I have a 20yr old Briggs 281700 series 12hp motor on a Murray riding mower. Lately, it is hard to turn over. Sometimes when I turn the key, the starter doesn't engage at all. Other times, I have to rock the mower back and forth and then the starter will engage but barely rotate the flywheel. Sometimes it will engage and get stuck. I thought the battery was done (even though it reads 12+v on a multimeter -- bad amps??) so tried jumping it from the car. The starter definitely works better and cranks the motor but still appears to struggle to turn the flywheel 3-4 rotations (the rotations look slow) before finally spinning fast and firing up. So it appears the battery is weak but I thought I remember the starter working better than this. I replaced that starter 4-5 years ago but do they get weak? Anything else to try or check? Thanks.