has start to bottom. Ilooked at dealers for a heavier rate rear spring but can't get them. Any ideas out there if I can use any other model spring (eg xr250r) on the existing shock or if any suspension company offers any higher rated springs