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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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    Hi All!
    It's been several years since the trusty steed's last let me down, but that day's come.

    Anyway, here's the problem...
    AFTER a rebuild of my stock 1972 XS2 carbs (rebuild kits from Mikes XS), the bike idles perfectly, but goes flat (severe hesitation) when engine speed is increased beyond idle. This happens BOTH under load (i.e., attempting to ride) or just revving the engine out of gear. If I engage the cold-starting choke (enrich the fuel mixture), it'll then rev properly out of gear (no load), but still goes completely flat under load (attempt to ride).
    So I think it's safe to say that something's causing one or both carbs to be running too lean. But I've been through the carbs so many times since the rebuild (which I've done many times over the years without issue) without finding any problems or making any improvement.
    At this point, I'm thinking it's gotta be something simple/obvious that I'm just too worn-out/confused to see.

    A bit of background...
    The bike had been running perfectly until this year. Due to scheduling and life, the bike's spent the past year sitting in the garage, without use (tank full of premium unleaded, petcocks off). When I finally took it out last weekend, it'd developed an INTERMITTENT flat spot under load (i.e., idled and revved great, but INTERMITTENTLY hesitated during acceleration and/or when maintaining ~40~50 mph). Oddly, I noticed that, again INTERMITTENTLY, if I let it sit and idle for ~25 minutes, the idle would suddenly increase to ~1,500 and become perfectly smooth. Then, the flat spot was GONE. A few nights ago, after this happened, I took the bike out for a long drive and it ran perfectly! But, the next day, the flat spot was back. So I figured there was either tarnish or debris blocking one of the carb fuel journals and Seafoam might clear it. Unfortunately, the Seafoam didn't help.

    As this flat-spot problem had happened previously and was remedied by a carb rebuild (with kits from Mikes XS), that's what I did this time. AFTER the rebuild, the bike started right up with a perfect idle, but it was now CONSTANTLY going flat--no more INTERMITTENT!--at anything beyond idle.

    Just to cover all the bases....
    I just replaced the NGK plugs with same NGK plugs, but that made no difference. I've checked the petcocks/fuel lines, but both're clear and flowing great INTO the carbs. I've replaced the float bowl fuel balance tube, which was clear.

    Again, the only thing that changed between the INTERMITTENT and CONSTANT hesitating was the carb rebuild. So it's gotta be carb-related. This sounds like a classic fuel starvation issue, but I'm completely confused as to what to do now?
     
    Last edited: Sep 7, 2020
  2. freddy3

    freddy3 XS650 Addict

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    A couple of additional detals...
    The diaphragms are perfect and I set the circlip on the main needle to the default position (fourth indent), as was the previous setting. So, other than swapping old carb parts for new, everything's the same.
     
  3. grizld1

    grizld1 Grumpy old man Top Contributor

    1. I'd guess, given your level of experience, that you drained all the old fuel out of the tank and poured in fresh stuff.
    2. Never underestimate the ability of the ignition to fool you into blaming the carbies. A longish sit in humid conditions can corrode breaker points. Check idle timing; if it's too far retarded a flat spot can result. If you find idle timing too far retarded, check timing at full advance as well. If idle timing can't be set correctly without going past 40* BTDC at full advance, the advance governor (AKA automatic timing unit) is in need of attention (note: new springs will not correct the advance curve).
    3. Components in carb kits are often questionable. Your carbs take 4JN19 needles. If the kits contained needles that were either longer or fatter, that could cause your lean condition.
    4. The next item that comes to mind is float valves, which are usually included in rebuild kits. When new float valves are installed, float level must be inspected and if necessary adjusted. If float level is too high fuel level will be too low, possibly causing a lean condition. Your floats should be set at 24 mm.+/- 1 mm.
    5. Did you tune the pilot mixture screw setting or just turn the screw out to whatever Clymer, Haynes, etc. wrote?
    6. Did you install new pilot jets? If so be sure that they're #42.5, series BS30/96 (small hole in the thread end of the jet, not the barb end).
     
    GLJ likes this.
  4. freddy3

    freddy3 XS650 Addict

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    Yup and thanks for the quick and useful reply. More below..


    I checked the ignition/timing. All Ok.


    Like minds. In the mid-70s, I trained as a Jaguar tech, so I've gotten into the habit of always comparing what comes out with what goes in. In this case, the needles were the same size and contained the same stamps.

    I guess I'll have to pull the carbs back out again and check that. I've been sitting here wondering if it could be the float level, but didn't measure it because I've never had it change after any previous carb rebuild, especially since the old float valve was pristine and appeared (by sight) to be the same length as the replacement valve.

    Yes. I've got both the OEM Yamaha Service Manual and the Clymer book. If I recall correctly, they both recommend 3/4 turn for the mixture screw. Does that sound right? However, I've always (for the past 40+ years) done it by ear. That is, I start at 3/4 turn and then adjust for smoothest idle without fouling the plugs, which's generally ~1.5~2 turns out. I think both were set to ~1.5 turns for the past several years.


    Yes. The rebuild kit part numbers matched what I took out. These are what I took out (the needle's stamped "Y22")...
     

    Attached Files:

  5. grizld1

    grizld1 Grumpy old man Top Contributor

    The correct needle for your carb set is 4JN19, not Y22. The Y22 from generic kits is a known source of trouble. You'll have much better luck if you buy individual parts made by Mikuni rather than Chiwanese brass from generic kits. Niche Cycle Supply has a good selection of genuine Mikuni parts at decent prices.
     
  6. freddy3

    freddy3 XS650 Addict

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    Point taken. According to my Yamaha parts book, 4JN19-4 is the Yamaha needle part number. So I just ordered one from Niche Cycle. Thank you!

    However, Y22's the needle that's been in there for the past several years and, until last week, the bike--and the needle--worked perfectly. Although I get where you're coming from, I really do hesitate to throw a new variable, even if it's technically correct, into the mix at this point. At least until I've exhausted other reasonable possibilities.

    Your mention of the needle jogged a thought. The rebuild kit did not include a replacement main nozzle (for the needle), so I didn't remove/inspect it when I rebuilt the carbs. If my memory serves me, I did replace that nozzle in previous rebuilds and now I'm wondering if the port holes in the side of the nozzle might be clogged?? I did inspect and blow carb cleaner and air through the center of the nozzle, so I know that was definitely clear. But I didn't check those port holes..

    Anyroads, when I was cleaning the parts, I remember debating whether to remove and clean it, but since there wasn't a replacement, I decided to leave it alone....just in case it snapped or bent or something. Of course, with hindsight....
     
    Last edited: Sep 7, 2020
  7. 5twins

    5twins XS650 Guru Top Contributor

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    The Y-22 generic needle matches the original 4JN19 in length but I can't say about the taper. By the way, that "-4" on the end isn't part of the needle designation or I.D. number, it simply indicates what clip slot it was run in stock (#4 from the top). Even if the needles aren't the source of your problem, I think you are better off with the right ones.

    The rows of tiny holes down the side of the needle jet are air bleed holes so if plugged, you wouldn't run lean, you'd run rich.
     
  8. freddy3

    freddy3 XS650 Addict

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    Thanks for the clarifications!
     
  9. grizld1

    grizld1 Grumpy old man Top Contributor

    The last thing I can think of is checking for air leaks at the carb boots, throttle shaft ends, and air box, but you've probably been there and done that.
     
    freddy3 likes this.
  10. freddy3

    freddy3 XS650 Addict

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    There is this seal on the choke lever that's torn (photo taken BEFORE rebuild). Could this be the issue?
     

    Attached Files:

  11. 5twins

    5twins XS650 Guru Top Contributor

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    I don't think so. That's just a dust seal.
     
  12. freddy3

    freddy3 XS650 Addict

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    Update...

    Since, in the ~45 years I've had the bike, I've only ever replaced the throttle valve shaft seals (not the shaft or valve), a friend suggested I just replace the entire throttle valve assembly--throttle valve, shaft, screws, bushings and seals. So I've ordered a Mikuni throttle valve kit and, while everything's apart (again), I'll exchange MikeXS's Y22 main needle with the Mikuni 4JN19 and check the float settings. Other than the diaphragms, which're pristine and appear to be holding vacuum, everything else within the carbs'll be new Mikuni parts. Since the carb bodies've been cleaned and inspected--no cracks, holes, dents, etc.--if this doesn't make the carbs sing (in tune), I'm gonna be REALLY surprised. It'll be about a week 'til all the incoming parts arrive, so, as always, I'll report back.
     
  13. 5twins

    5twins XS650 Guru Top Contributor

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    I'm surprised you were able to get those parts. I've never heard of them being sold before. Got a link to them?

    Your early carbs take a #125 butterfly plate. The '74 and later BS38's take a #120. The numbers probably indicate some sort of sizing but I'm not sure what. I'm also not sure if the two can be swapped.
     
  14. freddy3

    freddy3 XS650 Addict

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  15. grizld1

    grizld1 Grumpy old man Top Contributor

    I don't know how the throttle plate numbers correspond to actual dimensions, but as in slide type throttle valves, lower numbers indicate less cutaway and thus a richer pilot circuit mixture. I hope the rebuild works for you, but if the throttle shafts are worn, the carb bodies may be wallowed out.

    Edit: That kit is for the Super BN carburetors used on jet skis, etc., not for older BS series carbies. Have you checked the throttle shafts for play?
     
    Last edited: Sep 8, 2020
  16. freddy3

    freddy3 XS650 Addict

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    I didn't feel any unusual play--end shake or looseness--when I worked on them, so I can't imagine the shaft openings're badly ovaled/worn. That said, isn't normal shaft "play" what the seals're meant to address?
     
  17. grizld1

    grizld1 Grumpy old man Top Contributor

    Nope, the seals aren't there to limit play any more than an oil seal is present to limit play. If there's excessive play the seals can't do their job, which is to keep air from being drawn in around the shaft and to keep out dirt and grit. If you don't feel radial play, you don't need to be concerned, just test to see if the seals are holding and replace them if they're not. Seals from Mike's XS used to be and probably still are low quality repops. Get the real thing from Yamaha if you need it, it's a current part.
     
  18. freddy3

    freddy3 XS650 Addict

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    Sorry, I may not've explained myself well. I think we're saying the same thing.


    As mentioned above (based on the Mikuni needle recommendation), I ordered the Mikuni kit. Are the Yamaha kit parts better than Mikuni? I kinda reckoned they'd be the same (i.e., Yamaha simply re-packages Mikuni parts). No?
     
  19. grizld1

    grizld1 Grumpy old man Top Contributor

    Yamaha and Mikuni parts are the same, but the kit you ordered is not for early round slide BS38 carburetors. The shaft seals may be the same, I don't know.
     
  20. freddy3

    freddy3 XS650 Addict

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    So the throttle shaft in the Mikuni kits I ordered won't fit the XS2 OEM carbs?
    Marvelous. I thought I did well.
     

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