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78 xs 400 E rebuild and modify

Discussion in 'The Garage' started by Tyler Shiells, Mar 30, 2017.

  1. Decided to pull the carbs apart and clean em.
    Is there suppose to be any kind of gasket or ring with the float needle and float screw?

    I'm also changing all the Philips screws to Allen heads. I hate those Philips head bolts on carbs.
  2. And why is there big white stoppers on top of the pilot screw? So ppl can't adjust it past a certain point? Does anyone remove those?
  3. xschris

    xschris a lifestyle not a trend Top Contributor

    The needle valve should have a gasket that seals it when you screw it down. Yes you can remove the caps for the idle mix screws.
  4. Figured so. Prbly need one under the pilot screw too? Nothing there either.
  5. xschris

    xschris a lifestyle not a trend Top Contributor

    The 77-79 sohc xs400 don't use any washers or o-ring under the idle mix screws. Just make sure the tips on them are nice and pointy. They tend to brake off into the carb bodies. If they are worn, replace them as they will brake the next time you seat them to turn them out for tuning.
  6. Thanks. Good to know.
  7. Wow. Those needle valve washers are way over priced. Does anyone know the exact dimension and if a regular hardware store washer would work?
  8. Piston rings and gaskets set showed up.
    What I didn't realize was the gasket set came with valve seals. I already ordered the Viton ones. So damn. So I'll prbly sell a set. 20170413_091857.jpg

    Calling for some cold temps Sunday. Garden will be pushed back another week. And more bike work.

    I'll borrow one of those piston ring sleeves. Is it also advisable to use a piston ring installer or gently rolling them in by hand is sufficient?
  9. The local Canadian Tire didn't have the honing tool or ring compressor so that job is still on hold.

    Carbs apart for cleaning.
    Wanted to remove the butterfly valves to remove the seals and then this happened.
    20170416_154031.jpg Stripped the screw. So on to the drill press. Damn.


    Drilled the head out. Then slowly turned the remaining piece thru with needle nose pliers. No threads were stripped so just need to find a new one of those screws.

    Also looks like one of my butterfly seals have small crack in them. Other two look OK.

    I suppose the right call is replace em all?
  10. BBS360

    BBS360 XS400 Guru Top Contributor

    They're cheap enough. Replace them all. You can still get the seals from Yamaha.
    The screws are less easy. I think there was a Suzuki part# still available.

    There are alternatives though:

    Can also get regular seals from XS650direct but there have been some quality complaints in previous threads.
  11. Thanks for that link BBS360. I'll send an email and see what price I'm looking at. Also call yamaha dealer, see their price.

    Chris, that ebay sale does have exactly what I need.

    I sent the seller a msg to see if it could be sent with a cheaper shipping cost, cause $18CAD for that to be shipped is pretty steep, especially it could fit in and envelope.

    I'm going to try the hardware store for a screw first. If not we have a specialty shop that has every damn nut,bolt, screw imaginable. (With a $10 minimum.) Reason to buy more hand cleaner or WD-40.

    I did installed a few gaskets. Both side covers. Breather cover. Sump pump.

    I also have these that came with my gasket set.
    I know where the two in the middle on the top row go but trying to figure out the rest?

    The bottom 4 are the same diameter. Top left is slightly smaller and top right is obviously small.
    Anyone with knowledge of there placement would be appreciated.
  12. xschris

    xschris a lifestyle not a trend Top Contributor

    Valve tappet covers for the bottom and the neutral switch for the top left. The tiny one top right looks like the rocker arm end o-ring on the right side of the motor but there should be two of them.
  13. Right the valve tapper covers. Forgot about those.
    I did try the top left for the neutral switch but was red so I thought no.
    I'll check the rocker arm end for that size o-ring.
    Thanks again.
  14. xschris

    xschris a lifestyle not a trend Top Contributor

    The oem neutral o-ring was red but all the after market ones are black.
  15. Got the tapper cover gaskets replaced. Neutral switch. Also Chris was right, the small gasket was for the rocker arm right side end. The other small gasket was hidden inside a valve seal. Exactly the same size ad worked it's way in there during shipping, I guess.

    Is there suppose to be a gasket between the cylinder top cover and where the cam shaft sits?

    Also new rings installed.
  16. xschris

    xschris a lifestyle not a trend Top Contributor

    The cam cover get a thin bead of yamabond sealant. I use yamabond-4 and it works great.
  17. Honed the cylinder. Straight forward job.


    Pistons installed and move smoothly.
    Wow is that a tedious job to align to pistons and slide the rings in.


    What is the proper orientation. For the cam shaft? The single hole facing the top or the 2 holes facing the top?


    The manual shows a little timing mark but I don't see that on the cam. Just either one hole or two, 180 degree from one another.

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