XT500 can t stay at idle...

Discussion in 'Old's Cool' started by Readymeal, Sep 2, 2011.

  1. Readymeal

    Readymeal Been here awhile

    Joined:
    Nov 19, 2006
    Oddometer:
    113
    Location:
    Townsville, NQ
    Hi there,

    here s some information
    XT500 81
    new 540 kit and head job done recently (was working very well on another bike)
    new points and condenser
    new timing chain
    stock XT exhaust
    Uni Foam air Filter
    stock carb

    start well cold on 2nd kick with choke on
    does not start at all hot with choke off but does start again with choke on...but i m getting a lot of kick back in fact...even after "purging" the cylinder few times.
    the main symptoms is: the engine died if stays at idle...it looks like it s chocking from too much or not enough air/gasoline?
    a lot of back fire seems to build up even at idle and getting worst when reeving up.
    reeving up very nicely though
    when the engine died the spark plug looks good with no carbon deposit or even trace of fuel on it.

    i tried to adjust the air screw mixture and the idle screw..
    i tried to change the advance...
    i tried with another carb...
    valves are on the loose side...
    i cleaned the carb and all air/fuel passages

    Thank you in advance.
    #1
  2. Airhead Wrangler

    Airhead Wrangler Long timer

    Joined:
    Mar 4, 2008
    Oddometer:
    7,034
    Location:
    Back in Seattle, FINALLY
    I can't help a bit with your XT, but you don't live in Canada anymore. Change your profile or I'll tell everyone here that you're a male nurse. Oh woops. :rofl

    I wanna see photos of your new ride(s).

    EDIT:

    Alright I'll be a little helpful.

    "start well cold on 2nd kick with choke on
    does not start at all hot with choke off but does start again with choke on..."

    This tells me you need to pull your carb and flush out the idle circuit really well. Check any rubber intake boots for cracks or other air leaks and make sure they seal up to the carb well. It sounds like you just aren't getting enough gas at idle.
    #2
  3. Readymeal

    Readymeal Been here awhile

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    Nov 19, 2006
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    113
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    Townsville, NQ
    Thank you Senior Spencero, but i think you are only allowed to post in the battle scooter section nowadays... :) did you get you little Vietnamese mopeds going? :)

    it s not that simple as it does exactly the same with another carb...what are the odds that it s the same problem in both carbs? but i will check it out.

    In fact i can manage to get the bike running at idle by tuning up the air screw mixture and the idle screw but after a little while(10-15s)...i have to readjust or it s dies...etc...until there s nothing i can do to avoid the engine from dying.

    Furthermore the back firing and the several kick backs i m getting after the bike stall is telling me there s a lot of un-burnt gas in the cylinder but the spark seems good enough tested outside but could be too weak with the pressure at TDC.
    however the spark-plugs is coming back very clean even from running mainly with the choke on...so yeah it could be a lack of gas going into the cylinder...bizarre...not enough gas but a lot of back firing???? it could still be the ignition point...but it can t be that far off...i ll readjust it.
    #3
  4. DoubleBit

    DoubleBit Been here awhile

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    Apr 10, 2008
    Oddometer:
    305
    Location:
    Applegate, Nor Cal
    Still could be carb/intake/vacuum leak trouble.
    Also, back up to basics, how's the "new" engine's:
    Compression?
    Valve timing?
    Valve lash/clearance?
    Ignition timing?

    And, good luck!
    #4
  5. redprimo

    redprimo Been here awhile

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    central coast of California
    It sounds like an intake boot/ manifold leak. That would acount for the back fireing and for the lack of adequate vacume for the low idle circut causing you to have to use the choke- which isn't really a choke but rather a low pressure bypass which dumps more fuel into the engine than the normal idle/low rpm circut does.
    #5
  6. stteve

    stteve Been here awhile

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    Maybe try starting it without the airfilter. Also, i heared several stories about bad running XT500's, because the ognition was mounted wrong.
    #6
  7. motomike14

    motomike14 Thumper Crusader

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    I have a similar setup on my 1980 TT500 (540, cammed up, etc.). And, at one time, I had a similar issue. My air intake boot had a good 3/4" tear in it. Start there.

    Also, as mentioned, you may have a low speed idle circuit clogged. Yamaha carb cleaner and compressed air will fix that. Soak it for a good day or two (pull every piece out; jets, slide float, all of it) then let it dry and blow out every circuit you can find.

    You may also want to check that your needle is set correctly. As soon as mine runs even a bit lean, it runs like shit.

    Just a recommendation, trash that stock carb and get you a good 36 or 38mm Mikuni TM. Makes the setup a bit easier and improves power/response everywhere. I ride mine almost daily and it hardly ever has a starting issue.
    #7
  8. motomike14

    motomike14 Thumper Crusader

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    This too; check the points gap (0.014 in) and your plug gap (0.028-0.031; though I run mine around 0.038-0.040). AND, run a BP7ES NGK; extended tip helps.
    #8
  9. Rd650

    Rd650 Been here awhile

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    You said it has the 540 kit. I would assume that came with a high compression piston. If this is the case, do yourself a favor and get a 36 or 38 mm Mikuni carb kit and your choice of aftermarket exhuast. In the long run you will be much happier.JMO
    #9
  10. Readymeal

    Readymeal Been here awhile

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    Nov 19, 2006
    Oddometer:
    113
    Location:
    Townsville, NQ
    thank you for all the info...i ll get back into it asap

    will check the idle circuit and will most likely dip the whole thing into a carb cleaner.(still bizarre that it happens to both carbs)
    will have a look at the intake manifold
    points and spark plug gap are within range
    valve clearances are good too(on the loose side)
    running 540 kit normal compression

    i try to keep the bike looking somehow stock and quiet. i had the opportunity to get a 540 kit in while we were looking over the engine after purchasing the bike and the air box was just trashed...I will most likely make some modification in the future but i rather get it working and ride around before making too many change.

    thank you again
    #10
  11. Readymeal

    Readymeal Been here awhile

    Joined:
    Nov 19, 2006
    Oddometer:
    113
    Location:
    Townsville, NQ
    Alright...

    an update

    valve clearance good
    intake manifold in great shape with only some very few superficial cracks.
    idle jet and idle circuit in perfectly good condition.

    i moved the ignition a bit and i don t get kick back anymore but now the bike does not want to run at idle for more than 10s even with choke on and still does not want to start with the choke off while warm (air temp around 25C or high 70's F)
    i tried with and without the air filter.

    i even checked if the camshaft was not a tooth misplaced...nope there s a perfect timing with the crankshaft.

    I just don t know anymore...maybe play around with the ignition..but it can t be that bad if i managed to get the engine going...
    #11
  12. motomike14

    motomike14 Thumper Crusader

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    What jet sizes are you running? Position of your needle? Sounds like something with your jetting is a bit off (or, way off). Done a plug check?

    It's not uncommon for them to not want to run on the choke. A lot of times, I actually don't have to use choke with mine; kick it without, then grab it quickly.

    What cam are you running? Stock or aftermarket? I have a megacycle and think it may have changed it's behavior a bit.

    This also sounds stupid, but was a major PITA until I discovered it; check your gas cap. If it does not vent properly, you will have problems galore. For whatever reason, these things are picky with the way the tank vents. Once again, all from personal experience. I may have an extra I can send you if you don't have another to test it with.
    #12