Measure the distance from the rear axle to the seat bolt, with the rear wheel off the ground. Gear up and get on it, have someone measure it again. Should be 4 inches of difference. Adjust it if it is not. Then measure the static sag, the bike just standing alone under its on weight. I do not recall that number, but it will tell you if you have the correct spring. Some manuals give you a measurement of the adjusted spring. You really need a manual, there is a lot of model specific , priceless information. An oem shop manual is the best, a clymers works pretty much. Do not get the parts micro fiche cds, rip off.