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svejkovat 09-25-2012 10:07 AM

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?

JeffS77 09-25-2012 11:10 AM

isnt this why mikuni makes stepped needles ??

In the four stroke world this is why pumper carbs and preferred.

Shocktower 09-25-2012 03:17 PM

Sudco is your friend call and ask them also the pilot jet changes things and the needle jet there are tons of variables with Mikunis

anotherguy 09-25-2012 05:54 PM

At the top of the needle is a straight portion of varying diameters and lengths. This is what covers from just above 1/8 throttle to about half depending on the individual needle. The diameter is chosen to accommodate overall fuel demand and the length of the straight portion is chosen to cover demand at the higher end of the throttle opening,i.e. transition into the taper and main jet range.

The needle jet/emulsion tube determines the raw amount of fuel available across the range and the needle taper is used to fine tune delivery in individual portions of the range.

To answer your question the combination of slide cutaway (determines air available just over idle to just under 1/2),pilot/pilot air circuit,emulsion tube and needle taper tailor the fuel curve to the demand of a particular combination. You have to remember all circuits are additive meaning they all combine to determine the fuel delivery curve.

Sometimes the pipe,state of tune and carb combination leaves you with a flat spot that cannot easily be tuned away. It's either too rich or lean,the right amount cannot be delivered across the operating range. Sometimes if the advance curve is adjustable you can tune it away/minimize it if the fuel curve cannot deliver.

sieg 09-25-2012 06:27 PM

There all kinds of different tapers on needles. Although I never seen anything like you are talking about. But bottom line is a bigger carb will take away low-end torque and all the needle mods in the world will not get it back. If it was that simple all the little bikes would have big carbs with funny needles.:lol3<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

third eye 09-25-2012 07:18 PM


Foot dragger 09-25-2012 07:36 PM

I have sat on a pickup tailgate and sanded a needle down to make it richer,beer in one hand and fine sand paper in the other,we were out in the middle of the desert,the bike was too lean and no way to get another needle.
I kept sanding/drinking,then let the bikes owner test till we got it right.
It worked out fine but.........the needle is anodized so it doesnt wear away,once you sand a needle its soft and will probably wear out pretty quick.

Ive also heard of guys clipping a needle to make it shorter and thus richer,Ive never tried it.

JeffS77 09-25-2012 08:06 PM

The needle jet/emulsion tube determines the raw amount of fuel available across the range and the needle taper is used to fine tune delivery in individual portions of the range.

The tube is a pretty damn important piece I think a lot of people over look and just chase with jets and needles.. I learned the hard way in the past that this can be a deciding factor to weird mid range issues.

caryder 09-25-2012 08:22 PM

Big carb = low air velocity at low rpm = low vacuum to pull fuel into the air flow. Smaller needle dia won't fix the low vacuum issue. Bottom line you give away low end to gain top end because of physics.


stainlesscycle 09-26-2012 04:53 AM

carb plate is semi-effective in solving the big carb/low air velocity issue. (on intake side, not just airbox side.)

profiling could be effective IF there was only 1 needle clip position. if you move the clip around, the profile would move into unknown ranges.

since a carb is so linear in operation (needle raises in orifice/slide raises to allow more air) it's the nature of the beast. accelerator pumps solve some of the issues on 4 strokes. in theory you could add fuel via another jet or accel pump that activated at lean range... (kinda like mikuni power jet for example).

direct injection with eg monitoring/sensors could solve all these issues. but that adds a myriad of sensors - tps, o2, egt, etc..

KennyV 09-27-2012 05:27 AM

There is a Mikuni tuning manual available from Sudco that has a chart / graph that shows a lean / rich comparison of all available needles at different throttle openings.

anotherguy 09-28-2012 05:52 AM

Hell if you pay attention the Sudco catalog alone can be an education. But nothing beats a bunch of jets and needles,a box of spark plugs,a nice Saturday afternoon at the track and a notebook for learning how to extract the most from an engine. Budds Creek or High Point are great dirt dynos the weekend after The Nationals. Those big ass hills straight out of a corner will show any weakness in your tune.

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