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Engine revving while stopped.

Discussion in 'The Garage' started by David Thompson, Aug 30, 2021.

  1. David Thompson

    David Thompson XS400 New Member

    Newbie here. I have an original 1982 Yamaha XS400 Maxim that my dad bought brand new. I just had the head gasket replaced as it was shooting hot oil out the left cylinder. After I got the bike back, it is doing something it never did before. When I stop at an intersection, the bike just starts revving until I shut the engine off. I am not moving the throttle at all and the choke is off. It just starts at idle speeds and then increasing revving faster and faster and faster all by itself. I don't know what the RPM's are because my tach is not working.

    Any ideas what would cause this? Bike.jpg
  2. xschris

    xschris XS400 Guru Top Contributor

    Air leak? Carbs out of sync? Bad carb diaphragm? Throttle cable mounted wrong causing it to get hung up? Lean condition with the carbs? Just a few things to look for.
  3. David Thompson

    David Thompson XS400 New Member

    Thanks. The throttle cable is clean and working and I know it was running lean before the head gaskette was replaced. I guess I'll need to pull the carb and check it.
  4. David Thompson

    David Thompson XS400 New Member

    From what I have read, it sounds like the diaphragm is the culprit.
  5. David Thompson

    David Thompson XS400 New Member

    Hey all, I was able to get new parts to rebuild the carbs and I remove them and low and behold there were small holes in each diaphragm so THANK YOU xschris for leading me in the right direction. Ok my problem now is that I cannot figure out how to replace the diaphragms. I have seen people break the plastic parts but the new diaphragms are original OEM so they are not replacement ones and I am afraid that if I do this, it will destroy them completely. I have tried to pull the old ones out but they are sandwiched in so tight they rip at the bottom and don't come out clean. I have seen people patch the holes but now I cannot on one because it is ripped off. Last thing I saw people do was to grind down the lip holding the plastic in place enough to pop them off the cylinder. None of these examples use the Maxim 400 carbs so I am struggling with how to fix these. Anyone out there replace just the baffles on their Maxim 400?

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