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Old 03-23-2012, 06:58 PM
crazy steve crazy steve is offline
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Carb Identifier Guide

This comes up on a regular basis, 'How do I tell what carbs I have?' Well, here it is, in color no less... Note that this isn't a rebuild guide, just what you need to identify what you have so you can be sure of getting the right parts....


First, I'll note that Yamaha only used two types of carbs on the XS/XJ bikes, with two additional minor variations which will be noted. There's very little parts interchange between the two types, so it's important to know. I'll also note that each type used a common main body casting for all four carbs, but each body is machined to a specific position, so they can't be easily swapped around. I'll cover this more a bit later...

Ok, the carbs are on the bike and you want to know which you have. You don't have to disassemble anything, just take a quick look at the right side carb for this....



Arrow 1 is pointing at the outer undrilled bowl vent on the '78-79 type carbs; if you see this, that's what you have. And you'll find this on both sides of the carb, but it will be drilled on one side only. The body on the right is flat in this area, and that makes that a '80-82 type; these are vented through the carb inlet. Arrows 2 and 3 are pointing at the idle screw towers; as you can see, the late carbs are taller and the screw won't be visible like the early models (no screw is fitted to the early carb here). You may find a 'anti-tamper' plug installed in the late towers if the carbs haven't been rebuilt. Also note the 'extra' mounting point cast into the diaphram bell on the late carb.



Here's a view of the inlet side; the '80-82 type at the top, the early type on the bottom; the 'extra' hole in the inlet on the late carbs has to do with the bowl vents. You can see the idle speed adjustment screw between 2 and 3 on the early carbs. Here you also can see the 'extra' bracket attached to the late carb mounting points noted above. Note there's four hose connections on the early carbs, only two on the late versions; the 'extras' on the early carbs are the bowl vents and should connect to the airbox. If your inlets don't look like one of these, you don't have XS/XJ carbs... A handy tip to check the diaphrams is to push the slide up with your finger, then cover the oval hole with your thumb. If when you remove your finger while keeping hole covered and the slide stays up or drops very slowly, the diaphram is likely good. If it drops quickly, the diaphram is probably bad.



Engine side of the carbs, again with the late type on top. If you look closely, you can see the idle speed screw on the bottom of the late carbs. You can also see the early idle mixture screws; these still have the factory plastic 'anti-tamper' caps on them. Note that the early units have cast aluminum diaphram tops while the late ones are stamped steel; don't count on this as being a positive identification as you can find these swapped. Also note the choke linkage; all '78-81 will look like the early unit shown here, the upper set is XJ-only linkage.



No matter if you have early or late carbs, both should have this hose connection on the engine side of your #2 carb; if this is missing either somebody swapped a #4 carb in it's place, or you don't have XS carbs. This is where you connect the vacuum hose for the ignition.

Ok, for those who may have a 'box o'parts' carb set and to illustrate the two minor variations mentioned above, I'll note some internal details....



The late carbs have this tube moved about 1/16" closer to the center of the body compared to the early carbs, so float bowls won't interchange between types. Also note that the float valve hole isn't threaded on the late carb, but is on the early model so these won't interchange either.



Minor variation 1. While this is an early carb, some '80 carbs have this passage drilled. If you find this is drilled on your late carbs, you need special rubber plugs to close the smaller hole. These won't come in a rebuild kit, you'll have to buy them separately. Most late carbs won't be drilled like this, but you should check.



Float bowls and floats. There's three types of bowls; the '78-79, '80, and '81-82. The first two types look like the left side one (and are almost indistinguishable from each other outside) but won't interchange because of the moved tube noted above. The right side bowl was used on the '81-82 carbs, but will fit the '80 versions. The brass floats were used in the '78-79 carbs, will fit the '80 carbs/bowls, but won't fit inside the '81-82 bowls. The plastic floats were used '80-82, but can be swapped into the early carbs; this is common for replacing damaged brass floats, so this won't always indicate the type of carb you have. If you have brass floats, yours should look like this one; if it's mashed, collapsed, or leaking it should be replaced.



All '80-82 bowls have this 'bump' in the bottom of them, a positive identifier.



Minor variation 2. All XS carbs should have this hole drilled in the #1 and 4 carbs. This is for a small spring and ball detent for the choke linkage. These holes are not drilled on XJ carbs as they use a cable-type choke linkage, so if trying to use XJ carbs on a XS you'll need to drill these for the detents.

I can't begin to list all the small variations in some parts, so I won't try. Best bet when disassembling carbs is to make sure you keep all the small parts from that body with that body. Some parts swapping is possible, but carefully compare parts before you do. Slides won't interchange between types because they're two different diameters, although the diaphrams will if you can get them off without damaging them. You also can't mix early/late types on a rack, so don't try. If you're trying to piece together a set from more than one damaged set, it is possible to swap a #2 carb into the #4 position (but not the other way), trade a #1 for a #3 (or vice versa) on a late set, or a #3 for a #1 (only) on an early set. This will require removing the throttle shafts and drilling and/or tapping some holes, but is doable. Careful comparison of the two carbs will show you the differences.

I want to thank Mack for the pics of the bowls/floats...
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Last edited by crazy steve; 03-27-2012 at 01:14 AM.
 

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