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Thread: Jetting help

  1. #21
    Join Date
    Apr 2009
    Location
    Los Angeles, CALIF
    Posts
    134
    To review: stock 400EX HMF slip-on exhaust and K&N air-filter and I just re-jetted from the stock 148 to a 152

    Welp....just fired it up and took it for a test spin. Fires-up just fine, idles really nice. Low-end & mid have a really smooth & nice pull...BIG-TIME difference from stock. Everything feels great with a noticable improvement...up until I go WOT and try to wind it out.

    I put it in 2nd and pulled the throttle wide-open...it went from idle all the way to about 85% wound-out, but then it starts to hesitate & "stumble" and it won't wind-out all the way. Again, the low & mid are great, but once I get towards the top-end it just doesn't want to wind out. I'm guessing lack of fuel and I need to jet-up to a higher number????

    Hmmmm....

    Originally posted by zrpilot
    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.
    Reading this, it sounds like I've got to large a jet? cause I still have some "stumble" at WOT. Ya know, the air-filter instructions say I should remove the air-box lid. I did not do this yet, maybe that would make the difference? I'm still pulling air through that elongated snorkel...removing the lid should provide more air, so if I'm over-jetted and running rich, the additional air (or ease of air, eh?) might compensate for this. Am I on the right trail??

    Dude....THANKS!

  2. #22
    Join Date
    Sep 2008
    Location
    Maryville, TN
    Posts
    85
    Originally posted by BikeSwimLaugh
    To review: stock 400EX HMF slip-on exhaust and K&N air-filter and I just re-jetted from the stock 148 to a 152
    ...
    it went from idle all the way to about 85% wound-out, but then it starts to hesitate and it won't wind-out all the way. Again, the low & mid are great, but once I get into the top-end it just doesn't want to pull all the way. I'm guessing lack of fuel and I need to jet-up to a higher number?

    Some help please....
    Yes I think you are on the right track considering a different main. I would try one size larger (154) to see if it helps. If not then go one size smaller (150) and see if that helps. Whichever direction makes it better, try going one or two more sizes in that direction 'till you've found the one that runs best at WOT.

    You should be able to get Keihin mains for a few bucks a piece at any well-stocked motorcycle shop. Notice I said _shop_ not dealer. You want to find a place that caters to racers and modified bikes, if they do that kind of work they will have a large stock of jets.

  3. #23
    Join Date
    Apr 2009
    Location
    Los Angeles, CALIF
    Posts
    134
    Thanks for the lickety-split reply. I've edited my post after re-reading from page 1, please check it out.

    The instructions say to go with a big number jet until the engine stumbles, then size down. Since I'm already stumbling, I'd figure I need to go to 150....but the guy at the parts place was so certain the 152 was tuned for my pipe/air-filter.

    Well, as I wrote, I'm still running my air-box lid and this restrictive issue is likely the culprit. No doubt I should remove the air-box lid and check it all out again. Pulling air through a 2-foot elongated 1.75-inch tube is a whole lot different then having the entire top of the air-box wide-open! Best I try that before even thinking of tinkering with the jet again.

    But yeah, for the price I'd pick-up a 150 and 154 (one higher, one lower) and see what that does.

    I've got to say, changing the air-filter and exhaust really does make a big difference. Love the sound too!

    It's all fun so long as 1) Nobody gets hurt, 2) no crashes, 3) No flat tires and 4) Everything reassembles with no extra parts and things go together right.

  4. #24
    Join Date
    Feb 2008
    Location
    Lake Tapps, WA
    Posts
    256
    I would start with the 155. I've got a similar setup to you, K&N, Air lid on, Slip-on. The only other thing that I have is a CDI Box that advances the timing. I ride at about 600ft -1000ft. and I'm at a 155 main, 2 1/4 turns out on the mix screw.
    Ragin' Redneck

    '07 400EX
    iShock 3500 +2 A-Arms (Just added!!!)
    DG X2 Slip-on,
    Moose CDI,
    K&N Air Filter (With Cover)
    Pro Armor Nerfs with Heel Guards
    DG Racing Baja Dull Skid Plate

    "When you are tough on yourself, life is going to be infinitely easier on you." -Unknown-

    "The best argument against democracy is a five-minute conversation with the average voter." -Winston Churchill-

  5. #25
    Join Date
    Apr 2009
    Location
    Los Angeles, CALIF
    Posts
    134
    Thanks for the tip!

    What are your thoughts on the air-box? I'm not sure I want to run with absolutely no lid at all...so I was considering getting an aftermarket lid with a built-in filter or just mod'ing the stock one myself.

    I'm riding anywhere from 1,500 to 5,200-feet....probably averaging about 2,000....any thoughts on how to account for that on the main jet?

    The CDI ignition box...what's that mod all about? I've only heard vague references to it...what's the pro's, con's, etc. Thanks!

  6. #26
    Join Date
    Feb 2008
    Location
    Lake Tapps, WA
    Posts
    256
    As far as the Air Box, it pretty much all depends on what conditions you ride in.

    As far as the altitude, someone else on the board is probably more knowledgeable on that subject. I believe that the higher the altitude the lower the main due to the reduction in air mass, but I'm not sure at what level that starts.

    The CDI box is basically the brain. On my '07 its located under the hood behind the headlight. You unplug the stock one and plug in the new one. The moose CDI box I have advanced the timing, increased the spark intensity, increased the rev limiter and removed the rev limiter in reverse.

    It was right around $120 for the CDI box w/ a complete Jet kit. So I think it was well worth it.
    Ragin' Redneck

    '07 400EX
    iShock 3500 +2 A-Arms (Just added!!!)
    DG X2 Slip-on,
    Moose CDI,
    K&N Air Filter (With Cover)
    Pro Armor Nerfs with Heel Guards
    DG Racing Baja Dull Skid Plate

    "When you are tough on yourself, life is going to be infinitely easier on you." -Unknown-

    "The best argument against democracy is a five-minute conversation with the average voter." -Winston Churchill-

  7. #27
    Join Date
    Mar 2009
    Location
    WASH DC
    Posts
    88
    Originally posted by Wheelie
    ... KrackerG--
    Start at a 42 pilot, 4th notch from the top
    on needle and 172-175 main.
    thanks Weelie, i will start out with the 175 main and 42 pilot. most of the others i've spoke with said to start around the same place.
    2002 yellow Honda 400ex
    motor specs:
    -440 stroker kit -Trinity StageIV full exhaust -CDI box -170/42 jets -K&N air filter/airbox mod -Outerwears lid

    frame/suspension:
    -FullFlight +2 black a arms -450r shocks -DG front bumper -AC Racing nerfbars -AC Racing skidplate -Armadillo swingarm skidplate -Maier black plastics -Renthal bars -PowerMadd brushgards -gripper seat

    wheels/tires:
    -ITP Pro T-9 black wheels -IPT Holeshot GNCC front/rear tires

  8. #28
    Join Date
    Feb 2009
    Location
    Dirty D. NC
    Posts
    66
    400ex with the followin Mods,

    Curtis Sparks X6 exhaust
    Curtis Sparks advance key.

    Im at 390ft elevation.

    My jetting is a 42 pilot and a 165 main, stock needle setting and 2 turns out on the fuel screw. All keihin jets BTW.

    I dont know anyone else with similar mods. I would just like to know where they are at and what jets they are using.

  9. #29
    Join Date
    Mar 2009
    Location
    Nowhere, MI
    Posts
    547
    There is some great information here. I do have a question, though. Why is it that the needle is considered the most important part of the equation, yet it gets paid so little attention?

    I see a lot of jetting recommendations involving a new main and piolot, but stock needle position. These recommendations come from both forum members and pipe manufacturers(Curtis Sparks, for example)...

    Is it because of the fact that when you adjust both the main and pilot that it usually has a big enough effect on the needle? Or is it just that most people don't understand enough about the needle to mess with it?
    Sold my quad, but I'll be back... Looking for something different.

  10. #30
    Join Date
    Apr 2009
    Location
    Los Angeles, CALIF
    Posts
    134
    Beave....

    That's a GREAT question. I have to admit, when they start talking about 2nd or 3rd position on "the clip", I have NO IDEA to which part they are referring. I'm sure it's simple, I've just not seen any pics or diagrams showing this particular area/part.

    I really enjoy answering questions and helping people....but I have to digress...I just don't know about this area, let's hope someone else does!

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