KTM 640 A Shock Spring Replacement

Discussion in 'Thumpers' started by IronDawg, Nov 15, 2005.

  1. IronDawg

    IronDawg Been here awhile

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    Hey guys,

    I've been steadily working on Mack's 640 getting back into running condition since it's been sitting for a year or more. I've found the problem with the dying motor. The floats were way off so that gas was flooding the engine everytime you hit the front brake or compressed the forks. If you held the carb level, the floats were no where near level with the lip of the carb. I did notice that the float tab contact point is very close to where the tab mounts to the plastic float. Is this correct? Here's a picture. The arrow on the right is where the needle contacts the tab. The arrow on the left is the end of the tab. Seems like it should be somewhere in the middle so you have more adjustability.

    [​IMG]

    Mack had bought a 8.9kg (had it measured) titanium rear shock spring, however, it was for the 950 or pds shock body. It's too wide and short to fit on the 640. Can someone recommend another spring? I checked Race Tech and they don't have or recommend an 8.9kg spring.
    #1
  2. creeper

    creeper Still alive...

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  3. IronDawg

    IronDawg Been here awhile

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    Thanks for helping out the nOOb creeper!! :thumb Looking at the float thread, I'm good to go. Wasn't sure at first if I was supposed to hold the 'cage' down but I did and got things within spec.

    I'll call Lowe's and check on a spring.
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  4. creeper

    creeper Still alive...

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    Let us know if your carb issues are fixed, and how the conversation with Lowe's turns out.

    I need to look into some stiffer springs myself.

    C
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  5. gaspipe

    gaspipe Wandering Soul

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    Slight hijack -

    I set the floats identical on both BST equipped 640 ADV's. Both have the same jetting and slide/spring mods. Both are '03s. Both run (or did run great until one blew up :bluduh ) great. One had a horrible bog in whoops, the other doesn't.

    The one with the bog is the one that is still, uhmmmm, serviceable. I even swapped back in an uncut spring and the same stuff. Freakin' wierd.

    Hijack off.

    An 8.9kg/mm spring is pretty stiff for a 640 ADV. Ya planning on hauling a lot of gear? :ear IMVHO, the fork is the worst offender - a feller like me (230lbs) needs at least a .50 spring and a revalve to make it work for offraod stuff. The rear shock is useable (unless you're hauling a lot of stuff) if you use just about all of the compression damping available. A revalve helps a great deal, and a spring would be good, but only if you're going at it pretty hard. I am still using the OE, 7.0kg/mm (I think) spring, but I travel fairly light.

    Again, IMHO.

    Carry on.

    :thumb
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  6. creeper

    creeper Still alive...

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    As odd as this sounds (or, maybe not that odd), I think some (not necessarily all) reports of bogging could be attributed to an air leak, either in the intake manifold or, in or around the vacuum diaphragm.
    Just a pet theory of mine.

    Good points on the spring rates GP.

    Ta,
    C
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  7. gaspipe

    gaspipe Wandering Soul

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    Hmmm....I never did think of that. I need to take a close look at the rubber boots, the diaphragm and check for cracks in the vacuum chamber cap.

    Thanks... :thumb
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  8. Stobie

    Stobie Mr. Motivated Supporter

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    Now look further down the thread to find this and this.

    I eventually cancelled the cut 'n' grind job Lowe's was going to do, and ordered some 43mm x 495mm .50 kg/mm springs from MX-Tech, which are 10mm longer than stock, but there are plenty of preload spacers to take out to make room. I then bought Cugino Pegaso's very slightly used 8.0/260 spring for the rear. The stiffer springs let me get proper sag with very little preload, which actually makes the ride more plush, since excess preload makes the suspension initially stiffer.

    My suggestion would be to contact Factory Connection for springs. They are a stocking WP dealer, and pretty much have everything on the shelf.
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  9. Stobie

    Stobie Mr. Motivated Supporter

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    I think some of the bogging could be a combination of drilling the slide and clipping the spring. Clipping coils off the spring raises the spring rate (basic coil spring physics), but drilling the slide reduces the damping of the slide motion. Stiffer spring + less damping = boingy slide motion. Although the stiffer spring would seem to be more resistant to the motion of the bike through whoops and stuff.

    However, clipping the coils does reduce preload on the spring, which would make it more responsive at small throttle openings, which would only be a problem if it made the throttle touchy in tight, technical situations.
    #9
  10. meat popsicle

    meat popsicle Ignostic

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    Thank you for the links fellas. :thumb Index under "suspension" and "gaspipe's dirty finger" (or how to set your float height) :D
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  11. gaspipe

    gaspipe Wandering Soul

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    Good thoughts Stobie, and I explored that too. It does it with an undrilled slide and unclipped spring. I have spares before I drill and clip :nod.

    Something else is happening - the float hanging up or something bizarre.

    #11
  12. overlandr

    overlandr Dystopist

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    Nice to see that the technical vocabulary being employed by 640ers to describe physical phenomena is of the usual high standard!
    :thumb
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  13. Velocibiker

    Velocibiker Adventure Antagonist

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    My '02 is a "non-bogger". Always has been. (slide NOT drilled, spring NOT cut) Given your experience, I have to think it has to do with something besides (or addition to) the carb. Maybe there's an extremely fine line that causes the bog. Even though your settings are the same, there's bound to be some variation. Go figure....
    #13
  14. IronDawg

    IronDawg Been here awhile

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    gaspipe - I ride off-road quite a bit...most of it in Big Bend and west Texas where we get a mix of open desert and tight, rocky trails thru canyons and hills. I haven't taken the 640 off road much, but I can tell from the seat of my pants that the rear is not stiff enough. I could problably make it work by increasing preload/compression but then ride quality suffers. Getting suspension dialed in, IMHO, is top on my mod list vs motor/ergos/farkle.

    Mack had the forks fitted with .50 springs. I'm not sure about the revalve though.

    The 8.9 I thought might be too stiff also, this is what Mack had and I was going to try it. Like you, I'm thinking an 8.0 should do the job.

    Since folks are having a bit of trouble with Lowe's I'm going to give my local suspension guru a call. He did the work on my DR and it went from a bottoming out scary head shaker into a super plush, big jumping machine. He works with a shop that custom winds springs so getting the right size and weight shouldn't be a problem. The springs won't be titanium but they'll work. There's a bit of a turn around to get the spring, but it's worth it. Might be worth having the shock revalved while I'm waiting.

    Stay tuned....
    #14
  15. meat popsicle

    meat popsicle Ignostic

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    I tried to fix the link to the Factory Pro Float Height page (dunno WTF happened to it in the first place) and couldn't find the one it originally pointed to... I found one with a broken pic link and a MaxAir page with the tool on it and discussion. And Factory Pro's webpages for tuning CV carbs are added - all next to gaspipe's dirty finger. :wave

    EDIT: FOUND IT! :freaky Links in the Index. Here is a pic from the Factory Pro page:
    [​IMG]

    hope that stuff helps fellas.
    #15
  16. Loadedagain

    Loadedagain making chips

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    so... on the subject of more spring... whadjoo guys find? huh? huh?
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  17. gaspipe

    gaspipe Wandering Soul

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    The 640 ADV really comes into it's own once the suspension gets dialed in. It premium stuff, just not set up for any HD offroading.

    I didn't realize getting a spring was a big issue. May want to check with the folks at Precision Concepts, AFAIK, they are about the best around for the WP products and they intimately know the Adventure and it's setup.

    I had one Adventure redone by MXTech, and it is waaaaaaay better than the stock setup, and the other done by Precision Concepts, and it's better still. Just my two cents.
    #17
  18. IronDawg

    IronDawg Been here awhile

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    Went to Full Circle Cycle and talked with the owner, David Lapman, and discussed a couple things. First was a spring for the rear shock, second was lowering the arse end by 1 3/4 inches. David is well aware of my d/s riding habits, i.e. loaded with camping gear one week, next week desert riding with tight trails thrown in.

    He suggested a 3/8 inch travel restrictor for the shock vs. lowering links to maintain correct geometery, and a 8.9/245 spring to keep the recommended .9 inch preload to drop the arse about 1 3/4 inches and move the forks up 3/4 inches.

    The spring will be here next week. I ordered it in white powder coat, I could have had any color I wanted, clear coated, black, orange, or hot pink but that takes another week or two and didn't want to wait that long.

    FYI, stock spring od is 84mm, id is 64mm.

    EDIT: As for the carb, I found that the boot was loose at the air box. I made sure everything was tight when I put the bike back together. I'm going for a ride tomorrow to see if I've got the carb sorted.
    #18
  19. meat popsicle

    meat popsicle Ignostic

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    BUMP! For some current issues and good info.

    :ear IronDawg?
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  20. IronDawg

    IronDawg Been here awhile

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    Sorry guys...totally forgot about this.

    I've replaced the spring with a 245/8.5 from the original 260/7.0 and had it powdercoated KTM orange. I've got pics but not at work. I'll try and get them up tonight. I've got a shot of the old and new sitting together for reference.

    I had the opportunity to try it out in a variety of terrain including a trip to Big Bend, some back roads out in the hill country, and single track.

    The seat height is about 1" lower with the correct preload set. To compensate, I raised the forks 3/8ths of an inch.

    Before the change, I could feel the rear of the bike wallow in turns and was very rough over desert-like terrain. Now it's firm but plush. It stays planted in the turns and will take square-edge hits like jumping railroad tracks or logs better. Before the rear was kicking out...now it goes over stuff nicely. I think a lot has to do with lowering the rebound and compression setting on the shock since the spring now holds the bike up rather than the shock. The shock valving seems to be very good and doesn't need a revalve.

    Now for the bad...like anything else on a motorcyle, if you fix one thing, other things will pop up...like the weak fork. Now that the rear of the bike is behaving, the weak fork springs and sloppy action feel 10 times worse. I tried playing with compression/rebound settings and the fork height in the triples...3/8ths was the best of the worse settings. (I can't remember the comp/reb settings.)

    In the dirt, the front pushes easily, you really have to use the throttle to turn the bike. I've tried increasing compression/rebound but, like the shock, the fork becomes harsh. I've also developed a head wobble when riding in sand at speed (70MPH+). This might be due to the Metzler Baja tires. The front knobby pattern has a kind of wave to it.

    Single track was a bit scary. Too slow turning and just too heavy. This bike wasn't really built for single track and I don't think I'll ride much single track with it. I've got my stump-puller DR for that.

    On the street the bike felt great even with the weak forks. I feel more comfortable getting the bike leaned over since I don't feel like I'm in the stratosphere. The fork does dive a lot and the front end hunts a lot on bad roads but doesn't feel like it'll lose grip.

    I haven't loaded the bike up with gear yet. But I'm betting it would handle the extra load much better than stock.

    All in all, I'm happy with the results. I figured I'd have some work to do on the fork. The same thing happened to my DR, but more pronounced on the KTM. I'm definitely going to get heavier progressive fork springs and a revalve some time soon and I would suggest this to others if they do a lot of dirt riding.

    EDIT: No more carb problems!! Fixed by setting float level and making sure all was tight...air leaks are bad!!
    #20