Bonneville America Forums Forums Technical Forums Performance Mods Needles and main jet sizes

I have been trying to pin down an answer to the question of why do people reccommend a smaller main jet with a switch from the Bonny needle to a Thruxton needle. I have even discussed this with Peter Jenks. It sounds like the determining factor in fuuel flow at full throttle is the annulus created by the tip of the needle and the needle jet. Thus as long as the main jet is large enough to match the area of this opening (lets say its a 125), then you could go up to a 300 and it wouldn't affect anything.What is wrong with this logic?
Stu

Probably because needles overlap each other at a certain point, and the thruxtons are very rich needles. About as rich as TBS with 3 shims. So considering that, and that all jets supposedly (and from my experience)affect the entire throttle range to some degree, albeit very small when far away, lowering the mains would probably be necassary to keep the bike from being too rich, especially at the point of overlap.