Hi !

I did a bit of reading/research on carbs and i'm a bit confused about the difference between a Needle Jet and Jet Needle

I'm aware of the large needle present in each carb (the one with the notches where you move the circlip).

I also know that when the throttle is wide open (100%) the main jet is no longer regulated by the jet needle (the long needle) because it's just sitting there way above the main jet not restricting the flow at all !

So if the spark plugs look lean (white) at 100% thottle there's no real point changing the jet needle, but rather the size of the main jet.

Ok so there's the MAIN JET and JET NEEDLE out of the way, what the hell is the NEEDLE JET ??

At first I thought the NEEDLE JET and MAIN JET would be the same thing .. not so apparently ??

Can anyone explain the difference ? Does the GS500 have both a NEEDLE JET and MAIN JET ?

Thanks !  

Stupid Carburetors...  If everyone would just go to EFI we wouldn't have to waste all these brain cells trying to understand what basically amounts to a buncha smoke & mirrors anyway...  

Oh, well.   :lol:

The needle jet is the brass tube that the jet needle goes into (why can't they just call them "tube" & "needle" ?).  Here's a graphic from Bike Bandit that shows an exploded carb.  The tube (needle jet) is item 30, the needle (jet needle) is item 32.  

http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=137685

Basically, the needle jet (the tube) must be matched to the jet needle (the needle) so that when the slide is part of the way up, the annular gap between the OD of the needle & the ID of the tube gives the proper size to meter the fuel at that operating condition - as the slide goes up, the opening gets bigger & the fuel flow increases.  At some point near the top of the slide travel, this annular opening will exceed the size (or flow capability) of the main jet (down in the bottom of the carb) & the flow will then be limited by the main jet size.

Hope this helps...

Hey I thought that was emulsion tube...I also refer to the float closing rubber rocket shaped doo hikey as the float needle. So more needles for you.
BTW whoever said the needle doens't matter when the throttle is wide open had no idea what he/she was talking about...That the carbs out into the wide open and lift up the slide by hand...you'll see that the needle never leaves the emulsion tube...it still has atleast 1/8 in the thing. In fact DJ needles are very very thin in this region...and that lets them get by with 134 mains for the same set up as we use with 150's.
Cool.
Srinath.

Ahh I get it, I really appreciate that Bob !  

One other question. How does a less restrictive exhaust lean-out a mixture. On a four stroke, the suck-stroke's intake valves are open and more air is drawn in with a less restrictive air-filter.

But the exhaust doesn't have anything to do with the suck-stroke that 'creates' the mixture.

How can the exhaust therefore lean out the mixture ?

Thanks again !  

Quote from: seshadri_srinath
BTW whoever said the needle doens't matter when the throttle is wide open had no idea what he/she was talking about...That the carbs out into the wide open and lift up the slide by hand...you'll see that the needle never leaves the emulsion tube...it still has atleast 1/8 in the thing.  

Um, Yeah, sorta.  Actually, if the carb is set up properly, the orifice between the needle & the tube will be larger than the opening in the main jet when the needle is all the way up.  When this happens, the greatest restriction in the system becomes the main jet, which is why it controls fuel flow rates at WOT.  The needle still influences flow some, but the main jet has the most influence at WOT.

Hey, Srinath, it's 6am in Cali - what are you doing on the list??  You're cronic, dude...   :lol:

Quote from: giraffeOne other question. How does a less restrictive exhaust lean-out a mixture. On a four stroke, the suck-stroke's intake valves are open and more air is drawn in with a less restrictive air-filter.

But the exhaust doesn't have anything to do with the suck-stroke that 'creates' the mixture.

How can the exhaust therefore lean out the mixture ?

Imagine the end of the exhaust stroke on an engine that has a very restrictive exhaust system.  The pressure at the inlet to the exhaust system is high at this instant in time because of the restriction (we're talking about maybe 20 to 50mBar over atmospheric).  This means that the pressure in the cylinder is also high.  This is the point where the exhaust valve is closing & the intake is opening.  As the intake valve opens the intake charge comes in at a rate determined by the difference between the pressure in the cylinder & the atmopheric pressure outside of the carbs.  As the piston goes down on the intake stroke the cylinder volume increases, so the pressure in the cylinder decreases & the delta betwen the atmosphere & the cylinder increases & air flows.  If the cylinder starts out this process with relatively high pressure the delta to the atmosphere is smaller & flow is less.

If your carbs are expecting lower air flow cuz of the restrictive exhaust then the jetting will be sized as such.  If you then remove that restriction, you get more air flow - but your jetting is still set up for lower flow, so you end up lean.  With a less restrictive exhaust, you get more air flow through the carbs so you hafta bump up the jet sizes to compensate.

Did I help or are you just more confused now?  

Well I was in CA for 2 days only...got back to NC on the 7th...Its 9 am...and what you doing up at 6 am...Yea I know main jet is the limiting factor...which is why we change the main jet when we want to touch the top 1/4 throttle opening...LIfting the needle more will make the 1/2 to 3/4 richer...but that is because of needle taper...The needle taper nearly flattens out in the bottom 1/2 inch of it .. indicating that lifting it wont have any effect. But in the DJ needles that last 1/2 inch has a drastic thinning of the needle. So what I am saying I guess is the needle does count...but since the stock one has no taper it is not effective when you shim it up to richen the top 1/4 throttle. It does matter...but cannot be controlled/altered by lifting it. Can be controlled by swapping the entire needle with one that is thinner...
Cool.
Srinath.

Quote from: seshadri_srinathWell I was in CA for 2 days only...got back to NC on the 7th...Its 9 am...and what you doing up at 6 am....

Oh, Ok, I thought you lived in Cali.  Well, it's after 9 here in Michigan, too...    

I thought you were Bob Broussard...similar avatar too but he's going the other direction isn't he..
Cool.
Srinath.

Effective diameter is what counts...The effective diameter of the stock emulsion tube with the needle sticking in it may be smaller than the main jet, but the DJ needle in the emulsion tube could be larger than it. Further the needle is up stream of the main jet in any case...it always affects. The real argument is where do we control it. If we are supposed to control it at the needle it should still have the taper. If it dont have the taper we will be running around in circles cos lifting the needle will ahve very little effect on the mix till we lift the thing clear out of the tube and then it will have a huge effect. Hence we control it with the needle. If the needle had a good taper all the way to its tip...we could very well control the mix all the way to the top with it. and if it wont pull out of the emulsion tube in a 1/4 inch off the top...we could never touch the mains. DJ's needle is extermely thin in its bottom 1/4 inch...that was dont so they'd give you a rich stage 1 with doing nothing but needles...easier to put in...dont have to take carbs off...(unless they have the brass plug on the mix screw). Which is also why 134 is sufficient for a rich stage 3. Stock needle the last 1/4 inch being so much fatter makes 134 very very lean on the Stage 3 type setting. Yea I know segway into side debate 135 vs 150.
Cool.
Srinath.

You guys must have PhD in carb theory ? Thanks Bob and Srinath ..

Bob I hate to simplify but are you basically saying that at the end of the exhaust stroke, because the exhaust was so restrictive there is still some gas pressure in the cylinder that does not have time to exit the exhaust port ?

Therefore when the intake valve opens some of that pressure is pushed back into the carburetor ?

No we dont...Just learned the hard way...by falling on my face...
Cool.
Srinath.

Quote from: giraffeBob I hate to simplify but are you basically saying that at the end of the exhaust stroke, because the exhaust was so restrictive there is still some gas pressure in the cylinder that does not have time to exit the exhaust port ?

Therefore when the intake valve opens some of that pressure is pushed back into the carburetor ?

Yeah, that's close enough.

The pressure drop across a restrictive exhaust system is gonna be higher than across a low restriction system, so with the more restrictive system there will be slightly higher pressure in the cylinder at the time the exhaust valve closes.  Since the amount of intake charge that gets in is gonna be related to the pressure difference between the combustion chamber & the atmosphere across the induction system, having the pressure in the cylinder as low as possible at the start of the intake stroke is a good thing.  There is a lot more going on than this, because the cylinder volume is constantly changing & air & exhaust are compressible fluids so acoustics come in to play, etc., etc., etc...  ...but all other things being equal, having a lower restriction exhaust is a good thing.

Quote from: seshadri_srinathEffective diameter is what counts...The effective diameter of the stock emulsion tube with the needle sticking in it may be smaller than the main jet, but the DJ needle in the emulsion tube could be larger than it. Srinath.

I just measured the needle jet diameter, the stock needle diameter, and calculated the cross sectional areas. The GS needle jet has an ID at the top of about 3.18 mm, for a flow area of about 7.9 mm**2. The needle has a cross sectional area at the portion remaining in the needle jet (dia. aprox 1.82 mm) at WOT of about 2.6 mm**2. This gives an orfice area of about 5.3 mm**2 for the needle jet with the needle remaining in it.  This is roughly equivalent to a 2.56 mm dia. orfice (1.28 mm radius).

A 150 Mikuni main has a radius of 0.75mm, and cross sectional area of 1.77 mm**2. This means the needle jet with the needle in it has 3x the area of the main jet.

Flow resistance in a capillary = R = (8*n*L)/(pi*r**4), where n is the fluid viscosity, and L is the capillary length. Flow resistance is proportional to the radius to the fourth power. If we assume the lengths of the capillaries are roughly equal (second order effect anyway), the main jet has roughly 8.5 times the resistance of the needle jet with the needle in it.

The needle will have a very weak effect on the fluid flow at WOT. The main jet is the controlling factor.

At smaller throttle openings, the needle jet/needle combination will become the controlling resistance as the needle descends into the jet (longer L in the capillary resistance equation), and the taper of the needle cause the orfice flow diameter to choke down.

As an additional note, changing from a Mikuni needle to a Dynojet needle (a pretty radical needle change), is about equivalent to half the effect of changing one jet step (ie: 147.5 to 150) at WOT. It of course has a much larger effect at 1/4 to 3/4 throttle.

The Area of the needle jet opening with the Dynojet needle versus the stock Mikuni one is about 16% larger. This makes the needlejet/needle opening 4x as big as a 150 main jet, instead of only 3.5.

There are number of assumptions here: the needle jet introduces air to the gasoline, which will increase the volume of fluid going through the needlejet/needle opening. The viscosity of air is essentially zero compared to that of gasoline. The lowered viscosity of the mixture should compensate for the increased volume.

Dynojet supplies 134 max size jets in their kits I believe. This should be about the same as a 135 Mikuni jet and needle.

One small note...The flow is steady laminar flow not turbulent flow...venturi flow is always steady laminar. Ergo 16% more area will be 16% more flow. The actuality is basically that the equations are so complex...most companies dont bother with the theory. They just polish the needle and ride it and dyno it and O2 sensor it and repeat. There is too many un calculable variables...The venturi is not a perfect circle, the air intake jets in the bell mouth actually also influence the amount of fuel sucked in, the bleed and non bleed pilot jets influence it...Way too complex. They can flow bench it for a bit easier analysis if you are doing a large number of bikes.
Cool.
Srinath.

It is complex, and I have made engineering assumptions. Most carb design work is an art form, but there will be lots of analysis done also.

The flow is not directly proportional to the area as you stated. I have studied fluid dynamics, and worked in the field for some time, and wall effects are huge in small dia. pipes.

Bottom line is the needle has very little influence on the mixture at WOT. The main jet is the controlling factor. It would be very difficult to ever set up a carb if the needle had much influence, and it would be stupid to design a carb that way.

I said lifting the needle would have nearly no effect...put in an entirely different needle and it does make a huge difference. If needle makes no difference why dont we drop in a needle that is the same dia as the hole in the emulsion tube. The jet kit was not mathematically created...it was made by trial and error...the only solution to this is to get independent confirmation from someone using K&N type pods and slip on/pipe to post a report as to how the bike ran with 135's and with 150's leaving the entire rest of the jetting unchanged on an 89-00 bike.
Cool.
Srinath.

Quote from: KevinCIt is complex, and I have made engineering assumptions. Most carb design work is an art form, but there will be lots of analysis done also.

The flow is not directly proportional to the area as you stated. I have studied fluid dynamics, and worked in the field for some time, and wall effects are huge in small dia. pipes.

Bottom line is the needle has very little influence on the mixture at WOT. The main jet is the controlling factor. It would be very difficult to ever set up a carb if the needle had much influence, and it would be stupid to design a carb that way.

Carb design?? we aren't talking about that at all...we are only talking jetting modifications, the design isn't getting altered at all.

Yes the wall will have an effect but the 16% increase in area you posted when the slide is lifted all the way up also is accompanied by less wall interference (thinner inside walls due to thinner needle) That will actually make for a higher % increase in effective flow area. Having said that steady laminar flow is steady laminar flow...reynolds number under 2000. Wall effect or not, small pipe or not (as long as its not open conduit ot open channel).
In the end all this doesn't matter at all. We just need to have someone try it to get independent confirmation. I did so for a whole year and have the O2 sensors to prove it but we need to get another trial from someone else. Also We could find out what the factory jet kit has and that may help us figure it out. Cos if they have just grooved the stock needles and supplied 150 mains as their stage 3 setup...there it is. There is more than 1 way to end up with the perfect jetting. DJ just chose one, factory may have taken the other. Now wasn't someone selling a factory kit...could you post what it has.
Cool.
Srinath.

You said "BTW whoever said the needle doens't matter when the throttle is wide open had no idea what he/she was talking about"

From the Mikuni tuning manual:
"The main jet meters the fuel flow at wide open throttle."
"The Jet Needle controls the fuel mixture in the mid-range (1/4 - 3/4) throttle position."
"needle jets and needles act together as the main system in controlling the amount and mixture of the fuel which is drawn in mid-range (1/4 - 3/4) throttle operation."

Guess Mikuni, me, or all the other tuning books are wrong.