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Flat Spot off Idle

Discussion in 'The Garage' started by freddy3, Sep 7, 2020.

  1. freddy3

    freddy3 XS650 Addict

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    Your re-bushing was brilliant, but a bit beyond my capabilities here. Any thoughts on my idea of tearing out the old seal and just inserting one of the rubber seals into the existing bushing?

    Following your guide above, I could probably lever the bushing/seal outta there if I had to, but then what?
     
  2. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    I think that Mailman just scraped the old seal out, cleaned up real good, plopped a new seal in there...
     
    freddy3 likes this.
  3. freddy3

    freddy3 XS650 Addict

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    Like so...
     

    Attached Files:

    TwoManyXS1Bs likes this.
  4. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Does the new seal set deep enuff?
     
  5. freddy3

    freddy3 XS650 Addict

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    The new seal sits just inward from flush. The seal cap/cover fits fine.
    I haven't done the final assembly (because I'm awaiting a set of new throttle valve screws), but the test-fit (with the nut just finger-tight) has everything fitting smoothly.
     
    TwoManyXS1Bs likes this.
  6. 5twins

    5twins XS650 Guru Top Contributor

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    What I've started doing is packing the new seals full of red rubber grease after install. I'm hoping maybe it will help keep them soft and pliable. It certainly doesn't seem to hurt them at all.
     
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  7. freddy3

    freddy3 XS650 Addict

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    As long as the grease doesn't break down the rubber (over time), I can't see any harm. That said, the seals I just replaced were OEM (i.e., nearly half a century old), contained no grease that I saw and only just recently gave up the ghost. So I can't imagine what you'd gain by adding grease where none was spec'd.
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    Red rubber grease is specifically made for use on rubber and won't harm it. It's not petroleum based it's vegetable oil based.
     
    TwoManyXS1Bs and freddy3 like this.
  9. freddy3

    freddy3 XS650 Addict

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    Update...

    With everyone's HELP, I've got both carbs back together and even reduced the throttle shaft side-shake a tad (added a thin washer between the end of the throttle shaft and seal). Everything's buttery smooth, to spec and with no wobble in the throttle shafts. The only possible issue I see is that one of the piston/diaphragms drops slightly faster than the other (~7 seconds vs ~15 seconds to bottom) when testing the vacuum (raising and dropping the piston/diaphragms with the oval air port covered). So, just to dot all the i's, I ordered two new piston/diaphragms so the vacuums in both carbs'll match.

    Also, I'm still awaiting one Mikuni needle (for some strange reason, I only ordered one last week), but I don't see any difference between the proper Mikuni and the needles I've been using from Mikes XS for the past 20 or so years, so I doubt it'll make much, if any, difference. Still, considering how many times I've had these carbs apart to solve this flat spot, I reckon it best to cover all the reasonable bases, if you know what I mean.

    So, presuming it doesn't rain, I'll try to get the re-rebuilt carbs back onto the bike and tested this evening, reporting back the final result.
     
    TwoManyXS1Bs likes this.
  10. freddy3

    freddy3 XS650 Addict

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    Update...

    The flat-spot problem's solved...Thanks to ALL!

    However, now, I think the right cylinder (viewed from the driver's position on the seat) is running too rich, but I haven't been able to affect it via the air screw adjustment. Regardless of whether the air screw's set to the recommended default (3/4 turn out) or where the idle sounds smoothest (~1.5~2 turns out), there're two issues:
    1) the engine RPMs're slow to return back down to idle. This exhibits itself mostly when revving in neutral or letting the throttle go whilst at cruising speed...the engine RPMs take several seconds to slow back to idle speed; it acts like the advance springs're shot (the advance springs're two months old, clean/freely rotating and the idle was fine until today).
    2) the idle and deceleration're both sputtery. Not quite back-firing, but not smooth and the spark plug's wet/black (left spark plug's clean and normal).

    The only difference between the two carbs is that the RIGHT carb--the side I'm now having idle troubles with--has the original Mike's XS needle, which looks mint (and is the same length, diameter and taper as the new Mikuni needle in the other carb) and has been in there for several years without any problem; and it's the carb with the better vacuum (it takes the piston ~15 seconds to drop when lifted and let go with the air port covered).

    Thoughts?
     
  11. freddy3

    freddy3 XS650 Addict

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    Update...

    Today, I removed and rebuilt the right carb for the fourth or fifth time. Unfortunately, I didn't find anything amiss from the previous rebuild, so it's essentially still at OEM specs. When I started the bike up, the right cylinder's no longer just "sputtering"....now it's clearly backfiring during idle, or when I try to maintain 2500 rpm out of gear, or when decelerating (driving). Out of desperation, I cleaned and reinstalled the previous NGK spark plug (the one that was in there for the past five or six years), but no change.

    Again, the only thing that changed between when I had severe flat-spots and now (with the backfiring, overheating and hanging idle) are the carb rebuilds.
    Am I correct that setting the needle clip to the bottom (fifth) indent enriches the fuel mix? If so, can I do that without changing the main OEM jet? I think one of the many XS "idle hovering" or "backfiring at idle" articles I read said something about having to match the main jet to the needle setting. But, at this point, I've read/tried so much that I'm just running in circles...Ugh.
     
    Last edited: Sep 17, 2020
  12. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, lowering the clip (which raises the needle in the slide) richens the mix. No, you don't have to match the main jet to it but sometimes it needs changing to smooth out the transition area between midrange and main. But usually, this is done in the opposite direction after installing larger mains. They bleed over into the upper midrange and make it too rich, causing stumbles, so you lean the needles a step to compensate.

    It sounds like you've got an air leak and it seems like you're making it worse every time you remove and install the carbs. Maybe one of the manifolds is splitting, more and more as you strain it with carb removal/installs.
     
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  13. freddy3

    freddy3 XS650 Addict

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    I think you're right about the air leak..
    Do you mean the carb joint (Yamaha's term)--the part (attached to the engine with two screws) the carb slides into on the engine side that's held in place with a metal band (that's secured with a Philips head screw)? If so, I did take a quick look at it when the flat-spot first occurred and didn't see anything obvious. Are you thinking a hairline crack that may require a closer inspection or something obviously broken?

    The band that fixes the carb onto the joint (manifold) has that single Philips head screw and I've been tightening it with a standard JIS screwdriver. So, unless it's specifically designed to be "snug" only, I can't imagine it's been over-tightened and I replaced it (the joint) when I did the last carb rebuild several years ago.
     
    Last edited: Sep 17, 2020
  14. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

  15. freddy3

    freddy3 XS650 Addict

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  16. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Ok, those "air ports" are the tiny air bleed ports at the intake bell, the rear of the carb, around the outer edge of the bell...
     
  17. freddy3

    freddy3 XS650 Addict

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    This one?
     

    Attached Files:

  18. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    No, that's the "moon" vent to the diaphragm.

    Here's a pic of my XS1B's "bleed air vents".
    71XS1B-Carbs-AirJets.jpg

    5Twins has better pics of these "air jets"...
     
  19. freddy3

    freddy3 XS650 Addict

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    Got it! I'll try robinc's pilot air port test tomorrow.

    Although I can't imagine the relatively recent manifolds on my bike are as dry/cracked as robinc's, since I think I'm dealing with a significant air leak, I'll definitely remove and inspect them carefully.
    Many thanks (again), TM!

    p.s. In case I need new manifolds, do you know of a source for NOS or good/new aftermarket manifolds?
     
    TwoManyXS1Bs likes this.
  20. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

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