Carb Identification

[Aches n Pains]

Bought another 1979 Special that came pretty much in a basket.

I'm beginning with the carbs which were completely torn down. There are two sets and I'm currently just trying to free things up and get the parts matched up with the carb bodies. Several pictures of one of the carbs I have assembled.









My problem is to identify this carb model. It is a Mikuni BS34 but doesn't look quite like the carb on my existing XS1100 Special (I believe to be unmolested). It may be off another engine. The major differences are:

- Idle mixture screw is stainless and sits under a cap on the outside of the carb.

- Plastic floats vs. brass.

- Drain plug is not the typical, brass hex head. It is hex head, smaller width, and has a Phillips fitting on the bottom. (4th picture above)

- Float needle seat is not threaded. It fits into the bore with an O ring and is held in place by a bolted on retainer that has a vertical part that I've not seen before and doesn't show in my parts breakdown (3rd picture above)



The good news is that, after reviewing lots of posts, I found a great way to remove frozen pilot jets. These jets were hopelessly buggered in three of the four carbs.

- First, remove anything with rubber or fiber
- Second, put PB Blaster down the bore that holds the jet (try to keep from overflowing much)
- Third, heat the bore with a torch - 4 seconds per side worked for me (8 seconds total). Might do steps two and three a couple of times
- Four, remove the jet using the #1 bit from a Screw Extractor Set (see pictures below). There is already a large enough hole in the jet. Just push the extractor in straight and rotate counter clockwise pressing firmly into the bore. Breaks loose pretty easily.

Thanks in advance for your help,

Doug

[crazy steve]

That's not an XS11 carb....

Not sure what that is off of, as it appears to be a 'mix' of early/late bits and the carb body is clearly not a OEM XS carb. It's got the plastic floats and o-ring float needle like the '80-81 type carbs, but the idle mixture setup is unlike any XS11 model. The cast diaphram cover is typical of the '78-79 carbs, not the stamped steel covers of the '80-81 carbs.

Maybe off a later model 650?....

The BS34s were used on a lot of different bikes in various versions and it's my understanding there's internal differences that can't be corrected by rejetting, so if it's not off a like-sized motor they won't work, assuming you can work out any differences in throttle linkage and 'carb rack' dimensions.

[DGXSER]

Hey Doug,

Overall, looks like you have a set of the 1980 carbs with the crossover tunnel as the earlier carbs had. The plastic floats, push in float seat, mixture screw down in a tunnel (under a cover from the factory). All point to the 1980-81 model carbs. The fact you have a screw there that covers the pilot jet would indicate it has the crossover feed from the main jet. They only did all of this on the very early 1980 model carbs.

as to your trick on the pilot jets. That works most of the time, however, I have a set of parts carbs where the extractor broke off in the jet..

[cywelchjr]

Those look like 1980 carbs with a single exception, that's the bowl. I've never seen one with the drain like that. As for the going from model to model, the differences are not with the passages, those are identical, but rather with the fuel delivery and venting and with the spring rates on the vacuum slides, that is the part that makes it extremely hard to move them, the spring rate, as those springs can be almost impossible to source. Everything else can be worked around with enough time (the question here becomes is it worth it).

Now that said, as long as the right spring rate springs are in there, and all the jets (both air and fuel, and some of those are not replaceable) are correct they should be able to be made to work.

However, with the other carbs that have been found to not only work, but in many cases work better than the stock carbs with only minor mods, it may or may not be worth the effort. It all depends on whether your are money or time rich

[crazy steve]

You guys need to get your eyes checked

The idle mixture screw isn't in a tower, but has a metal cap pressed over the screw (unlike either type of XS carb). Similar to the '78-79 carbs, but with a metal instead of the plastic anti-tamper caps.

Edit: Well, maybe not. Is that a vacuum nipple next to the idle screw? Wrong angle for a XS carb if it is, but this is definitely different than the XS version.

The linkage on the one side is different from any XS carb I've seen too (first pic) as that looks to be a large cast piece.

[cywelchjr]

Third picture in the OP at the bottom is the mixture screw, which is clearly a BS34III type. They were a standard carb with differences for the model bike they went on that included the throttle linkages (which can be changed) and the bowls (which can also be changed but why bother). I can clearly see the tower there for the mixture screw, that's NOT the screw. I did say that the bowl didn't look like any I had seen, which means your likely right that it's not off an XS, but was probably adapted. That's not that big a deal as long as what it came off of had the same size cylinders that flowed the same (a two cylinder with half the displacement is the same as a 4 with the same) although it would be a bitch to mount them. It's best to stick with something close that's already known to work with minimal adjustment, such as the ZRX carbs that several have used, which need only a vacuum barb added for the vacuum advance and for which it was finally found where the sweet spot was for that to be placed.

So the fact is, I both agree with you (it's not an XS carb IMHO) but it IS a BS34III with the tunnel for the mixture screw.

[Aches n Pains]

Thanks, folks, for all the help. You guys have probably forgotten more than I ever hope to learn.

I'll pick up the bike as soon as the title is clarified but I am starting to think that the engine may have been swapped out. Will post pictures as soon as I pick it up.

The engine has no shaft on which to mount the kick start - just a rubber bung. What does that tell anyone?

Aches n Pains

[natemoen]

Just the rubber plug means it is most likely a 80-81 model.

[natemoen]

I have a simple question. Did the 80 carbs have the same float bowls as the 81s?

[cywelchjr]

I have a simple question. Did the 80 carbs have the same float bowls as the 81s?

No, the 80's have carb bowls very much like the early ones with the bolt in the bottom, unlike the 81's where it's the screw in from the side kind of thing.

[Rasputin]

The 80 bastard carbs came with the same bowls as the earlier carbs on the bike that I have. Here is a pic of the slanty hole which defines those. If you have this hole that is what they are. I took 2 pics hopefully they come out. The one has a wire shown which is inserted into that hole. Hope this helps some.

[cywelchjr]

I don't know if the 80 bowls are exactly the same, cause they don't use the screw to block off the idle jets but rather a rubber plug that is held in place by a protrusion built into the bowl. I'm guessing the early bowls don't have that as that would hit the screw I'm thinkin. I know this cause all 6 of my carbs are like this (4 on the 11 and 2 on the 400). And the stupid gas in Ca can eat the original rubber plugs in the 80's carbs if they've never been replaced with the newer ones designed for the nasty gas we have today.

Had my 400 start running horrible (fine one day horrible the next) opened the carbs, and no plugs, just come black goo in the bottom of the bowls. Had to order new plugs from MikesXS to fix them.

[Rasputin]

My 80 carbs have a screw and the bowls are different than the 81. I will get a pic here in a minute if i still have enough battery in the camera.

[Rasputin]

The first pic shows the screws that are in place on my 80 carbs. The second pic shows the difference between earlier and later carb bowls. The one on the left is from my set of 80's and the one on the right is from my 81MNS. Note the difference of the small nub which would keep the rubber stoppers in. The carbs from the 80 2H7 00 79I marked on them.

[cywelchjr]

The first pic shows the screws that are in place on my 80 carbs. The second pic shows the difference between earlier and later carb bowls. The one on the left is from my set of 80's and the one on the right is from my 81MNS. Note the difference of the small nub which would keep the rubber stoppers in. The carbs from the 80 2H7 00 79I marked on them.

Those aren't 80 carbs, as the 80 carbs have the push in seat not the screw in seat. And the 80 carbs don't have the 2h7 number, that's for the early carbs. I'll bet those carbs have the two t's on them too. All Mic BS34III's have the push in needle valve seat with the o-ring and not the screw in one. I HAVE seen some 80 carbs with brass floats, but those are also very rare, most all of them have plastic floats in addition to the press in seats, but 100% of them have the press in seats. Do those carbs have the mixture adjust screw recessed inside a tunnel (maybe with a plug in it) or are they exposed?