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Carb slides slightly askew

Discussion in 'The Garage' started by Marlin72xs, Nov 12, 2018.

  1. image.jpeg image.jpeg image.jpeg image.jpeg
    Ok. I've had a wicked backfiring problem, probably from 1978, but I never really noticed it until I installed the shorty, glass pack reversed megaphones in 2014. Anyway, at the rally, Maxpete said, "hey let's change the low speed adjustment." So we did, turned it out to 1-1/4 instead of 3/4, and guess what, It didn't backfire nearly as much, but there was an initial hesitation on take off. Screwed around with the low speed to no avail.
    Then I read the excellent carb thesis, (several times to allow it to sink in), by Grisld1 and 5twins. And I mean excellent. Great job guys!


    I have a 75 and from their charts, I pulled the larger #45 pilot jet, bought 2 , #132.5 main jets and moved my needle to the #4 slot. Yes, it's supposed to be there, but I moved it to the #3 spot back in 1978. Why, I don't know. Nothing changed back then so I left it that way.
    Long story short, The slide rubbers on my 72 were showing signs of cracking, so in June when I did the 750 thing, I used the 75 slides. They worked great, like the old ones and still with the same backfiring.
    Now I'm ready to change all the stuff in the carbs and looking thru from the back, I notice that the slides seem to be slightly askew. Was it my eyes? Did I install them wrong and crush the little tab in the rubber. No, and only a complete idiot could do that. (No offense to those who have screwed that up).
    Look at these Picts. You tell me if you've ever seen this before.

    The 1st 2 Picts are of the 72 slides. You can see light coming thru the holes on one of them.
    The 3rd and 4th are of the 75 slides. Use the horizontal line on the slides as a reference and in your mind, draw a perpendicular line straight up thru the 2 holes to the tab on the rubber bellows. What do you see?

    Yes, it's crooked on the 75. It still runs fine. But what gives from Yamaha? Has anyone ever noticed this?

    But the biggest question is, (dramatic pause), can the bellows be turned? Has anyone ever tried to? At $185 a pop, I wouldn't want to break them.

    Last edited: Nov 12, 2018
    gggGary likes this.
  2. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Good question prolly only answered fer sher by an A-B test with a set of slides that index "perfectly". A gut feeling is they are "close enough" to not bother anything. IIRC the only one I ever found way off was on a Yamaha venture. That one was loose, I reindexed, glued and it never bothered me again til it went off to a new owner. :shrug:
    I have found more than one slide installed wrong, and have been able to uncrease and reuse them successfully.
    Deteriorated rubber is not a huge problem in the upper midwest. Rust-another story.
    Marlin72xs and TwoManyXS1Bs like this.
  3. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Good eye, Keith. Agree with the suspicion that misaligned slides could affect off-idle response. Have a look at post #3:


    Edit: The misalignment may have different effects on early "raised floor" BS38s, versus later "lowered floor" carbs.

    When I replaced my diaphragms, I carefully turned the slides on the lathe, slowly removing the top retaining plastic ring. I wanted to see how they were held in place. Clamped pretty tightly between the 2 plastic rings, I would say.


    The bottom 2 diaphragms in that pic are my undamaged removed originals.

    The part of the diaphragm sandwiched between the 2 plastic discs is quite thin, somewhat delicate. I can't envision those being easily twist/rotated to get the slides aligned.

    However, these guys offer replacement diaphragms that retain the plastic rings.


    I don't know if their diaphragms are cloth-backed, like the originals.

    Their 2 tutorials show how they replace those things.


    The way they work the diaphragm in and out from between the 2 retainers might give you ideas on how you could "reclock" your diaphragms. How good are you at brain surgery?

    Watching with interest... :popcorn:
    Last edited: Nov 13, 2018
  4. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    From the BS34's on the bench, funny I never noticed before, problem solved!
  5. Well, the slight low end bog only surfaced when I set the low speed to 1-1/2, 1-1/4, & 1. And, again after reading the excellent carb PDF, I realized that I have basically straight pipes, foam air filters and I've increased the displacement by 15%. I think that warrants some upgrading and rejecting of my carbs.
    Anyway, I would have to agree with Gary and say that with the crooked slides, it still ran great. Haven't gone over 6000rpm yet. Well maybe a tad over, but except for the minor bogg and the backfiring, it's fine.

    Excellent find on the rubber, Steve. Great site. $60 bucks is way freaking better than $370. But, either their video sucks, (it's a video of stills and no sound), or my I-pad can't handle it. Was that floss he was using? I will try the butter knife trick on my ripped bellows first.

    I should have my rejetted carbs back on tonight or tomorrow, but won't be able to ride until Friday. We're having monsoons here in Georgia. Urban flooding, flash floods and loads of wet slippery leaves.
    TwoManyXS1Bs likes this.
  6. Noticed what, Gary? The flat bottom slides? Ah, yes! No alignment issues!

    And Steve, I assisted Bones when he reconnected Spock's brain.
    TwoManyXS1Bs and gggGary like this.
  7. 5twins

    5twins XS650 Guru Top Contributor

    When swapping slides, I hope you retained the correct needles. The '74-'75 4N8 needles are much longer than the '70-'73 4JN19 needles. They would be way too lean if fitted to '72 carbs. The needle length change was due to the "raised floor" being lowered in '74. A longer needle was required to reach down into the now lower mounted needle jet.

    gggGary and TwoManyXS1Bs like this.
  8. Yes, I did install the 72 needles in the 75 slides. I'm guessing that the 75 needle would bottom out in the 72 carb? And again, thank you and Grizld1 for the excellent carb guide. I think I'm going in the right direction with the rejetting; what do you think? I have #135 main jets on hand if needed.
  9. 5twins

    5twins XS650 Guru Top Contributor

    Yes, for your mods you will need some re-jetting, but probably just minor changes. That's all most of these need for your typical "pipe and pod" mods, but they do need it. Usually that amounts to one or two on the pilots and one to three on the mains. Eventually, if you get several sizes up on the mains, the needles may need to be leaned a step to cure stumbling and break-up in the upper midrange under heavy throttle applications. That's typical, the larger mains bleed over into the upper midrange and make it too rich. You have to keep in mind that the 3 circuits in your carb overlap. Changing one affects a portion of the one next to it. That's why larger mains eventually muck up the upper midrange once you get big enough.

    gggGary likes this.
  10. Thanks. What do you think about my slides? Ever see any offset like that?
  11. 5twins

    5twins XS650 Guru Top Contributor

    No, I haven't. I don't know what affect that might have but maybe, as Gary said, it's not enough to hurt anything. And also like he said, testing back to back with a "straight" set is probably the only way you'll know.
    gggGary likes this.
  12. Oddjob

    Oddjob adsgads

    I imagine that you could cut off the indexing tab and align them manually so that they are straight...
    GLJ, grizld1, gggGary and 2 others like this.
  13. A very smart person once said, "If you want to find out how something works, take it apart!" Well, the butter knife ain't gonna work. As you can see, the top is rolled/knurled (?) and that simply presses it down and easily holds it in place. I guess, epoxy will work, but I'd also like to put small dings to roll out the tops. Not sure how to do that yet, with precision.

    Oh, and yes the crooked slide hits the raised floor earlier. That's why is wouldn't idle slower for me. The stops no longer touch. A quick test shows that a crooked slide doesn't hit in the lowered floor. Which brings to mind this question. Are the 75 carbs better in any way than the 72's?

    Attached Files:

    gggGary and TwoManyXS1Bs like this.
  14. Wait a minute! Yes, that's it. So simple. That's what I will do! Great idea. I'll just put a light coating of goop on it. Really, thanks.
    The dismantling was not for nought. That will be helpful when I purchase new bellows to replace my damaged ones.
    gggGary, TwoManyXS1Bs and Oddjob like this.
  15. 5twins

    5twins XS650 Guru Top Contributor

    I like the way you pressed that slide apart. An arbor press it looks like? Here's a suggestion that might work - don't press it fully apart. Just press it enough to roll that metal lip up a little and loosen the plastic rings. Then you can probably pull the old diaphragm out and install the new one. And then you could "crimp" that rolled metal lip back down to hold the new diaphragm in place.
    peanut and Marlin72xs like this.
  16. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    IMHO as rule the carbs improved through the years. But If I change from stock definitely consider linked carbs 78-79 BS38 or any year BS34. Doing that does create issues with lining up to the stock airboxes. But JRP1 and I got a set of 78-79's to "fit" into his otherwise stock 74 TX-A he's put ~1000 miles on it rigged like that.
    Marlin72xs and grizld1 like this.
  17. Yes, and I'm sure that arbor press is older than me. I will certainly give that a try. I have one more test piece. However, that aluminum appears to be hardened, and when pressing it off, it just popped. The word finesse doesn't appear in the Arbor press description.

    Any opinions on the 72 vs 75 carbs. The lowered floor on the 75's really opened up the mouth. It looks huge compared to the 72.
  18. What I said about the crooked slide hitting the floor is false. In fact you can spin the slide 360 degrees and nothing hits the floor. It hits on the lower plastic ring on the upper part of the slide. BUT, when it is turned, it seems to allow more or less air through one side or the other. While eyeballing the centering of the slide, with the butterfly open, I saw light differences on each side.
    Well, anyway, I'm putting them on right after I press, "post reply".
    Oddjob, or shall we call you random task? I cut the tabs and aligned the slides and the pressure test for both is excellent. I keep thinking, "I should have used my brain!" Thanks again.
    Oddjob, TwoManyXS1Bs and gggGary like this.
  19. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    That may call for a double-check. The JBM diaphragms don't use the plastic rings, and would set lower if so. 2 other things that could bottom the slide are the steps at the sides of the slides, just above the floor, and the projecting needle jet, contacting the recessed slide base. See if your "undiaphragmed" slide sets just as deep...
    Marlin72xs, GLJ and gggGary like this.
  20. 5twins

    5twins XS650 Guru Top Contributor

    As mentioned, the later carb sets are generally better, more refined and improved. The only issue you may encounter with the '75 set is if you need replacement needle jets. They're not available, or at least I've never seen any out there. The '70-'75 carb sets all use a Z-6 needle jet but the '70-'73 "raised floor" carbs require a longer version to reach through that thicker floor. This one is available but is too long to use in the '74-'75 carbs. The "short" version you need for them isn't available .....


    Marlin72xs and gggGary like this.

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