Wilsons Sandbox

Listers, To stiffen up the rear suspensionon my KLR 650 A13 I recently replaced the oem shock spring with the PS 500/560 spring. The bike feels better "planted" with no scraping on the panniers but a problem did develop with the new spring: there is now a harmonic or resonnance problem with the bike. Mostly this occurs between 40 and 50 mph, not dependent on rpms (any gear). There is no wobble or weave but like a soft jackhammer, about 3 to 6 up and down pulses in the handlebars followed by an equal time of smoothness, then the cycle of jackhammer/smoothness repeats. Above 50 mph there is little or none. Slightly worse going uphill. The suspension was set on 5, the hardest. I reset it to one and it was a little worse, it seemed, so I reset it to 5. There was no resonnance before the spring and no other changes were made except changing the spring. I weight 190 lbs dressed to ride and am running gripsters at 34 psi front, 36 psi rear. Bike has 3500 miles on it. I don't want to go back to the oem spring or spend the $$ for a whole new PS shock. Anyone else had this problem? Any ideas??? Thanks, Pat Rileyville, VA 94 ZG1000 COG 1887 99 KLR 650 FSSNOC 3610 88 R100GS BMWMOA 71601, ABC 1619 76 GL1000 CWC 295 AMA LIFE MSF INSTRUCTOR