Welcome, Guest. Please Login or Register :: View Members
Pages: 1
Send Topic Print
Some Jetting Basics (Read 401 times)
Serious Thumper

Posts: 956
Denver, CO
Gender: male
Some Jetting Basics
12/04/09 at 08:41:42
I ran across this in another isn't specific for our bikes but it is pretty good as a base line. Feel free to comment or edit as needed.

Carb Tuning
How is carburetion affected by throttle opening, temperature, humidity etc?
Throttle Position:
Idle Set at 1700 RPM + or - 100 RPM
Idle -> 1/4 Air screw Turn in for Richer out for Leaner
1/4 -> 3/4 Needle Jet Clip up =Leaner Needle Clip Down = Richer
3/4 -> Full Main Jet (bigger the number richer the mixture)
Air Temperature  High = (less air so you have lean out the mixture)
Air Temperature  Low = (more air so you have to richen the mixture)

Altitude High = (lean the mixture)
Altitude Low = (Richen the mixture)

Humidity High = (Lean out the mixture)
Humidity Low = (Richen the mixture)

How can I temporarily richen or lean my mix without fiddling the carbs
Colder Glow plug to Lean it
Hotter Glow plug to Richen it

How do I tune the carbs?
1.  Pick the main jet first.  You can take a plug reading at wide open throttle.  Accelerate over a long distance (preferably uphill) getting into 5th gear and keeping throttle fully open.  When fully tapped out, grab the clutch and hit the kill switch.  The motor should stop immediately, then look at the plug.  Should be tan, but not too dark.
2.  The pilot jet affects cranking, idle, and particularly acceleration from idle.  If too lean, the motor will hesitate when accelerating (in neutral) off idle.  If too rich, motor will smoke excessively, foul plugs, and be unresponsive off idle.  Driving along in first gear just barely above idle, there should (actually MUST) be some blubbering, but it should clear up immediately at about 1/16 throttle.  Air screw affects pilot jet somewhat.  It is useful in 1 to 2 1/2 turns out and within the extremes of this range, is about like changing pilot jet 1/2 step (if that were possible).
3.  Slide cutaway has a tremendous effect on throttle response.  Its range of operation is approx 1/16 to 1/4 throttle.  Too lean and the bike will be a bear to control.  Too rich and throttle response will be sluggish.  Slide cutaway and needle DIAMETER are very similar in their actual effect on jetting.  It's usually easier to leave the slide stock and try to get jetting optimum with the needle diameter.  
4.  The needle is perhaps the hardest part of jetting.  There are various aspects to selecting needles.  Hopefully, the manufacturer was pretty close so all you have to do is change the clip position.  This, however, only affects the range around 1/2 to 3/4 throttle. By selecting special needles, it is possible to tune any desired throttle position, but hopefully you won't have to do that.  It's not as easy as picking the right clip position.  If the clip is too lean, the bike will be VERY crisp but after warming up it will usually develop a hesitation when accelerating using mid throttle positions.  If too rich, the bike will be slow and unresponsive and you may even get the blubbers
typical of extremely rich jetting.  
5.  Note we have been talking about throttle POSITIONS.  It is useful to mark your throttle grip so you can SEE what position your hand is in when jetting.  Remember, it's the throttle POSITION, not the engine rpm that you're using to jet the motor.
6.  In general, lean conditions will overheat a motor.  It might take a few seconds (main jet or needle clip too lean) or 30 minutes of woods riding (needle diameter too lean), but an overheated motor will become harder to ride, lose power, and hesitate on acceleration.  If you're lucky and smart, you'll recognize this before you seize it.  A motor jetted too rich (in any one area) will suffer loss of acceleration and responsiveness, may smoke, and may foul plugs.  It is possible to have one jet too rich and one too lean.  The bike could display both lean and rich characteristics depending upon where you hold the throttle.
7.  To develop your skill at jetting, you need to experience too rich and too lean with EVERY jet (pilot, needle diameter, needle clip, and main jet).  Only after you FEEL what too rich and too lean is will you be able to jet your bike perfectly.  Start with the main and go way too rich and feel it blubber at full throttle, high rpm with low load.  As you come down, you'll feel the power and responsiveness build considerably and the bike will begin to rev out cleanly..  About the time that the plug starts saying "too lean" you should begin to notice hesitation on full throttle acceleration and a reluctance to rev very high.  Don't press your luck.  One main jet size or one needle clip position is often the difference between a bike that's unpredictable and miserable to ride and one that always has great power whenever you twist the grip.

What happens when I turn the airscrew out - richer or leaner?
If your airscrew is on airbox side of carb, turning out will lean mixture; opposite if airscrew is on engine side of carb.

What are the symptoms of rich/lean running:

>>>Typical lean conditions:
     Poor acceleration - feels flat
Engine doesn't respond when throttle is snapped open - picks up speed as throttle is closed
Engine runs hot, knocks, pings and overheats (end result- hole in piston)
Engine surges or "hunts" when cruising at part throttle
Popping/ spitting through carb when throttle is opened, or popping and spitting through pipe on acceleration with a closed throttle (classic lean pilot circuit symptoms)
Engine runs better in warm weather, worse in cool
Performance gets worse when the air filter is removed

>>>Typical Rich Conditions
     Acceleration is flat, uneven
Engine will "8 stroke" as it loads up and skips combustion cycles
Throttle needs to be opened continuously to maintain acceleration
Engine works better when cold
Black smoke from the tail pipe
Poor fuel economy
Engine performance improves when air cleaner is removed
If the pilot screw is overly rich, idle is rough and the engine won't return to idle without blipping the throttle
Black sooty plugs, sooty exhaust pipe

How do I diagnose faults "on the fly"
When you are limited to street riding, take note on how easily and quickly the engine reaches the proper RPM for shifting gears. The engine should accelerate smoothly and quickly through all gears. When the main jetting is to rich, the engine will feel sluggish and acceleration will 'feel' slow. You can check the main jet by quickly closing the throttle from wide open to 7/8 position when the engine's RPM is greater than 4500. If the engine accelerates slightly, the main jet is to lean. A larger main jet is needed. If the engine hesitates or misses slightly, the main jet is to rich. A smaller main jet is needed. If the engine just slows a slight amount, the jetting is very close to correct.
Another test is to accelerate through the gears at full throttle. If the engine backfires through the carburetor, misses, cuts out or quits running, the main jet is lean. Increase the size of the main jet. If the engine acceleration seems sluggish, does not react to the throttle or sounds flat, the main jet is rich. Decrease the size of the main jet. The main jetting can be set quickly at a 1/4 mile drag strip. For this test, you can ignore the ET (elapsed time). Making sure the engine is warmed up, make your run down the strip making note of your final MPH. Keep increasing the size of the main jet until MPH begins to drop. Then drop back one jet size. Your jetting should now be correct.

To determine if the bike is lean (to little fuel for the amount of air reaching the engine through the carburetor) at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich (too much fuel in the air-fuel mixture)), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich. If your bike is equipped with a manual fuel shut-off (or you're clever enough to disconnect and plug the fuel lines), take the bike for a ride with the fuel turned off. If it runs worse as the float bowl drains, it's lean. If it gets better, it's rich. If you rev the bike and the revs hang up after closing the throttle this = Lean; if the revs dip down then come back up = rich.

How do I adjust the carb?
The backfiring indicates air/fuel mixture or pilot jet leaness. I'd start, as you correctly stated with the idling mixture as it's the easiest and cheapest. Remember there's another carb under your tank which you'll also have to do. Manufacturers specs are normally one and a half turns out from the slightly seated position - don't force the screw it's got a taper on the screw you're turning as well as in the carb. Start the engine and get it to working temp. Light a cigarette or whatever you smoke as you'll be busy from here on. You'll be letting the engine run for a while so do it outside where there's air circulation or put a fan on the engine and put the beers within reach. Adjust the idling speed to the spec or to where you can hear it's running at a constant speed (the idling speed is adjusted by hand by turning the brass screw with the spring on it sticking out of the back of the rear carb). Now turn the pilot screw out one quarter from the manufacturers spec. If the engine speed drops, return the screw to where it was and turn it in one quarter. If the engine speed drops, you're OK (and I'm OK). Drink the beer.
Don't go for a test ride after the beer. If the engine speed increases as you turn the screw out, keep going until you reach the point of max RPM. If it takes more than 3 turns for the engine speed to level off, the pilot jet is too large. Install a leaner one. This will probably not be your scenario. If the engine speed increases as you turn the screw in, keep going until it levels off. If the pilot screw ends up less than half a turn open, the pilot jet is too small.Trick : When you've found the peak idle speed, turn the mixture screw in one quarter turn. If it doesn't change the idle speed leave it there. If it does drop, return it to the last setting.

Any "shade-tree mechanic tips?
Sure - if you dont want to go the trouble of rejetting, a colder plug will often compensate for a slightly lean mix!
Back to top

'06 lt Blue, Dyna Power pipe, air screw 2 1/4 turns, 52.5 pilot w/ bleed holes, 150 Main, 2/3 spacer, Pirelli MT66 tires, Raptor petcock, 412-4006 Progressive shocks
  IP Logged
Pages: 1
Send Topic Print

« Home

« Home
01/17/22 at 08:20:23

General CategoryTechnical Documents/Reference › Some Jetting Basics » Powered by YaBB 2.2!
YaBB © 2000-2007. All Rights Reserved.