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Thread: Spark Plug Chops and Jetting

  1. #1
    Join Date
    Aug 2008
    Central NJ

    Spark Plug Chops and Jetting

    Since so many have questions about this, I'd like to take the time to explain it some. Remember, Im not a know it all, so anyone who has conflicting info or better advice feel free to chime in.

    Jetting for many can be a daunting task. Where do you start at, how much should you go up or down in size, whats the best size jet for my application, where should the needle clip be etc?

    Best thing to do is only thing at a time. I cant stress that enough. Dont change 2-3 things then go read the plug, too much has changed for an accurate reading. Change the main, then check, change the needle, then check, etc.

    So the idea is that if you change anything that effects the way the quad will run with the engine, air intake, carb or such your jetting will need to be changed. Thing is that if you dont jet it, the quad will run-maybe for awhile or maybe it will blow-problem is you dont know really even if you read the plug sometimes as everyones judgement is different. So with that being said, any change you make, rejet. It maybe a pain, but it could mean the difference between everything being fine and a rebuild later.

    To chop the plug, what you want to do is get out a dremel or something similar and cut away the metal around the electrode. BEING CAREFUL not to cut the electrode itself. This gives a better view of everything so you can see exactly what the plugs reading.

    So what you really accomplish when jetting is making sure the engine gets the proper amount of fuel to air ratio. If its close or near close it will run very well and give you the power you want. There are only 3 real world ways the bike can run when speaking about jetting:

    1. Good/great: Your jetting is spot on or close, maybe a hair rich-but the quad runs well and you have no issues. The electrode of the plug should have a chocolate brown color or be close to it. Slightly darker is ok, it just means your slightly rich. Personally I like the err on the side of caution since depending on where you ride and elevation changes it can be beneficial as you dont need to rejet.

    2. Rich: Your jetting is causing to much fuel to be dumped into the engine at a time. While the quad will run so so, it will foul plugs (cause carbon deposits) left and right. This will cause the quad to have black smoke come out of the tailpipe (unburnt raw fuel!) if its really bad. the electrode of the plug will be black, sooty, and depending on how rich it is may have unburnt fuel on it and you may smell the fuel on the plug.

    3. Lean: This is the worst jetting condition of all. The problem is most do not realize its to lean b/c the quad is running really good (or so they think) b/c it has alot of power-more air but less fuel in this situation. This will eventually cause overheating issues, so much so if its really bad you can get detonation and may even melt the electrode of the spark plug. The electrode will be almost pure white with maybe a hint of brown. YOU DO NOT WANT THIS, IT WILL CAUSE ENGINE FAILURE AS YOU WILL COOK THE PISTON/RINGS DUE TO HEAT BUILD UP.

    Here is an excellent chart on how to read plugs.

    As well as a chart and explanation on the ATVRiders site:

    Ok that all being explain stated and out of the way lets talk about jetting.

    Ideally the engine should run well, getting it there even if its stock is something that needs to be done. These quads come from Honda running lean-this is bad imho. It also makes them SUPER hard to start.

    You have 4 things you change when jetting:
    1. The main jet. This controls the amount of flow of gas into the carb itself and will have the most dramatic effects when changed. Stock size is a 148, Id run a 150 personally just cause 148 is a bit lean.

    2. The Pilot jet. This controls the initial startup and will help cure cold starting issues possibly for your 400EX. Stock size is 38, if your really having a hard time starting it up you may wish to go up one size to 40.

    3. The needle this effects throttle power in a sense. More you pull the throttle in the more the needle raises depending on where you install the clip. The needle has about 4-5 (depends on if you have a stock needle or whatever) notches on it and a c-clip that you can move. The bottom of the needle is the point while the end with the notches is the top. Higher you put the c-clip more lean you go and lower you put it more rich you go. Stock position is 3rd notch from the top. Newer 400ex's may come with a non adjustable needle. Buy any carb rebuild kit and you can get one or buy it from honda for $$$. Its worth having the ability to adjust it.

    4. The air/fuel screw. This is a pesky little screw you can adjust to help with startup, idle and if you use the engine as a brake (downshift) and it pops when you let off the throttle. I generally dont mess with it to much, mostly b/c where its located on the carb is a real pain to get to without taking off the carb and really in order for it to be adjusted right you need to have the bike warmed up and running-its quite hot right there when warm. Newer 400ex may use a D shaped slot that needs a special tool or you can modify the screw with a dremel, older 400ex's simple have a flathead slot on the screw. Stock is 2.25 turns out from it being fully tightened into the carb. 2.5 turns out is what I would run.

    Now, jetting itself isnt hard. Goal is to make the quad run well with the smallest jet possible. Start with upping the main jet to a higher number depending on what you did to the quad. Always go up, as you tune the engine and carb more slowly decrease the size till your plug looks similar to the ones in the pictures-the electrode should be a golden/chocolate brown.

    Once you have the main set right you can play with positions of the needle to fine tune the carb. Obviously if you feel you have a lack of power when you have the throttle 1/2 open or more its a bit lean, move the needle down a notch and then ride around and check. If it feels better than your fine, still feels lacking go down another notch. keep playing with it till you find a happy medium.

    As mentioned earlier the pilot is really only good for 0-1/4 throttle and starting up. Leave it alone if your bikes mostly stock. Only time I would up it is if you really have starting issues and the air/fuel screw and rejetting the main cant cure them or if you do engine mods-the very least of these would be a slip on exhaust and maybe a better air filter.

    So finally we get into how the heck you actually change these items. I had an 04 honda 450r carb laying around, the EX is SUPER similar in design and how the jets fit and work. However please remember this is still pictures of a 450R carb and not an EX.

    Tools needed for carb disassemble/work:
    -Smaller magnetic phillips screwdriver for needle arm screws-dont want to drop them
    -17mm wrench or socket to remove float bowl plug
    -Larger phillips screwdriver to remove float bowl. Be gentle brass screws strip easy.
    -Needle nose pliers to remove/replace c-clip.
    -6mm socket or wrench to remove main jet
    -8mm socket or wrench to remove main jet adapter.
    -Small piece of wire to clean jets with. Note: main jet, pilot and jet adapter all have small holes in them for gas flow that may need to be cleaned.
    -Carb cleaner to clean off any built up gunk in carb.
    -18mm thin wall socket to remove spark plug. The plug is very hard to get out without the supplied tool from the honda kit. Either buy the correct thin wall socket from motion pro or similar or you need to go buy a socket and grind it to fit. My advice-buy the socket or use the honda tool, much easier.

    Pictures/explanation of carb/parts:

    1. Inside the carb, float bowl removed:

    2. Top of carb to access needle and components:

    3.Inside top, access to needle. Note the arrows point at the 2 screws you need to remove.

    4. Needle notches

    5. Needle alone

    6. Plug chopped and explanation

    7. Float bowl

    Food for thought:
    1. This is member honda400ex2003s jetting placement and explanation thread. Its very helpful in figuring out what jets, needle clips and pilots to run.
    If it's stupid, and it works, it isn't stupid. - Murphy's Law
    2000 426EX,sold
    2004 450R WIP
    -Longtime A.A.D.S. (Accessory Acquisition Disorder Syndrome) sufferer.
    400EX Frequently Asked Questions, CHECK HERE FIRST!

  2. #2
    Join Date
    Apr 2008
    W.P.B. FL
    Thats a good write up^^^

    Besides doing plug chops jetsrus has a very good way to adjust main jets.

    This is from their web site.

    4) Main jet: 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.

    This is where I took it from.

    I use this method and it works really well without doing multiple plug chops.
    400ex hybrid Finished!

  3. #3
    Join Date
    Aug 2008
    Central NJ
    ^ Good info!

    I also forgot to mention you can get any jet you want from jetsrus!
    If it's stupid, and it works, it isn't stupid. - Murphy's Law
    2000 426EX,sold
    2004 450R WIP
    -Longtime A.A.D.S. (Accessory Acquisition Disorder Syndrome) sufferer.
    400EX Frequently Asked Questions, CHECK HERE FIRST!

  4. #4
    Join Date
    Jun 2009
    Northeast PA
    Chopping my plug tonight...

    I had to do it...hahaha
    Seriously though, this is a very informative write up. Good job CJM.
    2008 400ex
    CFM airbox w/ K&N and custom outerwears lid, 162 main / 42 pilot, choke removed
    custom aluminum air scoops, Design Concepts vented hood, HMF Sport full system
    Sparks +6 Key, YFZ front shocks, CRJ rear link, DG nerfs, DG swing arm skid plate
    rattle-can black rims, more to come...

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