I ran 11.5's in a knucklehead, but I was able to turn the timing back with the left handlebar grip at any time. When accelerating I would roll the timing out, then roll it in as the rpm increased. But, that seemed to be the practical limit, not sure how 12.5 would have worked. I suppose if you have a lot of cam it will, but I would certainly suggest a left hand grip control for the timing. Didn't have a really hot cam in the knuck, just a bit healthier than stock.