Riding Lawn Mower Backfires And Wont Start . If it’s around 350rpm, you can rule out a weak battery. The motor found in your mower is considered to be a small internal combustion engine. Riding Lawn Mower Backfires While Running AGreenHand from agreenhand.com If it leaks and it’s a sealed battery, replace it. The motor found in your mower is considered to be a small internal combustion engine. Replace the broken rewind spring with a new one.