2003 640 Top End Power Problem

Discussion in 'Thumpers' started by d0gWateR, Aug 21, 2005.

  1. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    I have a “funny” top-end power problem when I’m doing 65-70mph at about 4750 rpm and I increase throttle position aggressively. It hesitates and won't produce power. If I'm really aggressive it sound sort of like someone is trying to chainsaw their way out of my engine. If I want to accelerate past 65mph I have to do so very gradually and it helps if I’m heading downhill.

    This problem is new, I have had 8000 miles w/o any problems. It started after running for about an hour at 5000 rpm on my way down I-5 last Sunday for the start of the Nor'Wester TSD Rally.

    The best way to describe the lack of acceleration above ~5000 rpm is it’s just like when on an old car with a vacuum advance distributor and the vacuum line has fallen off. Acceleration is almost impossible, it has to be done very gradually.

    Anyway, normal riding offroad the problem doesn’t affect me, it seems if I’m on the mainjet for for short bursts I have lots of acceleration. It’s only when I’m running 1/2 throttle for a long time and then crack it to ¾+ throttle that it balks. Oh and one more little gem, if I was experiencing this problem while climbing a small hill (typically) then slamming the throttle closed then back open again to almost WFO would make the problem go away (temporarily).

    The problem is intermittent, of course… Sometimes during last week (rarely) it worked fine and power-o-plenty rocking up to 80+mph on the MT21 (rear) and D606(front) without any hesitation.

    I just spent the weekend dirtriding, and the problem just doesn't present itself under those circumstances.

    I suspect I do have the fuel level in my carb too high (engine dies under stutter bumps) so I’m going to remedy that this week. Clean the carb thoroughly and reseal everything back up to check the possibility of fuel delivery issue. I'm suspecting and hoping it's a fuel delivery issue (stuck slide - I ride in the dust a lot and have probably f#@!'d up my canisterectomy long ago). Hmm, just typing all this out helps me get it a little straighter in my mind. I'll bet it's that damn slide, dust coming in through the vent on the carb and getting the slide all gritty... Or perhaps a hose or diaphragm problem?

    It's a 2003 640 Adventure: snorkel in, side airbox screen, one size larger mainjet, stock pilot, KTM Comp Silencer c/w drilled baffle, scratches on both sides of the tank :lol


    Any other suggestions? Thanks in advance.

    Paul
    [​IMG]
    #1
  2. potatoho

    potatoho Cheese and Rice!

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    I think several of us have had issues with gas tank venting. I inadvertently plugged mine when I rigged up a fancy check valve. The symptom was that on occasion the bike would just die at half a tank whilst riding continuous hwy speed.

    Suppose you could have a vacuum leak on the carb.. on either carb spigot, or perhaps the top diaphragm chamber could be sucking air.

    Chainsaw, hmm. You know I'm about to repack my silencer (as soon as the packing arrives) because it went downhill really fast. My top end is near non-existant at this point and it sounds rather obnoxious. Can't find any other problems, so I suspect it is all a packing issue with mine.
    #2
  3. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    Hmm, mine sounds pretty much blown out too! Does that cause a loss in top end potentially? <--prolly a noob question, but I don't care.

    Also, I was once a victim of the tank vent problem and it definitely isn't that. I'll repack the silencer anyway, it's just too damn loud right now.

    Thanks potatoho

    Paul
    #3
  4. meat popsicle

    meat popsicle Ignostic

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    i lost my top end when my main jet came unseated... doesn't sound like what yours is doing - mine was dead at 3/4 throttle and up. But its an easy thing to check and make sure its seted properly and the spacer is ok. :D
    #4
  5. potatoho

    potatoho Cheese and Rice!

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    To be honest I'm just going by what I've read. My understanding is that as the noise increases from blown packing, the mid-range and top-end go away proportionately. A few weeks ago I began getting annoyed at some cackle, and now it's pretty noisy beyond 3,000 rpm and it just feels all around mushy beyond half throttle.
    #5
  6. Jan from Finland

    Jan from Finland Been here awhile

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    Is the chainsaw noise like someone is trying to saw your engine in parts, or do you mean noise of two stroke engine? If it is the former, it could be a cam bearing failure. In that case it should be repaired immediately. If you are lucky, you have to pay only the bearing. If you are not (like me) the chain will break, jam the engine and smash the oil pump, counter balance etc. And if you are really unlucky, the valve train might fail.
    #6
  7. Esteban

    Esteban Banned

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    Power loss above 5000 rpm

    Check all vents:
    fuel tank
    float bowl
    slide diaphragm

    They must all freely flow to the atmosphere, not the air box or other pressurized area. They should not face into wind or turbulent area either.



    As for ¨chainsaw noise¨
    I cannot guess without hearing it in person.
    Try the ¨stethoscope method¨ with a screw driver handle on your ear touch blade to engine while running. Touch various places on engine and listen for odd sounds. Works excellent as long as you know how it should sound. Get a known healthy bike for comparison.

    Steve
    #7
  8. 1stworks

    1stworks dude

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    what to ride tonight? 2 or 3 hours at rampart?
    see post in rockies,can leave south side at 5:00pm

    firstworks:D
    #8
  9. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    Saw in parts, but I only heard that once. I never twisted the throttle that hard again after I heard that noise!
    #9
  10. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    Thanks Steve, I'll check all of those things. I'm probably going to rip apart top end for the peace of mind it will bring to know all is OK .

    Paul
    #10
  11. UngaWunga

    UngaWunga Mosquito bait

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    The chainsaw noise might be detonation. Are you running a cheap assed low grade gas?

    Also check for airleaks leaning out the mixture.
    #11
  12. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    It's possible that I had bad gas, but I had just tanked up with 91 octane(R+M/2 method). It could have been some detonation, the engine temperature gauge read correctly at 4 bars, but the case of the engine seemed very hot.

    OK, you guys are going to kick my ass for this but:
    I had just added the side airbox screen before this trip w/o making any additional jetting changes. I had never done any long runs on the mainjet before this one. Could it be that with the blown out packing, drilled baffle, side airbox screen, that I was just too lean on top?
    The stock mainjet is 154, and I had only jetted up 2 sizes (whoops thought I was running a 156.5) to a 160 w/o changing the pilot air jet.

    I appreciate all the feedback guys, and I'll be able to get out to the garage and start working on the bike tonight.

    Thanks again!
    Paul
    [​IMG]

    #12
  13. meat popsicle

    meat popsicle Ignostic

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    if its too lean it will run hotter and the bluing on your headers will start creeping aft.
    #13
  14. potatoho

    potatoho Cheese and Rice!

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    What is the procedure for drilling the baffle on an SXC competition silencer? Is it a hole saw deal, or just little holes or something.

    Pinging sucks. If it is too lean, you've got several ways to find out. You can put the stock airbox side back on, or tape up the airbox screen. The snorkel, you could put it back in if you want, but I don't recall it drastically changing the jetting requirements the last time I experimented. You could go the carb route and increase your main jet, or move your jet needle up, whichever works better for you.
    #14
  15. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    I just drilled a 1/4 pilot hole then used a uni-bit (step drill) to open it up to as big as I could fit. I then drilled one or two other smaller holes. I'll take a pic when I repack the silencer.

    The needle is up one position (clip down one) and I'm jetted a little richer on the main [160]. The problem appears when I'm on the mainjet, so I'll try recovering my airbox and repacking the pipe and see if the problem disappears.

    I have some creepage :evil

    Thanks all!!
    Paul
    #15
  16. d0gWateR

    d0gWateR Goat Stuntaz DSMC

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    OK first with regards to canisterectomy. I have two hoses coming off the right hand side of my carb, one large diameter that has a chunk of hose with a 90 degree plastic fitting w/integral airfilter (dust would still get through) See "A" in picture below. The other port "B" in picture, has just a hose on it going nowhere. Should "A" be plugged? Or should it just have a proper filter put on it? I ride in the dirt/dusty a lot. Should "B" be plugged?
    [​IMG]

    When inspecting the diaphragm it doesn't appear to have any tears, or holes, but there is a lot of grit, and dust and crap. The slide has a very gummy sticky surface about 1/2 way up. I'm guessing this is from using a newly oiled filter before the solvent carrier had a chance to flash off completely.
    [​IMG]

    You could see some marks on the slide from where it has been binding in the carb. The carb body correspondingly was dirty and sticky like hell.
    [​IMG]

    I am going to pick up some jets tomorrow (I'll try the KTM shop but will probably have better luck down at the Harley dealer). For the curious out there, I snapped a little pick of the jets. Interestingly, my air jet was a 1.2, not a 1.1 as anticipated. Guess I should check my manual and see what it says. I took a picture of my 152.5 Mainjet although I had the 160 installed.
    [​IMG]

    I'm going to do the standard BST40 mods, drill slide holes to .125, cut down spring, 1.2 air jet, 47.5 idle, and start at 162.5 and work my way up (possibly) clip in 4th position. Snorkel in, KTM side airbox screen, KTM Comp Pipe from '04 SM hogged-out.

    Please advise on canisterectomy plugging-off question. Thanks Team 640!

    Cheers!
    Paul
    [​IMG]
    #16
  17. Esteban

    Esteban Banned

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    I used to think that, but I have seen on a few of my bikes (LC4 included) that an over rich condition leads to excessive bluing on my head pipes.

    Maybe fuel still burning in header pipe ?
    #17
  18. Esteban

    Esteban Banned

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    Upon dissassembly of the rear cap on my SXC muffler I found it has overlapping tubes to help muffle sound. The exhaust gas is dumped into the rear inner chamber of the SXC muffler where it has to go backwards to enter the tube that leads to the atmosphere. It is possible to cut the exit tube (part of the rear SXC cap) off and achieve a more or less straight through flow. The muffler needs to be reassembled with stainless steel rivets for longevity. These can be hard to find except at specialty shops on the industrial side of town. You can find them though. New hi temp large o-rings can be found at the o-ring store. Use hi temp silicon when re-assembling.

    Cutting off this overlap in the inner SXC plumbing will net a definite gain in HP with a little rise in sound level.

    By removing the front and rear cap on the SXC the entire muffler can be repacked for a very mellow quiet note. A stainless screen can also be installed under rear cap to help with spark arresting. Obviously not USFS approved but would pass any test the USFS or AMA does in Colorado that I have seen. (The stick up the rear test.)

    My SXC competiton muffler is about 3 years old. I think some newer ones come with spark arrestors of some sort and maybe different internals -check this.

    Esteban
    #18
  19. Esteban

    Esteban Banned

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    "A" is the breather under the diaphragm. Small rubber hose and cotton gauze filter should be on that. Needs to be in unpressurized area and flow freely.

    "B" is the float bowl breather. Also needs to be in unpressurized area and flow freely.

    Some dirt under the slide diaphragm unfortunately is normal. One of the drawbacks with this carb. My guess is the wear and scuffing you are seeing on the slide is not an issue. Clean and dry it well. No lube hear it will just act like grinding past when mixed with grit. Get the breather filter back on there.

    [​IMG]


    I doubt you ran it lean enough to hurt it. They are tough.
    BTW, I run mine with way smaller #138 main jet in the FCR 41 carb. Gas analyzer and dyno results lead me to that jet. I ride between 5000 and 13,000 feet.
    #19
  20. potatoho

    potatoho Cheese and Rice!

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    That doesn't sound right. I've never heard of anyone using as low as a 1.0, though they do make them as small as 0.5. I believe it is recommended to either use a 1.1 pilot air jet, or increase to a 47.5 pilot jet.
    #20