XT600 carb question again

Discussion in 'Thumpers' started by l1ndon, Apr 5, 2013.

  1. l1ndon

    l1ndon Adventurer

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    When I rev up the engine - it suddenly hits a ceiling and wont rev any higher, (it sounds like an engine limiter but obviously isnt)

    Could it be the second part of the carb not opening (the vacuum operated bit)? whats the most likely explanation/fix

    (I checked the diaphragm on the second carb when I had it in bits last week and it looked fine - it moved the valve up and down when I sucked on the big thick tube - should that tube go anywhere or is it a breather)

    Thanks guys
    #1
  2. dtysdalx2

    dtysdalx2 PITA but useful

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    Try looking around in Thumpers, there is a XT600 thread with all the experts.

    If the bike worked fine before, I'd say you have a plugged jet (main jet and tube I'd say) in the carb or you didn't assemble it right. Those carbs are a PITA to keep clean. DAMHIK. :D
    #2
  3. l1ndon

    l1ndon Adventurer

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    Thanks I added the question there -

    actually it wasnt running when I got it - I've had to clean the carb out to get it to run at all -

    I'm pretty sure all the jets were nice and clean when I finished though - still probably will end up with it in bits again

    thanks for your help
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  4. dtysdalx2

    dtysdalx2 PITA but useful

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    The passages in the carb body itself are the hard part to clean. Mine sat for around 9 months and the gas in the float bowl turned green and gummed up the jets. Removed and cleaned the main and pilot jets, it worked but there is a flat spot in the midrange now. I've tore the bike apart 100 times and I'm just not really excited about doing it again.

    My advice is to drain the float bowl if you let it sit more than a month.
    #4
  5. motu

    motu Loose Pre Unit

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    Make sure the linkage is opening the 2nd carb - it's something like 1/4 throttle or less, certainly not 1/2 throttle like a car carb.
    #5
  6. l1ndon

    l1ndon Adventurer

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    Ill have a check again but its definitively mechanically operating - but there's another bit that operates via suction and its hard to know whether that's working
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  7. thumpthumpthump

    thumpthumpthump Thumpthumpthump

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    If it has not run right since you bought it lets look at your jet sizes. What are your main jets and the altitude you live at? When you cleaned the carb did you make sure that all the little holes in the emulsifier tube where clear? Have you pulled the petcock to make sure it is not clogged? Sorry to ask , but did you check the air filter. I just bought a bike that had a totally clogged air filter and they had turned the fuel screw in all the way so it would run. Check the woodrift key on your stator , if it has broken and the flywheel rotated it changes your timing and will not rev out. Ask me how I know. Good luck ! Its always air, fuel or timing. thumpthump
    #7
  8. l1ndon

    l1ndon Adventurer

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    I dont know what size the jets are (I live at sea level) I cleaned the emulsifier tubes -

    yeah I pulled the petcock of the tank and it was flowing freely - is there more to it than that?

    I've tried it with no air-filter and it makes no difference

    I'll have a look at the woodruff key - not really sure how that all works so I can look into

    Just to clarify what its doing - its not just revving up to a limit and then going no further but running smoothly at those revs - it actually hits a certain rev amount and then makes a noise like a rev limiter - ie cylinder fails to fire for one rev - at a constant rate
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  9. thumpthumpthump

    thumpthumpthump Thumpthumpthump

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    What year xt do you have? the 84-89 do not have a rev limiter , I am not sure if the 90-95 do or not. What rpm is it stopping at ? Red line is about 7000. The early bikes have a tach but the second gen have a dummy light. the early xts had a 125 primary and 130 secondary main jet . the second gen. had a 130 main and a 104 secondary. If it is not reving to 7000 and the carb is right I would say it is the timing. It is fixed and unadjustable on the xts. The woodrift key is a little half circle shaped metal piece like a piece of a coin. It fits in a groove in the crank shaft and a groove in the flywheel and keeps the synchronized. If it breaks the flywheel rotates on the crank shaft and changes the timing. Pull the left side cover of the engine and look for the groove between the crank shaft and the flywheel.thumpthump
    #9
  10. l1ndon

    l1ndon Adventurer

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    Its a 91 - so no tach, thanks for your help - I'll look into it :thumb
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  11. thumpthumpthump

    thumpthumpthump Thumpthumpthump

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    I checked and the 90-95 has a rev limiter set for 6900-7100 rpm. If you are used to multi cylinder bikes you may be used to higher red lines and do not have a problem. Can you rev it enough to make the dummy light come on?
    #11
  12. l1ndon

    l1ndon Adventurer

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    Yeah I'm wondering if it might just be my expectations - my last bike was a vfr 750 so that revved very high. I'll do some more investigations
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  13. thumpthumpthump

    thumpthumpthump Thumpthumpthump

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    With no tach you have to use the dummy light. Can you make it come on? If you can then you are at redline.
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  14. l1ndon

    l1ndon Adventurer

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    No dummy light is coming on - when I look at how far the throttle arm is moving when it starts to cut off - it looks like less than halfway

    Well Ive checked out a few more things, possibly know what the problem is - though not sure.

    I ran the bike with the little pipe that feeds the secondary carbie disconnected (and sealed) and it runs exactly the same (ie wont rev past a certain point)

    It has fuel up to that point as it freely flows out of the pipe (fed from the float chamber.)...and I've cleaned that jet thoroughly already a couple of times



    So for some reason I think its not getting fuel up into the secondary part of the carb.

    When I look through the airbox I can see the vacuum operated needle valve mechanism rising up and down as I rev the bike, so it would seem that that is working correctly.

    So what would normally cause the fuel to be sent up through the secondary carb main jet - is it vacuum- if so what creates it?

    Thanks again guys

    (I should also add that - just once or twice, among the many many times I've done it, it seemed to behave differently and rev past the normal point it stops - but then immediately returns to normal and I cant repeat it)
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  15. thumpthumpthump

    thumpthumpthump Thumpthumpthump

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    Have you checked the woodrift key on the flywheel? You have not mentioned that sence it was suggested on 4/9/13. This will keep it from reving out. The timing get retarded. Even on one carb you should be able to hit redline in neutral with no load. On the secondary carb if the linkage moves the slide diaphragm moves and the fuel tube is open , that only leaves the main jet and the emulsifier tube with all the little tiny pin holes in it. Are all the little pin holes open in both carbs. They are the first to clog from bad gas. thumpthump
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  16. l1ndon

    l1ndon Adventurer

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    Thanks for reminding me about the woodruff key - its next on the list - then I';m thinking maybe electrical as I think you're right about the carb -

    The only part of it I havent checked is the secondary emulsifier nozzle - but I dont think that would sufficiently explain the symptoms.
    #16
  17. l1ndon

    l1ndon Adventurer

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    Ive got the side cover off, but cant see the woodrift key, what now?!? (And my gasket all shreaded, any ideas where I can get one from?)

    Thanks
    #17
  18. l1ndon

    l1ndon Adventurer

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    #18
  19. thumpthumpthump

    thumpthumpthump Thumpthumpthump

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    Your picture is too small to tell. There is a slot in the crank and in the flywheel and they should be lined up with a half moon shaped woodrift key in the slot connecting them both. If the slots line up then you are ok. If they are not your key is broken. You would have to loosen the nut , realign them and put a new key in. Make sure all the outher timing marks are right . A key and gasket are available from oem suppliers on line thumpthump
    #19
  20. JensEskildsen

    JensEskildsen Long timer

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    #20