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.Originally posted by richards
Let's have a discussion about how to tune main jets...
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.
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.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.
OK so lets talk about the different circuits and what they do, see picture below:
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.
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.Originally posted by richards
I believe one guy posted dyno results showing a couple of HP difference between two adjacent main sizes.
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!!!!)
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.
When I started the dyno run, I was jetted a follows:
Ended up at:
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.
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....
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