You are giving me bad flashbacks to my 6 volt 1963 VW Beetle. When you had a new battery, a new starter, and new cables it could grind the 36 HP engine slowly to life. However, give a few months aging to any component and the result was a grindingly slow crank and a weak spark. I usually parked it on hills and even learned to chock the front and start it by jacking and spinning a rear wheel by hand in 4th gear if I got stuck.

I finally solved the problem once and for all by finding an 8 volt battery at a tractor supply place - and bending the voltage regulator adjuster tab a little. The starter sure spun nicely then! And the lights were nice and right too, and didn't really seem to burn out any quicker than they did under 6 volts.