Rojo,
You might have figured out from those earlier threads/discussions that a switch to an 8v battery might have chosen for various reasons.
1. weak starter
2. weak battery
3. bad grounds
4. wrong size battery cables
If you do get a new 6v battery, check the cables (12v cables are thinner and will result in not enough current getting to the starter). 12v cables might be 4 to 6 gauge; however, 2 gauge down to 00 (2/0) gauge is appropriate for a 6v starting system.
Also, check that the grounds are clean, wherever a ground cable is connected: especially from the battery to the frame, and from the frame to the clutch housing above the starter.
Only after taking care of items #2, #3, and #4, should you concern yourself with the starter being weak. Also, taking care of all of the above is not wasted $$$ - they will all work towards obtaining a reliable starting system.
