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1976 bs38 pilot screw

Discussion in 'The Garage' started by anthony44, Sep 14, 2020.

  1. anthony44

    anthony44 XS650 Enthusiast

    From what i heard, the 1976-77 bs38's dont have an o-ring and little washer against that sping on the pilot? Im having a problem where the bike hesitates and breaks up down low when its accelerating. I noticed the pilots looks like they had a tiny bit of gas around them as i was trying to adjust them. Would this cause the problem im describing? Any fixes for this? thanks
     

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  2. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, no little washer or o-ring, just the spring. The '77 and older carbs are a bit cruder than the later ones so it wouldn't surprise me if they seeped a little fuel around the mix screws, but I doubt that's the cause of your problem. I see from your avatar pic that the bike has straight pipes, probably pod air filters too? Have the carbs been re-jetted for those mods? If not then that's probably the reason for the break-up and hesitation.
     
  3. anthony44

    anthony44 XS650 Enthusiast

    Ok then im not gonna worry about that pilot screw. I have 2 xs650s and the one in my profile pic has bs34s and that bike runs perfect. This other one i have has a set of bs38s i put together with misc parts but now that im thinking of it, i may have not put the vm22 pilots in. Just noticed the * in the jetting chart that says it takes that vm22. I gotta recheck. It runs great and idles perfect and goes through the gears but hesitates down low then picks up once the rpm goes up. Gonna have to order some vm22s of various sizes and play with it. I hate taking these carbs off!!! Any way to take off float bowl without removing carbs on these bs38s? Or do i gotta take em off everytime to do the jets
     
  4. 5twins

    5twins XS650 Guru Top Contributor

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    No need to remove the carbs, just drop the bowls using a stubby screwdriver. The main and pilot jets mount in the bowls on the BS38s. You'll want at least one size larger pilots (27.5), but might need two (30). Stock size in the '76-'77 carb set was a #25. And yes, you need the VM22/210 type.

    You said you put this set together from parts? Are the needles and needle jets correct (4M1 needles, Z-8 needle jets)? They could be the cause if incorrect because besides controlling the midrange, they will effect the idle to midrange and midrange to main transition areas. Also, are the float bowls '76-'79? Earlier bowls were made to take the BS30/96 type pilots.
     
  5. anthony44

    anthony44 XS650 Enthusiast

    Great thats good to hear. I did make sure the needle is the correct # and got that z8 needle jet too. The needle is the type that has that little donut shaped plastic holder btw. Cant find much info on that little donut spacer thing but im sure you know about it. What years used that little donut on the needle assembly? Pretty sure there is a little washer on em too but i dont know until i take em apart tomorrow
     
  6. 5twins

    5twins XS650 Guru Top Contributor

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    Up through '77, the needle was held in the slide by a large plastic disc. The disc was simply held in place by the large slide return coil spring .....

    [​IMG]

    The '78-'79 carbs got a more complicated set-up. There was a small plastic spacer on the needle but the needle was retained in the slide by a golf tee shaped plastic part held in by a snap ring. The needle also got spring-loaded with a little spring. This held it more stable and kept it from bouncing around like the older one can. It smoothed out the carb operation particularly in the lower RPM range .....

    [​IMG]
     
  7. anthony44

    anthony44 XS650 Enthusiast

    Nice thats the exact pic ive been looking for. That 78-79 slide is what i have in the 76 carbs. i have that exact spring loaded setup with the thick plastic spacer ring and thin metal washer below the e clip with that needle retainer plate and all that. So will this slide setup work with the 76-77 carbs if i make sure it has the vm22 jet and all the specs of the 76 that is on the chart?
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    Well, it might work if used with it's matching 5O2 needle. The needles in the older slides mount lower so are shorter. The 5O2 needle is longer but needs to be because it mounts up higher in the '78-'79 slide due to the little spring on it. But both types of slides, when used with their correct needles, end up having about the same amount of needle hanging out of the bottom of the slide and sticking down into the needle jet.

    I don't think the set-up you've got is going to work, using the '76-'77 4M1 needle in the later slide. Since it sits higher and is a shorter needle, it's going to run too rich. That's probably what's causing your midrange issues.
     
    anthony44 likes this.
  9. 5twins

    5twins XS650 Guru Top Contributor

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    Let's talk a little bit about Mikuni needles. They come in different series and the first number in their code denotes which one they are. Each series falls in a 10mm length range. 4 series needles can be between 40 and 50mm long, 5 series between 50 and 60mm, 6 series between 60 and 70mm, etc. Here's a pic of the various needles used in the BS38s. '77 and older carbs with the old style slides use shorter 4 series needles. The '78-'79 carbs, due to their redesigned slide that mounts the needle higher, use a longer 5 series needle .....

    [​IMG]

    Notice the big difference in length, probably a good 6mm, between the 4M1 you're trying to use and the 5O2 that's supposed to go in that later slide. Now, considering the fact that needle clip positions change the needle height by 1mm, that's a huge difference in length. It would equate to running the needle with it's clip moved 5 or 6 slots. Just moving the needle one clip slot can make a big difference in how the bike runs so just imagine what moving it 5 or 6 might do, lol.

    Now let's talk about the needle and needle jet combos used. Each needle was paired with a certain size needle jet, and the rest of that carb's jetting was tailored to work with that. The 4M1 needle was mated with a Z-8 needle jet. This was the largest (richest) used in any of the 650 BS38s. Because of this rich midrange, the mains used were quite small, 122.5 in the States and as small as 117.5 in some European countries. By comparison, the 5O2 needle got mated with a Z-2 needle jet, the smallest (leanest) put in any of the BS38s. Because of this leaner midrange, the '78-'79 carb set got 135 mains, the largest put in any 650 carb set.

    So, where am I going with all this? Well, to get that '78-'79 slide to work in the older carb body, I think you're going to have to use the 5O2 needle for sure, but you may also need to go with the rest of the '78-'79 jetting specs - Z-2 needle jet, 135 mains, and 27.5 pilots. At least to start anyway, then re-jet for any mods basing that on having the '78-'79 jet sizes as stock.

    Needle taper may come into play here too. The 4M1 and 5O2 needles appear to have a similar taper but the tip of the 4M1 is fatter. That means using the smaller tipped 5O2 in the Z-8 needle jet would probably give a richer midrange than the 4M1 would. I'm not saying this wouldn't work and in fact, it may be beneficial with mods. You'd just have to try it and see.

    [​IMG]
     
    anthony44 likes this.
  10. anthony44

    anthony44 XS650 Enthusiast

    Great info. Super useful. Just took the slides out and I do have the 502 needles in there. The 76 needles were shorter and silver in color. I still have em in a parts bag. These 502 needles poke out just past the needle jet, the others would have been way too short so That mistake is ruled out. Im about to get a stubby screwdriver and check if i put the vm22 in there now.
     

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  11. 5twins

    5twins XS650 Guru Top Contributor

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    There's still the question of whether the taper on the 5O2 needle is going to play well with the Z-8 needle jet. I guess running it is the only way you'll find out.

    Something else that may be affecting your set-up is how much the needle hangs out of the bottom of the slide. The 4M1 in it's proper old style slide and set in it's middle clip slot hangs out about 33mm. The 5O2 in it's newer style slide and set in it's middle clip slot hangs out 34mm. So, that's like a one step leaner setting. It could be the reason for your flat spot. Maybe raising the needle one step, making it richer (and a match to the 4M1 exposed length), will fix your hesitation.
     
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  12. anthony44

    anthony44 XS650 Enthusiast

    I see a #2 on that needle jet so i think its right. Found a big problem too, the small washer that was supposed to be under the e clip, was ontop of it. This made the needle sloppy and loose and not even have any spring action to it. Just reinstalled em right and about to put em on. It has 140 main jets too so those should be good
     

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  13. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, without the washer under the e-clip, it probably dropped right through the spring and was sitting too low (lean). Yes, that looks like a Z-2 needle jet. The rest of your jetting sounds good. I see a #30 pilot (correct VM22/210 type too) in one of your pics. That's one up from the stock '78-'79 27.5 and should be good. A 140 main should be OK too, at least in the ball park. I assume you have the "pipes and pods" mods? I run the 30 pilot and 140 main in my '78 carbs. I have an aftermarket, freer flowing exhaust but it has mufflers. If you have straight pipes then eventually you might want to try 142.5 mains to see if the bike runs any better on the top end and in the higher RPMs.
     
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  14. anthony44

    anthony44 XS650 Enthusiast

    Yep it has brand new uni pod filters and the stock pipes are cut so basically a straight pipe. This has to solve the problem! Gonna get the rest of it together now and try to start it.
     
    gggGary likes this.
  15. anthony44

    anthony44 XS650 Enthusiast

    It runs perfect now. Had a hanging idle at 1-1/2 turns out so i made the mixture adjustment 2 turns out on each and it solved it. No more hesitation or stumble. Idles very smooth and accelerates perfect. Carbs definitely in sync. Almost every part of this bike is from different bikes but it all works perfect and starts first kick thanks for all the expert input!
     
  16. 5twins

    5twins XS650 Guru Top Contributor

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    The mix screw setting spec for the '78-'79 carb set is 2.25 turns out so it sort of makes sense that you might need near that being that you're running that carb set jetting specs. I think the leaner Z-2 needle jet may be partly responsible for needing the bigger mix screw setting.

    Glad to hear you got it sorted.
     
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