Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: Braap257 on August 16, 2010, 09:10:36 AM
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Okay. So I know my bike is running too rich. And I Know it may need to be jetted. But I don't really know where to start? How do I go about leaning the carby out???
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Search the site there was a long descussion on this subject. It ended up with spanky"s guide to jetting your bike. Coop must have the link some place.
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Search the site there was a long descussion on this subject. It ended up with spanky"s guide to jetting your bike. Coop must have the link some place.
Ask and you shall receive. I did not write this article, Chokey did:
"Spanky's Jetting 101:
A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband.
A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. Hard starting when hot or cold, poor response when opening the throttle, reluctance to idle, all of these are symptoms of an improperly sized pilot jet or incorrectly adjusted air screw.
The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using.
A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, leading to deposits and wet fouling of the plugs.
Do you have spooge? There are the rare instances where a mechanical issue, such as a leaking wet-side crank seal, can cause spooge. But, by and large, this isnâ??t the case. In most instances, spooge is caused by rich jetting. It has nothing to do with how much oil you mix in the gas, or how hard you ride. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, resulting in deposits, plug fouling, and spooge. Spooge is nothing more than unburned fuel and oil entering the exhaust.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Someone with a good understanding of jetting can get you in the ball park, but you need to do the testing to determine the correct jetting yourself if you want it right.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit, because the pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to re-jet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel.
One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.
All jet testing must be done with the engine at full operating temperature.
As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. If your engine doesnâ??t respond to air screw changes, then you either have a dirty carb, or the pilot jet is way too rich. When the pilot jet is way too rich, you are forced to use the idle screw to open the slide so far in order to keep the engine running that the pilot circuit is partially bypassed, and the engine is actually starting to draw fuel through the needle jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong. A too-rich needle can often be felt simply when revving the bike on the stand. The bike will sound rough and raspy when blipping the throttle on the stand. A correctly jetted bike should rev cleanly and crisply.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Please note that, when reading plugs, the tip of the insulator, threads, etc. are meaningless for jetting purposes. They can tell you a lot of things, but jetting isnâ??t one of them. Only the mixture ring at the very base of the insulator, inside the threads, can tell you anything about the jetting.
These links should help you to understand reading plugs:
(http://cdn.thumpertalk.com/3/35/35507100_sparkplug.gif)
(http://cdn.thumpertalk.com/d/db/db7f02ab_sparkplug%20b-w%20.gif)
The slide is also a tuning variable for jetting, affecting the throttle range from 1/8 throttle to approximately 1/3 throttle. If you canâ??t clean up the small-throttle jetting on your bike no matter how lean you go with the pilot or the needle, the slide is the next step. But few bikes need leaner slides.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit.
Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer. Their purpose is the same, they just do it in different ways."
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Some would disagree with one aspect of that write up-but it's close enough :)
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Some would disagree with one aspect of that write up-but it's close enough :)
Everyone seems to have their own twist on this matter...
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Braap,
Jetting is really an individual thing, by that I mean it depends on your climate, track conditions, engine mods, pipe/silencer set up.
I had a 04 yz250 for the 04/05 season and the only jetting changes where to go up 1 size on the main and drop the needle a clip or 2.
If your bike is standard, start by raising the needle clip (dropping the needle) a clip at a a time and perhaps one down on the main.
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Well the conditions around here tend to be pretty warm.. and its usually pretty humid "It's Florida" I'm gunna attempt to jet the bike soon. But I may try adjusting the air screw 1st. If i can find it lol.
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This all sounds like a major headache, I think I will get my bike professionally jetted.
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It's actually easy to do and once you've done it once you'll have it down.Besides,if you get someone else to do it you'll still have the drag it back to them whenever you ride in a different temp or elevation-and let them guess at what condition you might be riding in.
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It's actually easy to do and once you've done it once you'll have it down.Besides,if you get someone else to do it you'll still have the drag it back to them whenever you ride in a different temp or elevation-and let them guess at what condition you might be riding in.
True, but no mechanic who wants to make money is going to show me how to jet my carburetor, my dad is is principal, he has no idea how to work with a carburetor, and the guys I ride with all run the stock jetting(as do I, because I have no idea of how to do it), so essentially, I can either wait until my grade 12 year to take mechanics(2 years from now) or send my carburetor to be jetted by a professional. I want to do it myself, but there are so many variable with engines, even two strokes, so, it is always good to have someone who knows what they are doing to help you through the process your first time.
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You are right, it does help to have someone else there the first time to help out. But that's not always gonna be an option.
It may take some diligence reading a service manual, and it may be boring. One day the light will come on and you'll say what the heck was so hard about that!
But just like anything else in life you are only gonna get out of it what you put into it.!!
Start Here: http://members.cox.net/bostonbrothersracing/General%20Boston%20Brothers%20Stuff/tuning/2stroke%20tuning.pdf (http://members.cox.net/bostonbrothersracing/General%20Boston%20Brothers%20Stuff/tuning/2stroke%20tuning.pdf)
http://www.amazon.com/Motocross-Off-Road-Performance-Handbook-Motorbooks/dp/0760319758/ref=ntt_at_ep_dpi_1 (http://www.amazon.com/Motocross-Off-Road-Performance-Handbook-Motorbooks/dp/0760319758/ref=ntt_at_ep_dpi_1)
http://www.amazon.com/Motocross-Off-Road-Motorcycle-Motorbooks-Workshop/dp/0760335966/ref=pd_sim_b_6 (http://www.amazon.com/Motocross-Off-Road-Motorcycle-Motorbooks-Workshop/dp/0760335966/ref=pd_sim_b_6)
http://www.angelfire.com/zine2/boostajetski/carburetor_tuning.html (http://www.angelfire.com/zine2/boostajetski/carburetor_tuning.html)
Tuck\o/
Jetting search TSM : http://twostrokemotocross.com/forum/index.php?action=search2 (http://twostrokemotocross.com/forum/index.php?action=search2)
You Tube Jetting search: http://www.youtube.com/results?search_query=jetting+a+two+stroke&aq=f (http://www.youtube.com/results?search_query=jetting+a+two+stroke&aq=f)
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where is it rich? top. Mid, or bottom? if it's on the top drop a main jet size
in the middle, lower the clip on the needle, bottom drop a pilot size.
Duh,
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you might also have a leaky crank seal if you're oil fouling, look into that as well.