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Old 09-25-2012, 10:07 AM   #1
svejkovat OP
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Joined: Jun 2009
Oddometer: 856
Can a jet needle be profiled to add low end richness?

This is just theoretical. I'm not thinking of any specific size bike or carb at this point. Just wondering what the mechanics and tuners out there think. I looked on Google Image and did not see an instance of what I was contemplating.

Often a larger OD carb is put on a motocross bike to allow max respiration in the powerband. Bear in mind I know very little technically about this stuff other than tinkering on my own two strokes for many years. That big carb seems to trade off in low rpm leanness and poor torque. Maybe there are booster circuits in the better carbs like on automobiles?

But on a simple carb, without such boosters, would it not be possible to profile the mixture needle to provide regular mix at idle, a shallow profile just off of idle and up to about 1/4 to 1/3 throttle that allows richer mixture where it's needed, and then from 1/3 to full throttle the needle profile at the emulsion tube opening is as original.

Exaggerated modification to show my intention. 3d cutaway of original on left and 2d of modified profile on right ('cuz i suck at CAD and don't want to cut a profile from a 3d object).

Just a bit above the shallow cutout of the modified version is where the needle rests in the emulsion tube at idle.
Quickly off of idle there's brief rich dump up to approx 1/4 to 1/3 throttle that boosts low end torque on an otherwise oversized barrel.
From about 1/3 to full throttle the needle profile is as original in the emulsion tube opening.

Any thoughts?

svejkovat screwed with this post 09-25-2012 at 10:25 AM
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