If the idea is to move he weight further back then you will need smaller pinion not bigger (assuming mm4) as well as the fact you are effectivly gearing up with a smaller spur.
Hulk on here mentioned a few month ago at a chadderton meeting that he was trying out smaller spurs, the smallest pinion he could get on was a 16 to push the motor back, think he had 73 spur, was a while ago though and my memory is not what it used to be could have been slightly different
