Topic: Jet sizes and what they mean?

[78_SaltLick]

Slow jets, main jets....my slowjet has a #38 stamped on it, just out of curiosity sake can you put a #35 jet on there? What does the numbers mean? I saw a post where someone had a #35 jet in and was going to put in a #38 in its place, is this just to allow more fuel to go through? I am lost what the numbers mean, i figured if you had a #38 jet, it had to be replaced by the same......

[TwoTired]

For Keihin Carbs the jet sizes refer to the metering orifice diameter, expressed in millimeters.  A # 100 jet is 1.00mm in diiameter. A # 38 Jet is 0.38 mm in diameter.  A #35 jet is 0.35mm in diameter.

The smaller the jet size, the less fuel will flow through it at a given pressure difference across the orifice boundary.

Cheers,

[DammitDan]

The size of the jet is how much fuel it allows to pass.  A smaller jet has smaller holes, so it lets less fuel through.  The vacuum pull from the engine is the same (a constant), so to make up for it your mixture is going to have more air in it.  The same goes the other way... With a larger size jet, there are larger holes, more fuel is allowed to pass, and in turn the vacuum pull from the engine pulls more fuel into the mixture, richening it.

I believe the sizes are determined by the metric diameter of the holes?  Or something like that.

Now what I'm trying to figure out is if there's some sort of scale for your mixture.  For example, if you put a jet one size jet larger vs 3 jet sizes larger, will the 3 jet sizes larger allow 3x the amount of fuel or will it be logarithmic?  (Trying to figure out just how high I need to go on my mains  )

[techy5025]

I doubt if it is linear...or a function of the area.  Maybe we need someone who
majored in fluid mechanics to tell us.  The friction of the aperature will have a
major affect at these flow rates.

You could always devise a test to measure it.

Jim

[DammitDan]

A test would involve lots of jets, which requires lots of $$$...  I'm just a poor college student!

Any of you old retired rich guys wanna do it?   

[seaweb11]

You may see posts here from people who have replaced a stock air box with pod type air filters and now their bike runs like crap!  This set up allows more air to get into carbs and you now need more fuel (bigger main jets) to balance it out.

Not sure if that answers your question any better than the answers already given.

[g8ts]

Two really good reads below. If you make it through both articles you will have a better understanding on where you might wanna go with your carbs.



http://www.aircooled.net/gnrlsite/resource/articles/jetting.htm

http://www.motocross.com/motoprof/moto/mcycle/carb101/carb101.html

[78_SaltLick]

You may see posts here from people who have replaced a stock air box with pod type air filters and now their bike runs like crap!  This set up allows more air to get into carbs and you now need more fuel (bigger main jets) to balance it out.

Not sure if that answers your question any better than the answers already given.

hmm, just trying to picture how to balance things, say if you change your exaust to a 4 into 1 then you change the clip setting on the needle, then adjust the air screws some, but if you change to pods then you adjust your main jet size....why not change the jet size if you go to 4 into 1 exaust instead of changing the clip height on the needle? Its all related to richening and leaning out the mixture, just in different ways i guess. i guess i just dont see the big picture yet, something that will come with time i suppose. gee now if i can only get that broken slow jet out of my carb.....

[DammitDan]

Think of it this way... Your engine is providing a constant pull on the carbs depending on RPM.  If you're at idle, the only thing that the vacuum effects is your pilot jet, because it provides the least resistance to the pull.  As your RPM increases, the pull increases.  This causes more air to be sucked into the mixture, and the carbs compensate by moving up to the next step in jetting, the slow jet (which provides slightly more resistance than the pilot jet).  Now keep in mind, the pilot jet still continues to allow fuel to pass, but the slow jet is making a "correction" to the fuel/air mixture. 

As you continue up in RPM, the same thing happens again.  At a certain point, the slides begin to move and the main jet needle (the next step up) becomes involved.  It's tapered, so the farther up the main jet holder it goes, the more fuel it allows to pass.  This is why the clip is important...  A lower clip setting and you allow more fuel to pass from the get-go since it stations the needle farther up, while a higher setting allows less to pass since you're stationing the needle closer to the orifice (it's like sticking your finger into a hole in the bottom of a boat... take your finger farther out and it allows more water to get in).  And when you're at 3/4 to wide open throttle, the main jet itself becomes the major player, and its size determines the mixture at that throttle setting.  But remember, at WOT it's not just the main jet that's providing fuel... the pilot and slow jet are also still in the mix, they've just been "overtaken" by the correction to the mixture.

So, to get back to the original question, changing the clip on the needles will only affect the mixture at a certain RPM/throttle setting.  Doing this for an exhaust change MAY be sufficient, because a new exhaust won't necessarily affect a particual area of the mixture.  However, if you make the change to pods it will affect the airflow across the entire board, and you will have to change pilots, slows, main jet needle clip position AND main jets.

Understand now?

[Bodi]

Generally the first step in rejetting is to do a plug chop at wide open throttle. This tells you if your main jet is OK. Experience has shown that going from 4-4 to a 4-1 pipe doesn't change the WOT reading much at all; the main jet is fine. Then you check 1/2 to 3/4 throttle; again experience has shown that the pipe change needs a bit richer mixture and you raise the needles a notch or two, or shim it if yours have no extra notches. Finally you try to get correct low throttle mixture plus a good idle and smooth response to throttle changes at lower RPM via the pilot jets and airscrews.
Pods make a huge change in the intake flow characteristic and consequently more changes are needed for carb adjustment. To get good performance you probably need bigger main jets, smaller pilot jets, and a reshaped needle taper. Sometimes the slide cutout even needs to be altered.