Carb Identification

[petejw]

a closer look at the pic, u can just see the vent 't's,
looks like the early model carbs.

[cywelchjr]

Oh, and not all 80 carbs have the cross passage or the plug either. It's early late model year related, early 80 carbs have the plug and the cross passage, and late model year carbs don't have the passage or the plug, otherwise, except for the fact that they are jetted differently, they are pretty much the exact same carbs and take the same kits with different size jets based on the cross passage existing or not IIRC.

[Rasputin]

These carbs have the slanty hole thing. They do not have the rubber plugs. They do have the screws in them, which makes me think older carbs. They also have rubber plugs in them which suggests the bastard carbs. To add to it all they have different jetting in the 2+ 3 carbs whih of course could have happened many years ago. Just wondering if I have not a set of bastard carbs then what do I have?

[cywelchjr]

These carbs have the slanty hole thing. They do not have the rubber plugs. They do have the screws in them, which makes me think older carbs. They also have rubber plugs in them which suggests the bastard carbs. To add to it all they have different jetting in the 2+ 3 carbs whih of course could have happened many years ago. Just wondering if I have not a set of bastard carbs then what do I have?

If they have vent lines they are 78-79 carbs, 80 and later didn't have vent lines as they vent to the bell. Only early 80 carbs have the slanty hole thing but all earlier carbs have that, and use a screw to block off the idle jet, where the 80 carbs use a rubber plug for the same thing, as the only difference other than jet size on early vs late 80 carbs is the slanty hole and the plug, and there are no threads in the 80 carbs for a screw to block the idle jet.

As for how they are jetted, ANYTHING could have been done by a PO. But the two main indicators of carb year are the venting (either vent t's or through the bell with only one t to each carb) and if you have vent t's they are early carbs, if you don't they are late carbs, and if they have the cross passage with no vent t's, they need the rubber plug and also MUST have that nub inside the bowl to hold the plug in the hole.

[DGXSER]

Hey Rasputin,

Those are almost definitely 78-79 model year carbs. As Cy mentioned, look at the mixture screw, is it tucked down inside a tunnel similar to the pilot jet? or does the top of it sit up on top of the tunnel it threads into? Also, look at where the idle adjustment screw is on the bank. If it points down and is under the engine side of the carbs, that is the newer style, if it points up and is on the airbox side, that is the older style.

[Rasputin]

OK learn something new every day. The screws in these look different than any others i have seen and have a different head design as well as differing washers etc.

[cywelchjr]

These carbs have the slanty hole thing. They do not have the rubber plugs. They do have the screws in them, which makes me think older carbs. They also have rubber plugs in them which suggests the bastard carbs. To add to it all they have different jetting in the 2+ 3 carbs whih of course could have happened many years ago. Just wondering if I have not a set of bastard carbs then what do I have?

Wait, I'm confused (I know, I get that way a lot). You say they don't have rubber plugs, and they do have rubber plugs. Which is it? Externally 80 carbs look almost identical to 81 carbs except for the bowl drain. Internally they are almost identical except for having the cross passage of the early carbs and a rubber plug in place of the early carbs screw over the idle jets.

Pretty simple really. Basically the carbs look one of two ways externally, either early or late (setting aside the black and gold of the MNS carbs). Internally are pretty much the same as well, with early 80 carbs having the cross passage and a rubber plug and different jetting if they do (and I don't mean on 2 and 3, I mean between what they would be without the passage). Also in some rare cases it's reported that some early 80 carbs also have brass floats, my XS400's BS34III's which are otherwise identical to my 11's carbs have brass floats, and they are factory and still pretty much pristine. In addition, some machines have 2 and 3 jetted richer, I believe just on specials, and not even all of them, and after 30 years, you don't know if finding one that way means it was that way originally or not, or the finding one that way means it wasn't that way to start with either.

[cywelchjr]

OK learn something new every day. The screws in these look different than any others i have seen and have a different head design as well as differing washers etc.

After 30 years, that probably just means a PO changed them at some point after having lost the originals. I know there are more than a few screws and bolts on my XS400 that are not original due to me or my uncle having lost them over the years when working on it and having to source a replacement, often with something that was "good enough" rather than an OEM replacement part.

[Rasputin]

When I took them apart they had the screws, they also had the rubber plugs. I thought I had finally come across a set of bastard carbs. Maybe I am wrong. Not like it would be the first time.

[Rasputin]

Went out to the shed and got the other two carbs that were on this rack this morning. The two that are in the pictures were part of a set that came on a parts bike I had. Had never had it running. I looked and yes the other two indeed have the rubber plugs in them and no threads for screws.
They also have different numbers which I had not noticed before. They are numbered with a 3 U9 and the rest I can not read. They also do not have the vent line.
So I guess what I have is a home made rack of carbs from two different years cobbled together. No wonder they were jetted differently.

[cywelchjr]

Went out to the shed and got the other two carbs that were on this rack this morning. The two that are in the pictures were part of a set that came on a parts bike I had. Had never had it running. I looked and yes the other two indeed have the rubber plugs in them and no threads for screws.
They also have different numbers which I had not noticed before. They are numbered with a 3 U9 and the rest I can not read. They also do not have the vent line.
So I guess what I have is a home made rack of carbs from two different years cobbled together. No wonder they were jetted differently.

Not bastard carbs, bastard rack! That rack will never work. You can use either type of carb on any year bike, but you can't mix types in a rack. But that does explain why you were confused.

[DGXSER]

You can use either type of carb on any year bike, but you can't mix types in a rack.

Ok, this has been stated several times, and I may have even stated it myself once or twice. But the more I ponder on it, the more I do not see how this is true. From a fuel standpoint, these bikes are independent systems cylinder by cylinder. So if a carb will work on that cylinder, what does it matter what is on the others? Now admittedly, I believe the different styles have a different sweet spot, but you should be able to tune them to be pretty dang close.

It may take a little more thinking and knowledge to realize the these two need to be jetted one way, and the other two another way, but they should run their cylinder just fine.

[natemoen]

And if different carbs had different sweet spots as in rpm ranges maybe you could get it to have a more even powerband? Probably thinking to abstract here, sorry.

[cywelchjr]

Ok, this has been stated several times, and I may have even stated it myself once or twice. But the more I ponder on it, the more I do not see how this is true. From a fuel standpoint, these bikes are independent systems cylinder by cylinder. So if a carb will work on that cylinder, what does it matter what is on the others? Now admittedly, I believe the different styles have a different sweet spot, but you should be able to tune them to be pretty dang close.

It may take a little more thinking and knowledge to realize the these two need to be jetted one way, and the other two another way, but they should run their cylinder just fine.

The problem is that because they are different designs, they don't behave the same at different flow ranges. This is not an issue if all of the carbs in the rack are the same design/model as they can be tuned to, but if their not the same, the different cylinders will be getting different fuel flows and mixtures at the same times and won't be in sync, this is NOT good for the engine and it won't run quite right in all the ranges, you will have spots that just won't be right no matter what you do.

It would be like putting 36mm and 34mm carbs together in the same rack, even if you can get them to mount up, they won't flow the same. This is the same thing, the vacuum pistons the diaphragms are connected to aren't even the same size which means they don't react the same. I can't see any way you could tune for this difference. If it was only that the jets were setup different, I would agree, and I would say you could put the newer type with the cross passage and without the cross passage together because that's just jetting. Those have the same vacuum piston, same venturi size, the only difference being how the idle jet is fed, and frankly if you had to, you could change than jet and drop the plug and plug the passage. I'm not sure off the top of my head what material I would use to plug up the passage, but I'm sure we could come up with something, maybe one of those gas tank patching compounds? But I'm not sure that would be an issue anyways.

[DGXSER]

I get what your saying Cy, and maybe this is just the cynic in me or the "Missouri -Show Me" coming out. That is very theoretical and theoretically speaking you can talk about how the cams for the early engines and valve sizes are different and therefore the engine would run different with the different carbs.

You have a point, I see the issue being stated, I just have my doubts as to wether it would truly effect the engine performance at any noticable level. Kind of like my current bike when last checked had some big swings in compression from cylinder to cylinder. But it runs like a scalded dog, just does not idle as smooth as I might like. I guess I'm thinking that the different carbs would be similar.