Jetting help

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Originally posted by richards
Let's have a discussion about how to tune main jets...

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I was actually thinking of starting a thread regarding this very subject. Let me first start by saying I’m NOT an expert on this, but I have some dyno evidence and experiences I would like to share.
First some basics:
1) Every bike is different
2) Temperatures, humidity, barometric pressure, and elevation ALL have an effect on jetting
3) Jetting TAKES TRIAL AND ERROR, no one can prescribe the perfect jetting for you, you HAVE to experiment and find what works for your bike.
4) If during the jetting/tuning process you have made a number of adjustments (especially main jet or needle clip) to the carburetor and the engine has not changed. There is a strong possibility that something other than carburetion jetting is causing your engine to perform incorrectly. Jetting is a constant. When adjustments are made to a machine with all components working properly the engine will respond in some way. Depending on the adjustment made the machine will either run better or worse, but there is almost always some form of change. When changes are made without any response it is a sign of other problems. For example float level needs to be correct as it affects jetting and tuning if wrong.

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Originally posted by richards
Used to be you could read spark plugs but that was iffy back in the day and it is worse now as newer gasoline additives can affect plug color.

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Good point. I often want to chime in when I see posts about a fella that rode his quad for an hour and pulls the plug, looking at the color for jetting. This is not an effective way to judge jetting. It COULD be, but there is a specific method of doing this that involves looking at the insulator on the inside of the plug which almost always requires cutting the plug apart to see.
OK so lets talk about the different circuits and what they do, see picture below:

http://i94.photobucket.com/albums/l9...t/calchart.gif

Fuel SCREW (Pilot screw in diagram below): The fuel screw is a small slotted brass adjustment screw located on the engine side (closest to the motor) of the carburetor. This screw is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The fuel screw works with the pilot/slow speed system of the carburetor, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the fuel screw can offer direct feedback on the necessary setting required for the pilot jet. The fuel screw is adjusted in a rather straightforward manor.

The ideal procedure for setting the screw in the correct position is to warm up your ATV engine to the proper operating temperature. Then turn the idle up so it is idling about 500 RPM’s higher than normal. Next turn the fuel screw all the way in until it lightly bottoms out, once bottomed out slowly back the screw out a ¼ turn at a time (give the engine 10-15 seconds between each ¼ turn of the screw, to allow the engine to catch up with the adjustments). Continue backing the fuel screw out until the engine idles at its highest RPM. The preferred setting window is between 1 and 2 turns. If the engine idles at its highest RPM from 0-1 turns out this means the pilot setting is on the RICH side and a smaller pilot jet should be installed. If the engine idles at its highest RPM at over 3 turns out, this means the pilot setting is on the LEAN side and a larger pilot jet should be installed.

If you get no RPM fluctuation when adjusting the fuel screw there is a very realistic chance that there is something clogging the pilot/slow speed system. Clean the system thoroughly with contact cleaner and blow out with compressed air. Carburetor must be disassembled.

PILOT JET (Slow Jet in Diagram below): The pilot jet is a brass jet located inside the float bowl next to the needle jet/main jet location. The pilot jet meters the fuel required for engine starting, idling and the initial throttle opening 0-1/8. A lean pilot jet setting will cause your engine to surge at very low RPM’s, bog or cut-out when the throttle is opened quickly and have trouble idling down. A rich pilot setting will result in hard starting, plug fouling at low RPM’s, sputtering as the throttle is cracked opened. The pilot jet is not difficult to set. With proper fuel screw adjustment fine-tuning should be painless. Once set the pilot jet is not terribly sensitive. If adjusting the pilot jet gives inconsistent feedback, or does unexplainable things. Check and clean out the pilot/slow speed system thoroughly with contact cleaner and blow out with compressed air. Pilot jet sizes are numbered in the following pattern; #42, #45, #48, #50, #52, #55, #58, #60 etc. repeating the pattern.

NEEDLE: The jet needle is the most important component in determining your carburetors jetting. The needle functions have a large effect on the carburetors jetting from ¼ to ¾ throttle.

The safest way to set the clip position is to richen up the clip position setting until the machine loses a little power (feels lazy/unresponsive) then lean it back one position. Ideally you like to run the needle setting in either the 3rd or 4th clip position, if possible. The needle clip jetting is especially critical because on average more time is spent in the midrange than any other part of the throttle. Most machines pull very hard in the midrange, putting quite a load on the engine.

MAIN JET: The main jet affects the jetting in the upper quarter of the throttle position. Coming into play at ¾ throttle on through to full open throttle. Even though most people relate the main jet to their carburetor in general. The main jet is only responsible for the last ¼ of the jetting. The main jet does not affect the jetting for starting and idling. It plays no part on low RPM or mid RPM jetting either. The main jet is very important to your machines overall tuning, but should never be over emphasized at the expense of needle tuning or other facets of your carburetion tuning.

Start with the biggest numbered main jet and run at wide open throttle, the engine should stumble at wide open throttle. Install the next smaller size until the stumble is gone. Once the main jet is correct re-adjust the pilot screw first and the pilot(or slow) jet if needed until proper idle is obtained.
This were a timed wide open throttle run could indicate proper jetting.


http://i94.photobucket.com/albums/l97/zrpilot/Jets.jpg

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Originally posted by richards
I believe one guy posted dyno results showing a couple of HP difference between two adjacent main sizes.

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Actually that was me and you’re a little off. I showed back-to-back dyno runs with a 6 point main jet change and it did not make a significant difference in power output.

So how do jetting changes affect engine performance?

Here is a 6 point main jet change makes very little difference in power (assuming the jetting is close to begin with, rather a more significant change in the AF ratio. I went from a 152 to a 158 main. No a big power difference, but look at the AF. The 152, for me, was a little lean.

But I would venture a guess as to say the 400EX would be JUST fine running this lean (as long as it did NOT detonate!!!!)

http://i94.photobucket.com/albums/l9...ain_change.jpg

Now here is a graph showing what the needle change can do... notice how the whole a/f curve changes across the entire middle RPM range...also notice how I gained nice power in the mid-range yet lost it on the top. A main jet change would take care of this.

Note there was not enough warm up run once the needle was changed, so the picture maybe a little different after some warmup.

http://i94.photobucket.com/albums/l9...dle_change.jpg

When I started the dyno run, I was jetted a follows:

Main 175
Needle: #3
Pilot #52

Ended up at:

Main 158
Needle: #4
Pilot #52

Interestingly, I thought my 400 ran pretty good before on the larger jets. Of course it ran much better after!!!!

Notice the power and AF changes. This is why a dyno can be an invaluable tool for tunning!!!

Below is the begining run and final run.

http://i94.photobucket.com/albums/l9...nd_jetting.jpg

What does warming up have to? Here are a few runs with NO jetting changes, just successive pulls, notice the change. Is it warm-up? I think so...

Note all these runs were done in a 2 minute time span... no changes....

http://i94.photobucket.com/albums/l9...ot/warm-up.jpg




Obviously the best way to get jetting right on is to put it on the dyno, but I think that someone can get the jetting close by listening and watching how the quad performs while riding. Make jetting changes methodically one step at a time. I hope this helps people understand jetting a little more

Comments welcome!

I've been hoping you'd do something like this.
I think it will be a big help to those of us who could use some help in learning proper jetting.
Thanks, and good job.

Sticky!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Something else to add to this.

It seems as though a lot of people try to tune their machine in neutral by revving it. In turn they often mistake a flat spot when stabbing the throttle as not being tuned properly.


This is NOT the proper way to tune. Tune the machine in gear, under a load in real world riding conditions--NOT in your garage or truck.

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Originally posted by Wheelie
Something else to add to this.

It seems as though a lot of people try to tune their machine in neutral by revving it. In turn they often mistake a flat spot when stabbing the throttle as not being tuned properly.


This is NOT the proper way to tune. Tune the machine in gear, under a load in real world riding conditions--NOT in your garage or truck.

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Good point

WOW. ZRPilot is the MAN!!

Nicely done. Thanks for taking the time to do this.

STICKY THIS THREAD!!!

Great stuff, thanks zrpilot.

I'll add a bit of backround on what is going on in the carb, may help people think through what they are adjusting.

The chart zrpilot posted showing Effectiveness vs. Throttle Opening is showing what jetting changes affect what throttle openings. The 3 main things we are tuning on our carbs are the pilot circuit, the needle position, and the main jet.

In the chart "Air Screw, Slow Jet" is the equivalent to our pilot circuit. On most 2 strokes the adjuster is called the "air screw" because it is controlling the amount of air coming into the pilot circuit. On most 4 strokes it is called the "fuel screw" because it is controlling the amount of fuel coming into the pilot circuit. Note there are exceptions, so always double check the carb you are working with. Our Keihin carbs use fuel screws. In the chart the "Slow Jet" is what we normally call the pilot jet.

The fuel screw and the pilot jet work together. The pilot jet controls the max amount of fuel that can flow through the pilot circuit. Larger numbers mean a larger hole in the jet and thus a higher max fuel flow. The fuel screw fine-tunes the fuel flow in the pilot circuit. When it is fully screwed in it closes the circuit and no fuel can flow. When it is fully screwed out it will allow the max fuel flow allowed by the pilot jet. Fully screwed out is usually in the 3-4 turn range, backing out any further than that will not change anything as the circuit is fully open and limited by the size of the pilot jet. This is why you should switch to a larger pilot once you are over 3 turns out when trying to find the sweet spot. You can go up 1 size on the pilot and then start tuning again around 1 turn out on the fuel screw.

The needle and the main jet work together in a similar way to form the high speed circuit in the carb. The main jet, like the pilot, controls the max amount of fuel that will flow through the high speed circuit. The needle controls how much fuel can come through the circuit at a given throttle opening. If you look closely at a needle you will notice is is tapered. Looking at the carb and moving the slide up/down you can see the needle goes in and out of a tube at the bottom of the carb bore. The main jet is attached to the bottom of that tube. So when the slide is open and the needle comes out of the tube the circuit will flow the max fuel allowed by the main jet size. As the throttle is closed the needle starts blocking the hole in the tube thus reducing the fuel flow.

When you "raise the needle" or "drop the clip" you are moving the needle up in relation to that tube. So at any given carb opening a bit more fuel will flow. Thus raising the needle richens the mixture. The chart shows the range that the clip position affects - about 1/8 to 3/4 throttle. So clip/needle position will not affect your idle (that's the pilot circuit) nor will it affect WOT (that's the main jet size).

Hopefully now you can see what is going on and have a feel for what you are doing when making jetting changes.
[list=1][*]select pilot size[*]adjust fuel screw[*]repeat 1 and 2 until you find a good combination for idle and initial opening of the throttle[*]select a main size[*]test at WOT[*]repeat 4 and 5 until you are happy with the mixture at WOT[*]adjust the needle clip position[*]test for clean running through the 1/8 - 3/4 throttle range[*]repeat 7 and 8 until you are happy with the mid range throttle response [/list=1]

One thing not discussed much for the 400ex is the needle taper. Probably because the stock taper in the stock carb and the 450R carb seems to work pretty well. 4 strokes are more forgiving on taper than 2 strokes in general. If you look at a discussion on 2 stroke jetting you will see a lot more talk about which needle taper to use. The needle taper is just the shape of the tapered section of the needle. It can taper slowly from the tip to the full thickness of the needle or more quickly and this affects the amount of fuel that flows through the circuit at a given throttle opening. Some needles do not have a constant taper, the angle changes one or more times along the length of the needle.

It would make a great sticky, especially with all the tuning/jetting questions and misconceptions.

This Definately needs to be a STICKY! I am getting ready to rejet my 400ex and this just answered most if not all of my questions! Thanks Fellows!

PLEASE sticky this before it gets lost. Bump ttt. And thanks to ZR and the other fellas for putting some great information together!