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"Damineding" Carb rebuild kit

Discussion in 'The Garage' started by franticvike, Jan 30, 2023.

  1. franticvike

    franticvike XS400 Enthusiast

    Got the valves checked. Left side was all right in the middle of spec. Valves for the right cylinder were both way out. Intake was very tight. My feller gauges go down to 0.002 and I couldn't get that in. Spec on that is 0.004 - 0.006. That tight makes me a bit nervous. Exhaust on the other hand was very loose, between 0.009 and 0.010. Spec on that is 0.006-0.008.

    Any thoughts on whether that combination (very tight intake and loose exhaust) valves is a likely culprit for the hot running / smoking condition?

    Had to run between shops all over the island to try and scrounge shims, but think I got them. Will replace tonight and re-measure.

    When I go to put the valve cover back on I've read it's good to put a bit of grease on the gasket. Any particular kind of grease or is general purpose stuff fine?
  2. franticvike

    franticvike XS400 Enthusiast

    Re shimmed, all in spec, and fired it back up. Starts right away, seems to idle nicely, but once again I'm getting white smoke out of the right exhaust after about a minute. Cannot figure out what might be causing this. Guessing maybe just something in the actually exhaust pipes that is burning up...? Tempted to just put it on the road and ride it to see if it clears up, but don't want to make the problem worse.
  3. CaptChrome

    CaptChrome XS400 Addict Top Contributor

    I'd ride it and see what happens. Watch the oil level. If it continues, it could be a leaking valve guide seal.
  4. franticvike

    franticvike XS400 Enthusiast

    Smoke went away after a quick ride, must've been some debris in the exhaust or something. Next problem is a creeping idle. Starts great and comes to nice idle, but then over about 2-3 minutes the idle will creep it's way up eventually getting to 3,000-4,000. Must be a vacuum leak. I've sprayed starter fluid all over and no discernible change. I guess that leaves either the diaphragms or maybe an o-ring in the air fuel screw...? Are there any other potentially hidden air leaks to look for before I pull the carbs back off?
  5. JPaganel

    JPaganel XS400 Addict Top Contributor

    One common source of an air leak, especially one that changes with engine temp, is hardened throttle shaft seals.
    They look like o-rings, but are actually square seals with a lip.

    They are a kind of a pain to replace - you have to disassemble carbs removing the butterflies. Butterfly screws are peened over, and you better be damned sure to have the correct JIS screwdriver for them, or you will strip them out and hate your life trying to fix that.

    The nice thing is that you can still get the seals from Yamaha, even though they are not listed in the parts fische for the XS400. Part number 256-14997-00-00.
    CaptChrome likes this.
  6. franticvike

    franticvike XS400 Enthusiast

    Is there a way to test that before pulling it apart? Maybe let it start to get hot then hit that area with starter fluid? What about putting a seal swell flulid or something like that on the area to see if it goes away temporarily?

    Also, do I need 2 or 4? Have to order in from Toronto apparently.

    Thanks for the input, and yes I have some nice JIS screw drivers.
  7. JPaganel

    JPaganel XS400 Addict Top Contributor

    You need 4. Two per carb body.

    You can try spraying starter fluid around where the shaft enters the carb. I don't find it to be the most reliable test.

    I go off the general principle that a 40 year old piece of rubber is probably going bad in some way. :D

    franticvike likes this.

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