You might consider putting two of those batteries in parallel if you can figure out some place to mount the second one. That will give you twice as much cranking amperage with the same 6 volts. Be sure the generator is producing 7.5 volts against a fully-charged battery after 20-30 minutes of running time at fast idle after starting. 6V batteries take a long time to recharge after starting, especially if you have to crank the engine for more than a few seconds.
Jerry
Yeahhh...if stuck with slightly too small a Battery...adding a second one would be really good.
If all is well with the Engine ( and properly tuned up and so on ) and with the Electrics and Charging Circuit...
The Engine should start very easily, even in cold weather...just a couple Turns and "Vah-Room!" it should start and be running...cold weather maybe four or five turns to start...even if the turns are 'sluggish'.
So, if it is not starting promptly/smartly, look to the details of the Timing, Plug gaps, Points condition and their Gap, Coil...Spark Plug Lead Wires, Valve clearances, Carburetor and Accelerator Pump, and so on.
I know my 6 Volt "216" 1946 Chevrolet Truck always started with just a flick of the Floor Starter Pedal in ordinary temperature ranges. ( I kept things in good order far as tune-ups and so on ).
A few times, 6 degrees above Zero, it was 'sluggish' and barely turned over at all! But it started on like the fourth or fifth turn...so...if all is well, there is no reason why a 6 Volt system would not perform just fine, and for the Engine to start smartly.
