If the gears are not fully meshed before the motor starts to turn, you've got the pivot holes in the mechanical linkage worn oval, and/or the round linkage worn in diameter. Get the mechanical relationships correct and the problem will disappear. If you're unwilling to do that, put a Ford-type solenoid switch between the battery and the mechanical switch on the starter motor and use a push button to energize the starter motor AFTER pushing the foot pedal into full engagement. Fixing it right is a lot simpler.
Jerry
