Xr100 won't start conundrum

Discussion in 'Motorbike Technical Discussion' started by clay, Jun 29, 2006.

  1. clay

    clay Guest

    So my son has a 98 honda Xr100, and I can't get it to start. Probably
    the most popular honda ever made.
    It was running O.K. then suddenly nothing.

    Here are the details:

    Gets Spark (kind of weak looking, but its an xr100 so maybe thats normal)
    Fuel to the Carb
    Choke is working (see below)
    Valves/cam fine (cam chain not broken, cam rotates, valves open)

    But zero gas is getting to the chamber, the plug is not wet, on full
    choke. Compression seems to be O.K. (haven't put a gauge on it yet), and
    I am getting a puff of pressure out the plug hole, but also back through
    the carb. valve lash seems high at .025". What should it be? .0025?

    So what could possibly be wrong, to not have enough suction to pull the
    gas into the chamber?

    I failed to mention the obvious. I have completely cleaned the carb jets
    too. That was the first thing I did after checking for spark. The idle
    jet was completely clogged.

    One thing I did notice, that doesn't make any sense is. When on full
    choke, gas squirts back toward the air cleaner from one of the two brass
    inlet tubes at the mouth of the carb. I have never seen a carb that does
    this, on choke. Normally, shouldn't it (the piston) be sucking gas from
    the carb idle circuit? Or does this mean the intake valve is burnt /
    still open on the compression cycle. (A racer friend rode the bike last
    year, and suggested it ran like it had a burnt valve)

    Time to find my compression checker......

    It has been suggested the the intake valve is burnt, bent, or the seat
    is cracked/broken. Junk stuck between the valve/seat. But, with the top
    end off, it looks like it might have jumped a tooth on the cam chain.
    How do I set/check the cam timing?

    Anyone have a scanned page or two of the manual, showing timing details,
    valve lash, etc...

    I also have a fax machine to send pages to if neccessary
    clay, Jun 29, 2006
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  2. clay

    clay Guest


    Looks like it jumped teeth on the chain, and either bent or burnt the
    intake valve. I also noticed the cam shaft & bore, is pretty worn/gouged
    on the gear side. And there are no cam shaft bushings on this motor.
    Obviously contributing.

    So on a head rebuild, do we add cam shaft bushings, and undsersize grind
    the shaft?

    Is there a trick on this bike to pulling the head off, without dropping
    the whole motor? The head ALMOST comes off, but won't clear the studs
    without dropping the motor completely. Or do you unscrew the head studs

    Reply direct if possible.

    clay, Jun 29, 2006
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  3. Here we go again.
    The Older Gentleman, Jun 29, 2006
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