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Runaway Motor

Discussion in 'XJ Technical Chat' started by cturek, Nov 16, 2008.

  1. cturek

    cturek Member

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    Today, after rebuilding the carbs including new throttle shaft seals, I bench synced the rack, set my pilot screws at 1.5 turns ( I have the coarse threaded screws - I hope this is close to 3 turns out of the fine threaded pilot screws.) Installed the rack and connected the vac line and fuel line to the petcock. Opened the petcock to prime for about 10 seconds and turned to on. Started the bike. The engine ran at about 2K rpm while I had the "choke" on. After about 30 seconds she seemed to smooth a little bit so I closed the choke, goosed the throttle a SMALL amount and the motor ran wild. Had to hit the kill switch as fast as I could. Restarted with same result. the tach pegs before I can hit the kill switch.

    I was very careful while installing the rack to make sure the throttle linkage did not get hung up on the cylinder head. Assuming the throttle linkage is not hung up, what would make the motor run wild? I polished the slide bores to a mirror finish and my pistons hit the bottom so hard during the clunk test, I thought I was going to break them.

    I have all stock jet sizes. Stock airbox with foam filter, stock exhaust. Help me tame this beast.
     
  2. MiCarl

    MiCarl Active Member

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    Bottom line is it won't rev up unless it's getting extra fuel and air.

    Usual suspects:

    Throttle cable mis-adjusted.
    Choke and throttle cables switched.
    Throttle linkage hung up.
    Idle screw set too high.
    Butterflies significantly out of sync.
     
  3. manufan

    manufan Member

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    I'd try looking for an air leak.
     
  4. TheHound

    TheHound Active Member

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    You say it idled fine before you goosed it.
    Maybe the throttle is hanging up somewhere.
     
  5. HalfCentury

    HalfCentury Member

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    One of more diaphragms may be sticking in the up position.

    How did the "clunk test" work?
     
  6. Turkey

    Turkey Member

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    The last two times I put my carbs on the bike did exatly this. Both times one of the clamps on the carb boots were barely touching the throttle linkage. I'd check that first. Also check idle adjustment, and make sure the throttle cable isn't too tight. If you Idle it all the way down, and it still doesn't stop over reving, then something is holding the throttle open.
     
  7. cturek

    cturek Member

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    Thanks you guys. Got home from work this evening and found a honeydo list. Working on the XJ wasn't on the list. Should be able to take a look at it tomorrow evening.

    I'm hoping that it is a throttle cable issue. I didn't really check that it was allowing the linkage to retreat all the way when I connected it.

    I gotta say that, when it was running, the stock exhaust sounded great - a deep throaty growl. I can't wait to get it tuned and post a video.
     
  8. asphaltgecko

    asphaltgecko Member

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    WHEN YOU REBUILT, did you KEEP ALL THE PARTS TOGETHER WITH EACH CARB ? You can not mix them...at ALL ..... just a thought...
     

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