Quote:
Originally Posted by Bob KellyIII
Howdy Danno !
I agree with your statements on the shimming of the needle, I probably didn't make myself clear on that.... but like you say it is exactly like putting in a bigger mainjet because you are actually pulling up the needle out of the main jet when you do the shimming...(in effect anyway, the needle sticks into the emulsifer tube not the jet itself but it is close !)
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Take look at this photo. Main jet is at opposite end of emulsion tube and needle doesn't affect its metering as much as needle-jet/collar at other end. The gap between needle and needle-jet changes dramatically with height of needle from slide's closed position to fully open.
Shimming needle only affects mid-range because tip of needle doesn't change at all (see pics below). Most change in needle's diameter is in mid-range. Once you get to 10k+, needle-diameter remains constant-diameter. So even if it lifts up out of needle-jet more, it doesn't change fuel-metering because gap remains constant. This matches with engine's loss of VE in higher-RPMs and loss of torque (max-power is around 10K). So changing needle-position with shims won't affect high-end fueling at all.
However, changing main-jet will affect mixtures across board from mid-range onwards. Which is not what we want to do when adjusting only mid-range-fueling since top-end is already too rich anyway. No need to add even more petrol to already super-rich high-end.
Notice differences in factory new-gen needle vs. DynoJet's stg-II needle:
Mid-range on DynoJet needle is thinner than factory for extra fuel in mid-range. While tip is much, much fatter to remove LOTS of high-end fuel.