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80 XS400 high idle

Discussion in 'The Garage' started by mikeasis, Mar 29, 2019.

  1. mikeasis

    mikeasis XS400 Member

    Hows it's going everyone. I recently picked up an 80 XS400 special that was running rough. The intake manifolds were cracked as well as the boots from carb to filter. I replaced the intake manifolds and found NOS boots on Ebay. Pulled the carbs and blew them out. Drained the tank and put in fresh rec fuel and I'll be darned fired right up and idled nicely. Now it seems to want to idle up like the choke us on. Any tips appreciated
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    Sync carbs with manometer. Stock air filters and exhaust? Non-stock setups will cause tuning issues.
  3. mikeasis

    mikeasis XS400 Member

    sorry for the late reply. stock carbs with stock air boxes and pipes. mixture screws still have the plugs over them so they have never been messed with.
    I used compressed to blow out the jets and passages. The diaphrams looked good. I put a flashlight behind them to help look for pin holes or tears.
    I plan on checking the valves this weekend. The odometer shows something over 14,000 miles.

    Do you think I need to pull the carbs again for another cleaning and the mixture screw plug removal?
  4. xschris

    xschris a lifestyle not a trend Top Contributor

    Yes, I would remove the pilot mix screws caps and set them to at least 3 turns out from seated to start with. A valve adjustment and then a carb sync with a manometer is also a must after. Post some pics of the bike so we can see what your working with.
  5. mikeasis

    mikeasis XS400 Member

    Not a very good pic. This from the previous owner. Screenshot_20190227-054638_Facebook.jpg
    cbrfxr67 and buztabuzt like this.
  6. mikeasis

    mikeasis XS400 Member

    The chrome is pretty nice with no real putting. The seat needs recovered or replaced and she needs the forks rebuilt. Still need to figure out an electric start issue, it will fire right up if I jump the solenoid but right now my main focus is gettin it to run right.

    This is my first time with a multiple carb setup, a little intimidating but looking forward to it. Looks like i'll have to add a manometer to the tool list.
  7. you can make a device to function as manometer if you wish for under $5 worth of parts. It's on the youtubez and googlies.

    electric start is probably relatively pain free by taking apart the right handle controls. Either it is not grounding, or more likely the starter button flange has broken away. Here's a pic of my busted button that replaced with a 2fastmoto ebay purchase

    wise to focus on carbs first tho :)
  8. xschris

    xschris a lifestyle not a trend Top Contributor

    Those handle bars look like they are coated. The stock ones would have been metal chrome. The right hand control needs to be ground to the left hand control which happens through the bars. Most times this is the e-start issue.
  9. mikeasis

    mikeasis XS400 Member

    is this what your talking about? or an assembly?
    Last edited: Apr 5, 2019
  10. mikeasis

    mikeasis XS400 Member

    Good catch on the handle bars I do belive they are coated. I probably wouldn't have thought of that. the previous owner stated the electric start worked, at least intermittently until he changed the bars. I think he was wanting to make a scrambler style bike and put these endure style bars on it.
  11. mikeasis

    mikeasis XS400 Member

    Finally got around to adjusting the valves, Ive done a lot of diesels, id rather do a rear engine bus any day than a bike lol. The valves where pretty tight with the right intake so tight I couldn't get a feeler gauge in there. noticed the cap (unused vacuum port)on the left intake manifold was loose so I replaced that as well and changed the oil + filter and installed fresh NGK plugs. Fired it up and it died, cranked slowly and restarted. Sounded quieter but it didn't run long enough to get warm. The battery was dead so I put it on charge. I have the mixture screw plugs out but have not made any adjustments I figured I see what happens with the changes I made.

    I think ive got a line on a set of stock bars off a 79.
    Last edited: Apr 9, 2019
    cbrfxr67 likes this.
  12. mikeasis

    mikeasis XS400 Member

    Tried restarting and found its flooding out. pulled the carbs for another cleaning. Thinking I may order new float needles to see if that cures it, Im not aware of it flooding out before and thinking after disturbing the float needles they may not be sealing. Do these brass floats go bad often? They look ok not corroded up.

    Where would I hook up a manometer set? Im thinking the intake manifold ports. I ask because when looking at manometers it seems the 6mm Yamaha adapter is sold separately.

    Ok stupid question. Is there anyone out there that rebuilds our carbs, actually fully disassemble replace shaft O-rings ect?
    Last edited: Apr 12, 2019
  13. Correct on connecting manometer location after you get it otherwise running.

    What's your float height? Can find documentation here on proper measurement and assembly, but 26mm seems generally acceptable.

    Sadly seems not uncommon for some flooding. I am battling now, not only floods engine but pisses out the left Air box
  14. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    I would replace the needle and seat since they both wear together. If the little spring on the needle compresses at all when the float rests on it, then it needs to be replaced. A visible ring on the needle also makes the needle bad. Brass floats have been known to develop pin holes and also fall apart. Testing for leaks is easy, just hold the floats under water and youll know. One of the floats in my carbs, fell apart. One of the float canisters detached from where it was soldered on to.

    Both issues can easily be fixed with some gentle soldering.

    This is also the reason why we have fuel shut offs and I always make sure to turn the fuel off when I park.

    Floats can stick sometimes but I havent had this issue.
    buztabuzt likes this.
  15. mikeasis

    mikeasis XS400 Member

    None of my measuring devices have the correct metric readout LOL. Carbs are off the bike again so ill give them another look over and pay better attention to the needle and seats and try to put the floats in some water after checking the heights.

    Can the enrichment circuit plungers (choke) come out without separating the carbs?
    Last edited: Apr 14, 2019
  16. From memory, it requires separating the carb bank
  17. xschris

    xschris a lifestyle not a trend Top Contributor

    Choke plungers can be removed without separating carbs. Just remove the choke rod and hardware then unbolt the plungers. Just be careful not to loose the small balls and springs the goes behind the rod in the carbs.
    buztabuzt likes this.
  18. mikeasis

    mikeasis XS400 Member

    My yamaha manual states for the XS400G (I think i have the model right) float height should be set to 27.3 mm give or take 0.5 mm. Both look very close to 26mm . Seats look clean as well as the needle end but there a wear ring on the needle. I wonder if it's not sealing correctly there. Is it ok to test the floats in water?

    The bike is cleaning up nicely. Chrome for a 38 year old machine is looking good. No Idea why the pics posted up twice.
    20190414_185133.jpg 20190414_185448.jpg 20190414_185505.jpg 20190414_184240.jpg 20190414_185133.jpg 20190414_185448.jpg 20190414_185505.jpg 20190414_184240.jpg
    Last edited: Apr 14, 2019

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