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Jetting Tips For Newbies

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The idle mixture is quite sensitive as there is not much air flow. Since idle jets only come in 2.5 increments, the idle mixture screw allows fine tuning between sizes. That is all it does. Jet kits don't (necessarily) have an idle jet because it doesn't change if you cut the 3x3. Not enough air flow at low rpms. Tweaking the screw is enough.

If you install an extended idle mixture screw, you will need to change the pilot jet only because the aftermarket screw is shaped differently on the end which affects how much fuel flows through the circuit it controls.

Idle jet & screw affect up to 1/4 throttle max. If idles well you are done.

Needle affects about 1/4 to 3/4. If it runs well at mid throttle you are done. On the DynoJet kits the slide lift spring is changed for a special one. This is why there is such a difference between jet sizes between DJ and JD kits.

Main Jet affects about 3/4 to WOT. If it runs well WideOpenThrottle you are done.

THE PURPOSE OF JET KITS IS TO GET THE CORRECT MIXTURE. The purpose is NOT simply to get MORE fuel.

If you are not sure if needle or main are best, first remove airbox door and go for a ride. The airbox door being removed means there is slightly higher pressure at the throat of the carb. This means less pressure difference from the float bowl to the venturi so less fuel gets drawn up. If it runs better at mid throttle try raising your needle clip 1 slot (needle sits lower and less fuel flow). If it runs better at WOT try a size smaller main jet.

If that didn't change or ran worse put the door back on and try blocking off about 1/3rd of the 3x3 hole. This reduces the pressure at the carb throat, increases the pressure differential, and draws more fuel through the existing settings. If it now runs better at midrange, lower the clip 1 slot so that the needle sits higher and allows more fuel flow. If it now runs better at WOT try a size bigger main jet.

Repeat until you are satisfied.

If you have a problem at WOT don't touch the needle, it isn't your problem.

If you have a problem midrange, don't touch the main, it isn't your problem.

If you have a problem in either midrange or WOT, don't play with the idle mixture screw, it isn't your problem.

Q: can I cut it a different size than 3x3?

A: You can cut the hole any size you want, but the jetting recommendations that come with the kit are only valid with 3x3. If you use a different size a competent shop with a dynometer and a exhaust gas analyzer could figure out the appropriate needle and jets to use.

Q: Is my brand XYZ [highlight]QUIET[/highlight] exhaust considered OPEN?

A: No. An open exhaust is just that.

Q: Will a bigger jet give me more power?

A: No. The CORRECT jetting will give you the most power.

Q: Are all needles the same?

A: Not even close. Depending on the carb they will be different lengths, and they all have different starting diameters, taper, transition points. If this sounds confusing leave it to someone who understands and has the proper test equipment to figure it out, put together a JET KIT for your model of bike, and sell it to you :thumbsup:.

Q: I am a new DRZ owner and don't know how to adjust the idle mixture.

A: FAQ section. You need to spend some time and read the FAQ section for the many tips that have been put there. Thanks to the patience and advice of many users especially Eddie I have learned the basics, which I now pass on.

Now from the FAQ section, From the "Common Thread Reference" thread in the FAQ

http://www.thumpertalk.com/forum/sho...d.php?t=327405 tells how to adjust the mixture screw. I might add you want your idle turned fairly low while doing the adjustment.

Q: Why has my bike started taking several seconds before the idle will settle down?

A: Hanging idle caused by lean mixture. It can happen as the pilot jet gets dirty/plugged such as after the bike sits a while. Try reading http://www.thumpertalk.com/forum/showthread.php?t=962558&highlight=idle+mixture

Edited by slowriding
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if you get to your extreme ride altitude and find it too soft(rich),just remove the airbox door to let in more air to lean out the main jet some.then when you get back down reinstall the door to richen it back up.

Great idea

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The mixture screw is a parallel path for fuel to take in addition to the idle jet. Say your bike has a 25 idle jet but really needs a 26.39 pilot jet (which is not made). The mixture screw lets the little extra fuel to tweak the idle mixture. If the idle mix is correct the transition to the needle will be smoothest.

The small amount of fuel getting through the idle circuit will have nearly no effect on higher engine speeds.

The reason there is an idle mixture screw but not for higher revs is the idle mixture needs to be much more precise due to the low air flow.

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Very good post slowriding.

I will add the carb must be clean before you start changing things the pilot jet is notorious for getting clogged easily and in some cases the tiny carb passages (not just the removable jets) can be clogged too The service manual has a nice diagram showing the carb passages

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Many thanks for this info. Would make a great sticky topic.

Agreed, but it would need to be accompanied by a break down of the carb and it's components/jets. That way the new guy attempting this could get the much needed "visual".

Good job SR!

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I see this may have been inspired by my thread... I appreciate it. this should get stickied?

-Zach

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Agreed, but it would need to be accompanied by a break down of the carb and it's components/jets. That way the new guy attempting this could get the much needed "visual".

Good job SR!

Here is a start on that

Seeing as it is jetted, I would assume that he drilled out the plug that hides the mixture screw in stock form. You can access it on the bike if it is a S model. You'll need a small screw driver.

3.jpg

3.jpg

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Yes SR, that OffRoad thread is a good one. He did leave out a few things, so a little tweaking and you'd have a heck of a good combo jetting/carb disassembly & cleaning sticky!

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you wouldnt happen to have a right up like this for a keihn fcr-mx carb would you?

I don't have images at this time. The Jetting forum would be a good place to ask. The differences are the AP (accelerator pump) mods, and air jet. Of course all the jetting and needle selection is specific to each type of carb.

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