JNaws Carburetor Adventures

[Its been a long time]

Yep just checked part numbers on the parts diagram and it's a 75 year assembly. Different part #

75: 256-14940-00-00
79: 2F0-14940-00-00

My next question is how can I make this work, or am I better off sourcing a 79 complete assembly.

Bah.

edit: seems heiden tuning has replacements: https://www.heidentuning.com/xs650-...sembly-for-late-mikuni-bs38-carbs-detail.html

As far as I know, mixing parts from different carburettor assemblies can't work properly. I suppose it could work to a degree. But you'd need to have pretty much expert product knowledge to know what will not work versus what might work.

That's my take but there's experts here who know far more than me.

 

[JNaw]

Looking at some other posts: https://www.xs650.com/threads/my-20-year-project.50894/page-3

It looks like 5twins seemed to imply that the slides are compatible in the carb bodies, but the slide determines the needle and needle jet, which in turn determines the rest of the jetting.

This sounds like it could make sense if they used the same bodies throughout the early years and only redesigned the slides and float bowls.

It looks like I can either swap in the newer style slides and associated parts (which are a pain to source down currently) or swap in the correct needle jet and mains/pilots for that.

 

[Its been a long time]

Looking at some other posts: https://www.xs650.com/threads/my-20-year-project.50894/page-3

It looks lime 5twins seemed to imply that the slides are compatible in the carb bodies, but the slide determines the needle and needle jet, which in turn determines the rest of the jetting.

This sounds like it could make sense if they used the same bodies throughout the early years and only redesigned the slides and float bowls.

It looks like I can either swap in the newer style slides and associated parts (which are a pain to source down currently) or swap in the correct needle jet and mains/pilots for that.

Understood. I see you're in Canada so it might not help you. But I think these people in the UK have everything you need whichever way you decide to proceed, link below. I have used them and they're OK to deal with. Good luck.

https://nrp-carbs.co.uk/

 

[JNaw]

@5twins id like to confirm some speculation before I start dumping money:
1: The 4n8's required a z6 short needle jet, are the z8's the same? since short z6's are hard to get
2: Since the 4n8's and the 502's are nearly identical besides the 5mm difference in height, id be curious to see if that 5mm extra is due to that spring and extra fluff they added on the later slides? Theoretically a 4n8 with '75 model slide should react the same as a 502 on the newer model slide?
3. If thats the case, When i switch to a z8/short z6 needle jet, I am looking at rejetting both the mains and pilots now correct? Any idea of a short range of jets I should buy?

edit2: 4. For the pilot sizes, Your chart states that anything over 40 is the bs30/96 style, although my carb float bowl requires the vm22/210. Is the size going to differ much changing to the z8 needle jet than what i have currently?? I assume the factoring differences to determine the pilot size is floatbowl type, needle jet and needle. Unless you tune the needle height for pilot to midrange transition instead of tuning pilot jet?

Currently at 142.5 mains and 30 pilots with 4n8's and z2

edit1; more research tells me the z8's are slightly richer than z6's.

Edit 3: the more I think about this. I doubt there's a tuning issue with the carbs besides maybe the pilot section currently. I'm going to try increasing the size of the petcocks intake and fuel nipple, as well as raising fuel level a tad and see if that sorts it. The bike runs great all the way to redline on the street everywhere besides constant highway speed at 110km/h. If lowering the needle increases the surging problem on the highway I will know it's leaning out and it's a fuel supply issue.

 

[5twins]

I think before going any further or buying any parts, you should positively I.D. your carb set and the parts on/in it. You've already discovered an earlier slide and needle is installed. What makes you sure the carb bodies are '78-'79? Refer to my pic in post #9 of the bell mouths. That will tell you if the carb set is early or late. You'll also want to I.D. the float bowl. Two different types were used, one for the BS30/96 pilots and one for the VM22/210's. The difference is in how the air is delivered to the jet. The VM22/210 type bowl delivers the air after the jet, above it. The jet flows straight gas. The BS30/96 type bowl delivers the air down an angled passageway to the bottom of the jet. The jet flows fuel/air mix. This also explains why the VM22/210 jets are so much smaller (sized in the 20s) compared to the BS30/96 ones (sized in the 40s).

Even though the two pilot types will physically swap, you can't do that. You must match the pilot type to the right bowl, one made to use that type.

 

[JNaw]

I think before going any further or buying any parts, you should positively I.D. your carb set and the parts on/in it. You've already discovered an earlier slide and needle is installed. What makes you sure the carb bodies are '78-'79? Refer to my pic in post #9 of the bell mouths. That will tell you if the carb set is early or late. You'll also want to I.D. the float bowl. Two different types were used, one for the BS30/96 pilots and one for the VM22/210's. The difference is in how the air is delivered to the jet. The VM22/210 type bowl delivers the air after the jet, above it. The jet flows straight gas. The BS30/96 type bowl delivers the air down an angled passageway to the bottom of the jet. The jet flows fuel/air mix. This also explains why the VM22/210 jets are so much smaller (sized in the 20s) compared to the BS30/96 ones (sized in the 40s).

View attachment 327900

Even though the two pilot types will physically swap, you can't do that. You must match the pilot type to the right bowl, one made to use that type.

I'm 100% certain they are later model bs38's, as they have the the large float bowl vents and pilot vents on the bellmouth, exactly like your photo. Float bowls I will take a look again, but do not have the pickup tube in the bowl casting, and from memory, the pilot circuit casting is completely open without that "bridge" looking part.

Butterfly plate is stamped 120 as well.

 

[5twins]

OK, if there is no overflow tube in your bowls then it sounds like they are '78-'79 bowls. The 120 butterfly plate was used in all the BS38s from '74 on.

Yes, the short Z-6 needle jet is very hard to find, nobody is reproducing it. If you switched to the Z-8 then you'd probably want the matching 4M1 needles as well, then base the jetting on a '76-'77 carb set. But since the 4N8 appears to be very close to the 5O2 in every way but length, it should work OK. Since you have the older slide, the shorter length shouldn't be an issue. Just be aware that the stock setting for that needle was the #4 slot (from the top). That means the normal #3 slot (stock setting in most other carbs) will be a step leaner, slot #2 two steps leaner.

The P.O. probably put those older slides in because he couldn't get a '78-'79 slide/diaphragm replacement. Until recently, I don't think anyone was reproducing them. That one from Heiden is most likely just the slide/diaphragm and doesn't include the snap ring or plastic "tee" that holds the needle in (or that little spring and the thick plastic spacer for the needle). I'm not sure if you can even get those parts new, scrounging used ones may be the only option.

 

[JNaw]

OK, if there is no overflow tube in your bowls then it sounds like they are '78-'79 bowls. The 120 butterfly plate was used in all the BS38s from '74 on.

Yes, the short Z-6 needle jet is very hard to find, nobody is reproducing it. If you switched to the Z-8 then you'd probably want the matching 4M1 needles as well, then base the jetting on a '76-'77 carb set. But since the 4N8 appears to be very close to the 5O2 in every way but length, it should work OK. Since you have the older slide, the shorter length shouldn't be an issue. Just be aware that the stock setting for that needle was the #4 slot (from the top). That means the normal #3 slot (stock setting in most other carbs) will be a step leaner, slot #2 two steps leaner.

The P.O. probably put those older slides in because he couldn't get a '78-'79 slide/diaphragm replacement. Until recently, I don't think anyone was reproducing them. That one from Heiden is most likely just the slide/diaphragm and doesn't include the snap ring or plastic "tee" that holds the needle in (or that little spring and the thick plastic spacer for the needle). I'm not sure if you can even get those parts new, scrounging used ones may be the only option.

I will try slot #4 and see how that affects the ride on the highway.

That's my thinking too, in your photos the 4n8 looks every so slightly fatter but I can't verify. I have a 502 needle now coming on Wednesday that I'll stash away, but will do some measurements with a caliper and compare.

If someone has a bs38 style carb set, can they take a measurement of the needle length protruding out from the slide?

If they are Infact the same as an assembly, I theoretically should be okay with the z2 needle jet and the current jetting I have in the bike.

I will try and confirm on the highway if it's indeed a lean out condition by turning the petcocks off and see if it's similar symptoms. I never tried the needle in the 4th slot either so I wonder if that would cure the issue if it is Infact lean.

Now I'm just stuck wondering about the hanging rpm after blipping throttle when warm. Could this be caused by an overly lean needle too?

 

[Its been a long time]

Now I'm just stuck wondering about the hanging rpm after blipping throttle when warm. Could this be caused by an overly lean needle too?

Maybe you already did this - But with the hanging revs on closing the throttle, are you sure the butterfly is closing fully to it's stop when you close the twistgrip? I have seen it happen where the butterfly return spring has been weak and the butterfly plate doesn't close fully on closed throttle. My XS650D doesn't have a throttle closing cable to positively close the butterfly, it depends on the return spring.

 

[5twins]

A 5O2 needle with the clip installed in the 3rd (middle) slot will hang out the bottom of a '78-'79 slide about 34mm. Here's some 650 needle info. Yes, it looks like the 4N8 doesn't taper to quite as sharp of a point as the 5O2 does, so it probably is a little leaner .....

 

[JNaw]

Maybe you already did this - But with the hanging revs on closing the throttle, are you sure the butterfly is closing fully to it's stop when you close the twistgrip? I have seen it happen where the butterfly return spring has been weak and the butterfly plate doesn't close fully on closed throttle. My XS650D doesn't have a throttle closing cable to positively close the butterfly, it depends on the return spring.

I can check this but it does settle down to idle after a sec. Worth a look. I'm all ears at this point.

@5twins great chart, that will help immensely thanks. Worst comes to worst I may just grab a 4m1 needle with a z8 needle jet and tune around that.

Looking at the chart though, the "d" measurements are diameters correct? How come they are all the same all the way down the taper?

Got lots to test now, thanks guys.

 

[5twins]

Yes, the "d" spec is diameter and indicates what it is at the end of the taper. On a single taper needle, that would be at the tip, so the diameter spec is the same for all the "d" values. On a 2 taper needle like the 4NJ19, you'll find 2 values listed. The "K" listings (lengths) are set up the same way.

 

[gggGary]

5Twins
and that's not a joke.
saved several of those pics, 5t you are a priceless resource here!
JNaw I hope you appreciate how much knowledge has been dispensed.`

I'm in the basement salvaging anything that can be saved from a set of FJ1000 carbs that were left in a dirty wet shed,

the bodies are all junk. Couldn't save a single slide.
What a shame such beautiful engineering was neglected to scrap.

 

[JNaw]

Yes, the "d" spec is diameter and indicates what it is at the end of the taper. On a single taper needle, that would be at the tip, so the diameter spec is the same for all the "d" values. On a 2 taper needle like the 4NJ19, you'll find 2 values listed. The "K" listings (lengths) are set up the same way.

Gotcha, looked to me that at each respective k# and d# measurement, the actual size didn't change and stayed constant. I wasn't sure if it was a mistake but I understand how it's supposed to be read now.

@gggGary I am infinitely grateful for all the Information that has come out of this thread, I love learning about this stuff and chatting with everyone here so far has been an honor.

 

[JNaw]

Attached is a video of hanging idle with both choke on (first half) and choke off.

https://drive.google.com/file/d/1LiPJCI-StX8m-y-VwzOeCl48Zj0hoymv/view?usp=drivesdk

This only happens when at operating temp.
Throttle is closing fully.

It still hangs in both scenarios so I'm thinking the advance weights or something else.

What's people's thoughts? Will be checking highway run with the 2nd slot on the needle later.

 

[5twins]

Well, that doesn't seem to be hanging much and for very long. Maybe you can just tune it out with the mix screws (set them richer, at or near their factory spec of 2.25 turns out).

The needle info came from a giant document that I think member JP linked to a while back. But it's huge, listing over 1800 needles, so I "condensed" things and just showed the 650 ones, along with that XS400 needle I found that works. I left more blank space in case I discover other needles in the future that work (I'll add them).

 

[JNaw]

Well, that doesn't seem to be hanging much and for very long. Maybe you can just tune it out with the mix screws (set them richer, at or near their factory spec of 2.25 turns out).

The needle info came from a giant document that I think member JP linked to a while back. But it's huge, listing over 1800 needles, so I "condensed" things and just showed the 650 ones, along with that XS400 needle I found that works. I left more blank space in case I discover other needles in the future that work (I'll add them).

Played with the advance weights, they slightly stick when returning to normal. I also noticed one of the weights have a notch that causes it to stick..betcha that's the issue. New one will have to be ordered

Currently waiting for jbweld to cure as I stripped a float bowl screw hole last year and seems my ultrasonic loosened my previous repair.

 

[JNaw]

Good news boys.

Leaning that 4n8 needle to slot #2 solved the highway surging at 5k. Must've been fuel loading when the slide was at full lift and held at that rpm.

Fully stable all the way to redline.

I may try shimming it to keep it on the richer side since it is aircooled, but as it stands everything is top notch.

Lubing the advance weights system did help fix that hanging idle, so got a new assembly coming to replace. (Do these need to be lubed with grease periodically?)

Final tuning for anyone curious:
78/79 Bs38's
142.5 mains
30 pilots
4n8 needle
Z2 needle jet
75' style slides
24mm float height

Last question too, what's everyone doing for leaky crank seal? I've replaced the seal and it's still leaking on the highway. Covering the whole underside with oil. Wonder if the crank is worn down and not sealing correctly now?

Thanks for help @5twins and everyone else here!

 

[Grimly]

Last question too, what's everyone doing for leaky crank seal? I've replaced the seal and it's still leaking on the highway. Covering the whole underside with oil. Wonder if the crank is worn down and not sealing correctly now?

Not unknown for the crank to be un-centred, that would wreck a seal.

 

[JNaw]

So, took it out for a longer ride and it started doing the cut out but it died entirely. Just started surging and then straight bog, no amount of throttle or rpm change brought it back, just had to coast till it caught back up on the highway exit and it runs fine afterwards.

This is sounding like a low fueling issue still, what does everyone think? It was cruising fine for the first 15-20km then once I started giving it more throttle it surged and complete bog.

I'm gonna completely tear apart the petcocks tomorrow and see if there's a blockage somewhere.

Could this maybe be an ignition coil/condenser overheating?