Yfz450 with starting issues

[Yfz245]

Ok I need some help with this problem I have about lost all my hair. I have a 2005 yfz 450 and it has problems cranking it has a Athena 478 big bore kit and the starter just slowly rolls the motor over and it's not charging very good I've replaced the regulator and stator and it charges like 12.8v and the starting I can let the battery charge with a battery charger and it still barely rolls over but I can bump the charger up to 40 amps and it will crank right up. Is there something wrong with the machine or is the big bore to much? All help will be appriciated


Thanks