BS38 swopping needle jets and jet needles

peanut

peanut

XS650 enthusiast & inveterate tinkerer
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I have acquired a set of BS38 carbs which I would like to use as a spare carb set for my 79 special.
The carbs appear to be of the later 78-79 model but have been built with some parts from earlier BS38s carbs

They have 4N8 Jet needles (74-75?)
Z8 Needle Jets (76-77?)
BS30/96 type Pilot jets (25) with no air holes ?
No Main jets were fitted

What i'd like to know is
1. Are the slides early or late (see pic ) and will they accept the later type Z2 needle jets and 502 jet needles ?
2. Is the supplied gasket correct for the fuel bowls which also appear to be of the late 78-79 type (see pic)

I have a sizable quantity of VM22210 type main and Pilot jets and the diaphragms seem to be sound.
DSCF5322.JPG

DSCF5311.JPG


DSCF5312.JPG


DSCF5313.JPG

DSCF5318.JPG
 
You have quite the assortment of parts there, lol. I see the throttle cable arm is the early "claw" style. The slides are the '77 and older style. The 5O2 needle won't work in them, it's too long. The bowls appear to be '78-'79 type because of the VM22/210 pilot jet configuration and the missing overflow pipe. The bodies I can't be sure of unless you show us the intake bells and the inner side of each carb.

3mQOwp3.jpg
 
yep they are definitely Heinz57 :)
Thank you for the parts ID and date confirmation

The guy offered me a partial credit so I'm trying to work out if its worth the hassle and expense.
My option would appear to be to

Fit suitable mains Jets which I have.
Fit VM22/210 type Pilot jets (I have a range from 27.5 to 40)

If I refit the early type slide ,needle jet and jet needle would that work ?
78 79 Mikuni.JPG
 
Peanut, this definitely isn't my department, so take this with a grain of arsenic.

1- I believe that the later 76-79 float bowls can use the earlier 70-75 pilots, (with or without their side air holes), since their bleed air intermix zone is above the pilot, making the pilot design type irrelevant.

2- I believe this chart came from the Dutch website, identifying the Euro carb configurations. Note the 584 carb type (76-79), third from the bottom.

full


I'll go duck behind the couch now...
 
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TwoManyXS1Bs said:
Peanut, this definitely isn't my department, so take this with a grain of arsenic.

1- I believe that the later 76-79 float bowls can use the earlier 70-75 pilots, (with or without their side air holes), since their bleed air intermix zone is above the pilot, making the pilot design type irrelevant.

2- I believe this chart came from the Dutch website, identifying the Euro carb configurations. Note the 584 carb type (76-79), third from the bottom.



I'll go duck behind the couch now...
Click to expand...

:) good effort 2M .....5 out of a possible 10 ;)

Thank you for the chart I haven't got a copy of that particular one. There is however an excellent chart in our Tech section which is very similar but it takes a bit of searching out.

The carbs and bowls are 78-79 BS38 vm22/210 type which is correct for my 79 special

I have lots of the correct late type Pilot jets and Main jets so they are not I think going to be an issue.

The issue with these carbs is the slides , needle jets and jet needles which are early 76-77 type and are quite different to the later 78-79 slides needles and needle jets.

What I need to know is will these early slides with their needles and jets work in a later carburettor .?

I only have a very limited time within which to request a credit return to the seller . If I can I'll recondition them because late BS38s are very scarce and extremely pricey this side of the pond.

If I get stuck with these carbs it could turn out to be a very costly mistake :rolleyes:
 
I've never tried an early slide in the later BS38 body but I don't see why it wouldn't work. But, you will need to tailor the jetting around the needle/needle jet combo you've fitted. If you look at the chart above, you'll notice leaner needle/needle jet combos ('78-'79 carbs) get larger mains and pilots. The '77 and older carbs that use richer Z-6 and Z-8 needle jets get smaller mains and pilots. But wait, I know what you're thinking. The chart shows some much larger pilot jet sizes on some of the early carbs, sizes in the 40's. That's because those are the BS30/96 style and due to a different float bowl design, they are flowing the already made fuel/air mix. The VM22/210 style pilots used from '76-'79 flow straight fuel. The end result is that both styles deliver about the same amount of fuel/air mix to the pilot circuit even though the sizes are so different.

qj73qga.jpg


So, in your case, I would tailor the jetting around the '76-'77 spec since you have that needle jet (Z-8). Your 4N8 needle is a mismatch but may work OK, only experimenting will tell. It is slightly longer than the correct 4M1 needle but if you refer again to the chart above (and look at your pic), you see that is should be run with the e-clip in the 4th slot. That would make it the same length as the 4M1 using it's 3rd slot.

syuKACC.jpg


However, if you examine the taper diameters, the 4M1 does appear to be a little "fatter" than the 4N8, particularly at the tip. That would make it leaner. That means the 4N8 could prove to be a little rich when used in a Z-8 needle jet. Again, only testing will tell you this .....

g0cu2dB.jpg


The "no hole" BS30/96 style pilot you found is actually a different type, the N151.067 .....

https://www.rockymountainatvmc.com/p/930/45377/Mikuni-N151.067-Series-Pilot-Jet

2M may be right and these BS30/96 style pilots could very well function OK in the late BS38 bowls. It's something I never tried simply because if I needed to buy pilots, I just got the proper ones. Now, if someone was to give me a big bag of BS30/96 pilots for free then sure, maybe I'd experiment with them. But, the problem with either of them that could crop up is that they aren't available in sizes below #30. You couldn't match the stock sizes used in the '76-'79 carbs.
 
thank you 5twins thats a very detailed and informative response .
This is a complex problem which may be too much for my analytical ability these days :(. The main problem is going to be sourcing all the necessary parts in Europe.
I may just keep the carbs and wait until some used slides and jets turn up on UK ebay.

The reason that I bought these carbs was so that I could have a spare set of identical late BS38 carbs to swop with mine when cleaning and setting up every spring or when carrying out some tuning experiments . It would mean that I could take my time over the work and still ride the bike without a break.

I have two sets of GPZ500 (EZ500) Keihin carbs that I intended to use but they have not been prepared yet. I have all the necessary upgrade jets etc from following the thread on here, so that may still be my best option in the long term as parts are cheap and available.

I will try and source some used slides and jets on here and other sites before I give up.
Thanks again for all your help and time its much appreciated
 
I have a couple spare carb sets for the very same reason you want some. If my usual original stock set is "down" for some reason, I have spares to swap in and run. One set is a correct '78-'79 set, the other a '76-'77 set. It was interesting to be able to compare the differences in operation and performance between the two versions. The older '76-'77 set runs a little rougher at low speeds and idle, probably because the needle isn't spring-loaded like it is on the later sets. That allows it to jiggle around in the slide more and I think that makes it meter the fuel less precisely, at least at lower RPMs. If you check the fit of the needle under that plastic disc that holds it in the slide, you'll see it's pretty sloppy. There's enough play for the needle to move up and down about a half MM. Needle clip position changes alter the needle height by 1mm so that means this needle is free to bounce back and forth through about a half step of needle adjustment. I'm assuming it goes leaner when the slide lifts (and the needle drops), then richer when the slide drops (needle forced up). Other differences in the earlier set are better midrange power, probably due to the large Z-8 needle jet, and what surprised me, slightly better mileage. I expected the mileage to be a little worse because of that large needle jet. I guess the much smaller mains account for the better mileage. Still, due to their overall smoother operation, I do prefer the '78-'79 carbs.
 
I assumed that the later 78-79 carbs would be richer running but I guess that doesn't make much sense when Manufacturers were trying to reduce emissions and increase MPG .
I am very tempted to keep the carbs and try and source the correct parts but they cost me $170 and they would cost a further $200 to buy new slides needle jets and jet needles together with the needle holder bits etc and that would make them very expensive indeed !

I love the 78-79 carbs that are on the bike . They are still not quite dialled in but the engine starts first kick and pulls powerfully right through the rev range to 6000rpm with no hesitation whatsoever. The throttle response is very snappy but there is still a slight tendancy for a slow to return to idle when hot. I may up the pilot jets from 32.5 to 35 this spring.

Interesting observation about the differences between the jet needle holder assembly on performance. Definitely one for 2many to check out methinks ;):)
 
Pretty much all later E.P.A. mandated carbs got spring-loaded needles. Obviously, it's a more precise way to meter fuel and control emissions. Now that I have a BS34 equipped bike to work on, I can't wait to start playing around with them. Mark my words, in the future, possibly the near future, I think the BS34s are going to become the most valued 650 carb set. They're more modern, lighter and offer additional tuning options (idle circuit air jet). Their slightly smaller size shouldn't hurt the 650 much because this bike isn't about super high speed, near red line running. It's more about "real world" midrange running, and a smaller carb actually gives better midrange performance. Let me have a few months to toy with them and I'm sure I'll have some good things to report.
 
I don't know why BS34 have always seemed to be the second choice or poor relation to the BS38 . I suspect it is probably that old wives tale that the bigger the bore the more the performance.

I can't wait to try out the EZ500 carbs they are super light almost half the weight of the Mikunis
 
It used to be you couldn't tune them as well. They were more E.P.A. strangled and there weren't the variety of aftermarket adjustable needles available like there are now. No one had bothered to research and discover the Canadian and "world" versions of them with adjustable needles and slightly different jetting.
 
Meddy said:
Its seems that you have the same issue with that set of carbs as I have on mine Peanut, I wonder if the factory did a bit of mix & matching on the 79 models.
Click to expand...

interesting idea meddy but I rather doubt it. The BS38 78-79 slides , needle jets and jet needles are a matched assembly and not interchangeable with parts from the 76-77 BS38 .
Using early BS38 parts in the late BS38 carbs would cause massive confusion for dealers and owners alike.

the bits on my carbs have definitely been mixed and matched by someone recently

Do you have early slides and jets fitted to your late type 79 BS38s ?
 
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Those are BS34 slides. They are often listed by many different sellers as fitting Specials back to '78 probably because Yamaha's parts drawings are wrong. They incorrectly show BS34s instead of BS38s in those drawings. Of course, we know better, we know the BS34s didn't come along until 1980. But a seller perusing parts diagrams to try to determine what models a part would fit probably wouldn't know this.
 
5twins said:
Pretty much all later E.P.A. mandated carbs got spring-loaded needles. Obviously, it's a more precise way to meter fuel and control emissions. Now that I have a BS34 equipped bike to work on, I can't wait to start playing around with them. Mark my words, in the future, possibly the near future, I think the BS34s are going to become the most valued 650 carb set. They're more modern, lighter and offer additional tuning options (idle circuit air jet). Their slightly smaller size shouldn't hurt the 650 much because this bike isn't about super high speed, near red line running. It's more about "real world" midrange running, and a smaller carb actually gives better midrange performance. Let me have a few months to toy with them and I'm sure I'll have some good things to report.
Click to expand...
That's funny 5T when I got here, Ahem a couple years ago, I was all the BS34's are better but got poo-poo'd. I had previous experience with that carb so it was easier for me to overhaul. Now I'm pretty happy with with the 76-77 and 78-79 BS38's also, but headed for working on more of the early two cable carbs soon. I did do a 73 set a few years ago that ran fine even with points ignition.
Allison's 81 is running great with BS34's, a slight jet bump and washer mod, Mikes mufflers, the resto is working super on 78-79s perhaps the best carbureted bike I've owned. The survivor 79 is a just a bit lean at lower RPM with all stock jets. But never bad enough to make me pull and change the needle slot LOL.
I suspect the float bowl vents being moved to the intake in 78 also improved how the carbs work.
To date I have the best luck with stock air boxes, usually oiled uni foam filters on stock filter frames, and "larger" mufflers.
 

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