Need help tuning an FCR carb

Discussion in 'Thumpers' started by R-dubb, May 12, 2006.

  1. R-dubb

    R-dubb Dubbious Adventurer

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    Some of you LC4 guys might know what's going on here.

    For the past six months or so, I've had a 39mm FCR on my KLX650. Works great. Very light throttle action, instant response, decent mileage, plug color is light tan with a touch of grey, low-end response and pick-up is fantastic . All is well, except.... when I open it up (especially in higher gears) the engine sputters and bogs down. I've been playing with raising or lowering the needle and moving the main size up and down, to no real benefit. I'm in a tough spot cause living in middle of SF where it's hard to really whack it. PITA..

    Anyhow,

    Starting to get confused. Just when I think I'm moving in the right direction, it happens again. I think it might be the throttle pump that's throwing me off. Last night I went through everything just to be sure I'm not missing something.

    Current setup:
    Float level = 9mm
    Idle jet = 48
    Idle air screw = 1.5 turn
    Mixture screw = 1 turn
    Air jet = 200
    Needle = OCEMR, 4th pos. (feels lean)
    Main = 165 (also seems a little lean)

    Maybe a long spray on the pump is causing the mix to go rich and then lean. When I put in a bigger main the bike seems to run a little better if I open the throttle real slow, but the bog gets worse if I open quick. The strange thing is that the bike doesn't seem to come out of it very well even if the throttle is held wide open for more than a few seconds. Needle adjustments have similar effects, but I haven't been systematic enough to really know what's happening. Pulling the tank to access the needle is such a pain...

    The question is....does anyone have a clear method to sort this out without yanking my gas tank more than about five times? I've got about four different needles and a collection of mains that go both ways. I prefer not to do jetting reps the side of the road, but alas, that may be the best option.

    Pulling my hair out.

    -dubb
    #1
  2. Arch

    Arch Incurable Gearhead

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    Wish I could offer some firsthand info, R-dubb, but all of my FCRs have worked perfectly for so long that I can't even recall which bike's jetted which way. :lol3 However here's a favorite old FCR tuning link from my Ducati build-up days... http://www.factorypro.com/tech/carbkei.html
    #2
  3. JimC

    JimC Long timer

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    I think you might have the answer here. Most of the FCR carbs come set up with squirt times of 3+ seconds, most FCR dirt bike tuning write ups I have seen want the squirt time down around a second. I would start here and don't thouch anything else until you get the squirt time down around a second. I did this on my KTM 400 and it runs fantastic without the hint of a bog.

    Jim in Sacramento
    #3
  4. Flanny

    Flanny Flanny-it-up!

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    Yeah - when I did the TM40 on my Adventure, the squirt time was really messing me up. Back off on squirt time to the minimum, and see what happens.
    #4
  5. Arch

    Arch Incurable Gearhead

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    Glad you guys mentioned that! R-dubb, that's one thing I did do to the FCR on the 525 (along with JD's kit). Changed (shortened) the squirt time with the Honda diaphragm mod. Got it cheap from Service Honda, but have the part number and link at home, so if it doesn't get posted before I get home tonight, I'll look it up for you and post back here. Also, you can hit KTMtalk.com and search for "squirt time". Gotta run...
    #5
  6. Arch

    Arch Incurable Gearhead

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    OK, done early after all. Here's a cut & paste of an email I sent another member a while back...

    <!--StartFragment -->Another of JD's (and others) recommendations, this diaphragm reduces the pump's squirt time from 3 seconds to about 1.5 seconds. The motors simply don't need three seconds worth of accelerator pump squirt every time you grab a handful and use less fuel when set up the other way.

    Honda Part # 16021-MEB-771
    Or Yamaha part # 5JG-14940-19

    Another method of reducing squirt time is the BK mod..
    http://www.4strokes.com/tech/ktm/crockbkm.asp

    I went with the Keihin/Honda part 'cause it slips right in. All that needs swapping is the rubber diaphragm. Takes a couple of minutes.

    Anyway, hit the jetting forum at http://ktmtalk.com and do a search for "squirt time" or "BK mod". There's a ton of useful info there.

    Hopefully this will help with your problem, R-dubb, but even if it turns out to be something else, the mod is still worth doing.
    #6
  7. R-dubb

    R-dubb Dubbious Adventurer

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    Got it!

    Thanks guys. Looks like a BK mod is in order. I don't have the "MX" version carbie, but I'm pretty sure the squirt mechanism is similar.

    I'll let you know how it works out. Hopefully this is it. Certainly makes sense, given how resistant the problem is to jet and needle changes.

    cheers
    #7
  8. rufus

    rufus We're burning daylight...

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    I did the bk mod on my KTM 525 exc. I closed the squirt down to nothing. I then rode the bike and while idling along i whacked the throttle open and it bogged. I opened the squirt stop screw as little as i could, and whacked the throttle again. I repeated this procedure until the bog disappeared. I don't know how long the squirt is, but it is the minimum needed to prevent bogging.


    p.s. before doing this you need to set the start of the squirt so that it doesn't hit the slide.
    #8
  9. R-dubb

    R-dubb Dubbious Adventurer

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    Well, fuck me..

    My garden variety FCR has a metal tab which can be adjusted to affect squirt time. Did that, shortened the squirt. Made no difference.

    So I took the main from 165 up to 172. More power all the way to 3/4 throttle. Still wouldn't run at full throttle. Took the main up to 185. Still would not run at full throttle, but 3/4 was pulling like mad. After that, sput, sput, nothing. Sucker about throws me over the bars when it cuts out. This is getting completely anoying. I either need some qualified help or I need to spend a day on a deserted road so I can do some actual testing. Not being anywhere close to the mark is a real drag.

    -dubb
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  10. R-dubb

    R-dubb Dubbious Adventurer

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    I may be slow, but persistence has its rewards. Making the main bigger was helping the bottom and pushing the top further out of reach. So...I went to the smallest main I have in the drawer (162) and stuck in a stock KTM needle with a fatter tip at the bottom, lifted it all the way up (7th position from the top) and the bike goes like a banchee all the way to WOT. I still detect a little breaking up and flattening at the very top. This was a much needed breakthrough. All downhill from here. Yupp, I do realize I'm talking to myself.

    See ya!
    #10
  11. supercreep

    supercreep _______________

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    no you're not - congrats on the progress.
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  12. Arch

    Arch Incurable Gearhead

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    Progress! :D
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  13. twotyred

    twotyred Practising Recedivist

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    Found this while having a look around the KTM tech section in www.4strokes.com

    http://www.4strokes.com/tech/ktm/jetmods.asp

    "I did a guy's accelerator pump in a campground and found a LOT of mud on top of the diaphragm. The little rubber boot doesn't seal the plunger worth a damn and I don't know what you can do about it, so until someone comes up with a solution I recommend you pull your pump plate and diaphragm off to clean it once a month."

    Looks like something to check next time the tank is off.

    #13