[OT] Carb Theory - Main Jet, Needle Jet, Jet Needle

[The Buddha]

That applies only when you use the mikuni needle and yes I agree with that...What they are saying is the needle cannot be lifted at 3/4 + throttle to alter your mix. Use the DJ needle and a whole different set of rules applies. They may also say the same damn thing. Now here is the twist...We have CV carbs. When you pull the stock air box off and put in K&N's, and take off the stock pipe and put a aftermarket pipe on why do we need to touch the carbs at all...they are after all CV carbs and since their velocity is constant they should keep working fine. Yea what they dont tell you is...its CV per setup. Change the setup and the velocity changes at all throttle settings. Since jetting depends on throttle position you need to alter that. Same way change the needle and the metering changes. If I cut a tapered needle all the way to the tip, you will be able to lift it and get richer all the way to WFO however there are problems associated with that...some of which you have outlined in an earlier post. DJ, mikuni and factory all can say the same thing...they may very well say it in the same words too...You cannot lift the needle and richen the mix significantly at above 3/4 throttle. However they do not all mean the same thing. They are only talking about their needle. Put in the other needle and its output is different. If you are running at 12:1 with the DJ needle and 134 jets, put in mikuni's and you are instantly leaner say 14:1. But lift the Mikuni jets and you still at 14:1 . I have dyno runs and everything to show the 150's are just about what should be there. 135 is so small you can feel it when you ride it. 145+ it becomes more difficult to tell especially in a dry summer weather. On a cold winter morning 150's will be the best. Tried 152.5 and the performance fell off a cliff. If you are running 135's with the stock needle I have to ask you where is your floats set to. You could set that high...5-6 mm high and easily have the bike run right in most throttle/rpm ranges. That's all I can think of. My virago was lean as hell...and it was leaner in the low and mid range than up high. I lifted its floats 4 mm and it was an instant 2-3 jet changes everyehere. However on the GS that makes for a risky option cos the float cahmber is facing up and has the air jets and bell mouth too close above it. The virago's is more of a side fill chamber...no chance of overflow.
Cool.
Srinath.

[KevinC]

The needle jet area is 3.5 to 5 times the main jet area. The main jet is the determining restriction at WOT.

Srinath said "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"

If the areas where any where near equal, I would agree with you. With the area of the main jet 3 to 5 times smaller, and the length of the capillary also much longer than the needle jet/needle, the main is the determining restriction at WOT. The needle - Dynojet or Mikuni - has very little effect.

My float levels are fine. The bike makes more power if you run it near the right air/fuel mixture, not extremely rich. James was running similar sizes to me in his race bike also.

[The Buddha]

The needle jet area is 3.5 to 5 times the main jet area. The main jet is the determining restriction at WOT.

Srinath said "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"

If the areas where any where near equal, I would agree with you. With the area of the main jet 3 to 5 times smaller, and the length of the capillary also much longer than the needle jet/needle, the main is the determining restriction at WOT. The needle - Dynojet or Mikuni - has very little effect.

My float levels are fine. The bike makes more power if you run it near the right air/fuel mixture, not extremely rich. James was running similar sizes to me in his race bike also.

The bike makes more power till you hit the 150 mark...go past it and it drops off drastically. Many many people here have used the 150's (or close to) I am talking about including Chimivee just now. Practically I know what works...We just have to resolve the theory. Now you said the mains are 3-5 times smaller (is that in cross section area - if so probably) and the length of the capillary being longer than the needle...Well the needle isn't much shorter than the capillary (aka emulsion tube) The needle nearly drops to the main jet. However the flow isn't capillary action...That will be greater if the diameter is smaller...Emulsion tube Doesn't work like a capillary. It works like a pipe basically...flowing from high pressure to low pressure - venturi action - and the needle is a restriction in that pipe. The DJ needle is probably 1/16 the cross section area (1/4 the diameter) of the stock and hence is a much smaller impediment to flow. So same pressure difference will make for more flow. Hence the need to provide more resistance to that flow by using a smaller main jet. All I am saying is that they all matter to overall flow...DJ has 1 way and mikuni has another. So 134 works with DJ's needle and 150 mikuni's work with stock needle. I asked you about float level only cos it was on my mind from having seen 3 sets of carbs from 3 different sources...with levels set wrong. Are they right to the top of the float bowl and no more...If they are set 4 mm higher...your mains will work like they were 4 sizes bigger. Also you sure you have stock needles and are canadian needles different from US needles in profile. Now aren't you running Keihein FCR's.
Cool.
Srinath.

[The Buddha]

And James has a DJ kit in his.
Cool.
Srinath.

[KevinC]

I never said anything about capillary action! Small ID pipes behave differently than standard pipe flows, because the boundary layer occupies the entire pipe after a very short distance. This is called capillary flow.

Even using standard pipe flow calcs, which are wrong for IDs this small, the 3.5 to 5 times area of the neddle jet/needle means the main is the controlling factor at WOT.

The needles in a Canadian bike are standard Mikuni needles, but with a positionable clip so you don't need to shim the needles.

I am running 36 mm Mikunis, but I keep good notes of the jetting changes I make. I have 2 sets of 34's also. I'm finally up to 150s with the 36 mm carbs, high lift cams, 80 mm bore, 12:1 compression ratio, and 60 hp.

I thought your GS burned a ton of oil? That sure cjanges the jetting equation.

[The Buddha]

My jetting formula is only for stock carbs on a stock motor. 36's, 34's all are entirely different...even any other 33 say Keihein or other design mikuni's are different. I found the jetting by experiment...not by theory... S I cannot calculate what jets should be in a 34 mm when I am running 150's in a 33... No Idea.
Small ID pipe flow... We have covered this... with the stock needle the boundary effect is more than with DJ's needle...The DJ needle is much thinner especially in the higher lift range and not only does it give more effective cross section area for flow than the stock needle it also reduces the effective wall area.
My 89 uses oil now ... But I did this on that bike when it had 15K miles and was using a lot less oil. I also did the same thing on my 90 which at that time has 2K-4K miles and never used any oil. I have also done the same thing to many other peoples bikes, including Gino's bike right now (which for the life of me I cant figure out why it shoots the carb vaccum caps - but I digress). Further there are a lot of people here that have tried a bunch of other jets and finally ended up with 150's. Yet  others have put in 150's and left it alone. All of that makes no difference. Theory is theory and it has to be proven on paper as well and I believe 135's will work with the super thin needles DJ provides...and not with the stock. Also mains are the controlling factor in 3/4 to full throttle only because the needle is not tapered in the last 1/2 inch... To make changes to mix... you need taper ... else the effect is too little and screws up different things at other throttle ranges. Mains are the right way to meter at 3/4 to WOT. Cut a taper in the needle all the way to the tip and it will let you increase the flow to the WOT.
That's it.
Cool.
Srinath.

[KevinC]

I am of course talking about stock carbs back when my GS was stock.

Mikuni sells a huge range of needle shapes, that they state are for tuning the 1/4 to 3/4 throttle range. I'm not sure if any of them match the Dynojet needle - it does look pretty strange - but they do have a lot of diameters and different tapers. Their charts do not show changing the main even with a large range of needles.

[The Buddha]

There is also one experiment you can do...if you have a stock GS left to check or ride around...but put 135 mains and stock needles in it and ride it and see, and put a thinner needle and see if it makes a difference. I'll bet its night and day. If you look at a stock needle and a DJ needle you'll immediately know the 2 are vastly different...I was going to do the same or similar experiment... More like put an O2 sensor back in my pipe and put the DJ kit in it and try riding it and looking at the readings I get.
Cool.
Srinath.

[The Buddha]

Hey if mains are all that matter how come the stage 1 (stock filter and slip on) DJ setting recomends 118 DJ mains which we pretty much know is 1.18 mm dia when stock US mikuni is 122.5 which we believe is larger than 1.225mm.
So when you were saying mains are all that count, you are indirectly saying the stock US bike is too rich at WOT so go smaller...
BTW my virago 535 runs perfect with 137.5 mains and floats set 4 mm high. I believe that makes a bit more flow ~1-2 sizes at WOT. So I have 142.5's... DJ supplies 126's. The bike was lean as hell with 135's.
Cool.
Srinath.

[The Buddha]

Well dont jump on my case about the virago...I believe they are also suggesting different air jets... Thus compensating for samller mains... no so with GS though.
Cool.
Srinath.