Jetting 101 - Everything Explained!

Discussion in 'Carburetor' started by randelator, Jan 6, 2008.

  1. randelator

    randelator New Member

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    With this everyone should be able to do their own jetting, enjoy!!!

    Overview: Jetting 101


    Carburetor fuel circuits and functions:

    1) Pilot jet/slow jet - Affects mixture from idle to 1/3 throttle opening. The pilot jet meters fuel to the "bottom end circuits"

    2) Air screw - Meters air to pilot jet. It is usually located near the back or air box side of the carb. Turning in clockwise, will richen the pilot mixture. Turning out, counter clockwise, will lean out the pilot mixture. Average setting for most 2 strokes is 1 to 2 1/2 turns out. Refer to owner's manual.

    Note: If the adjustment screw is in the front of the carb, it is a fuel screw and not an air screw; it is adjusted the opposite of the air screw. Most 4-strokes are this way.

    3) Jet needle aka Needle - Affects the mixture from 1/4 throttle to full throttle. The needle is in the leanest position when the clip is on the top, and richest on the bottom. The needle calibrates the full to the change in throttle valve (slide) opening. The further down the needle is, the later the main jet comes on and the leaner the mixture at that point.

    4) Main Jet - This circuit affects the mixture from 1/2 throttle to full throttle. This is your full blast top end circuit. This circuit is most accurately tuned by checking the plug for the correct color, sort of mocha brown.

    5) Float/Float valve/Float level - Your owner's manual has the correct level and procedure for calibrating your bike. Here are some symptoms that would indicate the need for float adjustment.

    A) If float level is too high, the float bowl overflows out the drain hoses, and fuel often blocks the air passages that allow the carb to de-pressurize. This causes a bog, or hiccup over large hits.

    B) If float level is too low, the engine starves for fuel off idle, causing a "boooooowang" sound very similar to a pilot jet or air screw set too lean.

    Jetting Guidelines; Eliminating the variables.

    1) Never change more than one circuit at a time. You need this tool to tell you exactly which circuit is affecting performance and at what throttle opening.

    2) Use fresh gas. Octane ratings diminish with time and the higher the octane the slower the burn and richer the mixture. The quality of today’s pump gas is decreasing. To compensate for the fuel, richer jets are required. Also note that fuel with ethanol (aka MTBE) also requires a slightly richer mixture.

    3) Always start with a fresh air filter. Let a freshly oiled air filter dry at least 6 hours in advance so that the alcohol in the air filter can evaporate. Then match the carb to a motor that breathes properly. Otherwise, the engine will run slightly leaner when the clean air filter is installed.

    4) When performing a plug check, use a good used spark plug. New plugs are white and are hard to read accurately for mixture checks. If you have to use a new plug be sure to put at least 15 to 20 hard minutes on it before attempting to get a reading.

    5) Always check for clogged carb vent lines. Clogged lines will cause hard starting and bogging when the motor is hot. They can also cause erratic running that seems to mimic a mis-adjusted float, but worse. Performance will seem rich and acceleration will be sluggish off idle.

    6) If you’re trying to track down a jetting problem, don’t change your premix oil brand or the ratio. The pre-mix ratio will determine how much fuel is available to be burned because the oil isn’t burned, and in terms of liquid volume, the oil takes up a portion of the liquid (oil + fuel). While the ratio will have a minimal impact when making small changes, remember that you’re trying to eliminate variables.

    How to Jet

    1) Mark your throttle housing and grip in 1/4 turn increments. Use a marker, razor blade, etc.

    2) Now get the motor to operating temperature by riding around, away from the pits.

    3) Once the motor is warm, ride in 2nd and 3rd gear from the low RPM to high RPM. This puts a good load on the motor and is an accurate test of performance.

    * try to notice if the problem gets worse or better as the motor heats up.

    4) Now try to locate the throttle opening at which the problem exists. See details just below.

    Rich Jetting Symptoms and Solutions

    A) Motor won’t idle and idle set screw is ineffective. Try leaning out the air screw by turning it counter clockwise a quarter turn at a time. If the air screw has no effect, install a leaner pilot jet and return the air screw to 1 turn out. Proper air screw settings are usually between 1 and 2 1/2 turns out. The correct pilot for your bike is one that allows instant off-idle throttle response, and allows the air screw to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines.

    B) Motor stumbles off idle to 1/2 throttle, then cleans up. If everything in part A above is correct, check the needle. Stock position is usually in the middle, but check your owner’s manual to be sure. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and problem still exists, try a leaner needle.

    C. Motor will rev through the mid range then becomes gurgley (technical term) at full throttle and power is sluggish. This is a text book example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem still persists even though it’s improved a little, reinstall your original main jet and lean out the needle one position. Now fine tune the main jet with plug checks.

    Plug checks are the key to fine tuning the main jet once the other circuits are set.

    White Porcelain Chocolate Mocha Brown Black/Wet

    Lean Perfect! Rich

    Look for symmetrical burn patterns, smell for odors like burnt plastic, which is actually tranny fluid and would be indicative of a blown seal.

    Lean Jetting Symptoms and Solutions

    A) Motor hesitates off idle with a "Boooooooooowang" sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until motor revs without hesitation. If the motor "hangs up" or doesn’t come right back down to idle, install a richer pilot, and reset air screw.

    B) Motor knock knock knocks at idle when hot. Try solution A. If problem persists, perform a "leakdown pressure test" to check for ignition crank seal leakage.

    C) Motor revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When a motor runs excessively hot and lacks power, it is on the verge of seizing. If you are on the trail and don’t have the right main jet, try raising the needle by lowering the clip. It won’t be exact but at least you won’t seize.

    Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition.

    Thanks randelator!!!!






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    Last edited by a moderator: Mar 20, 2014
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  2. blasterman312

    blasterman312 New Member

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    Jabes!!!!.....sticky this please!
  3. cyphertext69

    cyphertext69 New Member

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    Here are some illustrations for those who can't read.
  4. randelator

    randelator New Member

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    great add-on
  5. rix005

    rix005 Guest

    Nicely stated and I have added to your rep. The only correction is that MTBE is not methanol it is methyl tert butyl ether. an oxiginator used to elevate octane and the primary replacement when lead was fazed out. MTBE is a carcinogen and found polluting ground water and has not been used in gasoline for 2 years.
    blasterfreek30 likes this.
  6. cirame1

    cirame1 New Member

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    vote to sticky
  7. bentobeto

    bentobeto New Member

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    i understand but how does it work what does it do:">:">
  8. mr_3_0_5

    mr_3_0_5 Member

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    someone sticky this please..
  9. Outlawed Genetics

    Outlawed Genetics New Member

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    could backfiring be a symptom of being lean? backfire is at about 3/4 - WOT in 3rd and 4th gear. if so would a click or 2 on the needle be enough or would i need to up the main jet? as far as i know its still the stock 230 jet and only mod is a reed spacer, and a cheap aftermarket air filter like a ready filter or some shit like that. i dont have 240 jet but do have 250

    i will also add that even with the backfire its got lots of power so if it is lean i dont think its by much.this bike is freshly rebuilt and i noticed it while performing the last bit of the break in process
  10. redranger

    redranger New Member

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    haha thanks a lot man!
  11. fineazell2

    fineazell2 New Member

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    Can you repost the .zip please?
    It doesn't work when I click on it.
    Thanks
  12. drodr033

    drodr033 New Member

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    hey,
    I have a yamaha blaster year 2000...does not idle....motor was running fine and suddenly refused to start. we emptied out the fuel tank, cleaned the carb, rebuilt the carb with new kit. Bike starts fine but now does not want to idle. we have been messing around with a/f screw but seems non responsive. upon rebuilding carb, we found that the valve throttle 1 was worn down on one side.
    The valve throttle I am referring to is image 8 in the picture.

    [​IMG]

    we have sprayed carb cleaner in intake boots. No signs of air leaks but I have no air or fuel adjustments. Only one screw found to adjust A/f. Any Suggestions?

    Thank you,
    Drodr033
  13. jostith

    jostith New Member

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    Can you keep it running long enough to ride it? Or does it die too fast? What I am asking is does it seem to bog down or is idling the only issue that it has?
  14. NierdelBlast

    NierdelBlast New Member

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    Awesome info,thanks man! turns out my needle was set way too lean and it had a stock main. Quick clip reset and bigger main, cleaned the pilot, accelerates with no hesitation and come back down really well. Plug came out nice and golden brown. Thanks again!I:I
  15. bcolella4

    bcolella4 New Member

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    hey guys i just sent my sclinder and head to ken occonor to get trail port and head mod i wa wondering the jetting i would use. i am bored .40 over fmf strait k/n airfilter advanced timing boysen reeds. i have a 290 in i right now with a stock pilot. i live in jersey. ken recommended a 290and go up to a 35mm pilot. does that sound right?
  16. beaten blaster

    beaten blaster New Member

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    that sounds about right.. i'd go with what ken says..he has many years of motor work under his belt,it's very safe to say he knows what he's talking about I:I
  17. tack

    tack New Member

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    I need help I can get my blaster to run right it has a new .40 over wsm piston new stock jets clean carb reeds are good but they are stock fmf silencer stock head pipe stretched 6 in in the rear tors completely removed no loose wires anywhere will not idle back fires spit sputters and no wotX(X(X(X(X(X(X(X(X(
  18. Blaaster

    Blaaster Member Technician Forum Supporter

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    Start here.





    [​IMG]
    Last edited by a moderator: Feb 6, 2014
  19. Wifesblaster

    Wifesblaster Built not bought

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    Check the reeds. Might want to start your own thread.

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