Carb Question

[Edisto]

I picked up a 78 XS a couple weeks ago at quite a bargain. My other bikes include a 80 XS Special and an 82 XJ1100. The previous owner had a local mechanic do some work to the bike, including some carb work and putting a lined XS Special tank on what I am fairly certain is a standard XS. The bike cranked up--compression was a bit low, but all 4 cylinders were within 5-8% of each other--around 115-120 psi. The bike would only run on choke and 2 of the cylinders were not firing, and shortly, there was fuel coming out of the air filter. So, I pulled the carbs to begin cleaning them. In breaking down the carbs, it was obvious some work had been done. The main jets, and the pilot jets are new. But--The carb closest to the choke slide is different than the other 3 carbs. I have no experience with 78 model bikes, so I didn't know if this was normal or not, or if it even makes a difference.

Any input would be great. Thanks!

[natemoen]

The right one in the pic i am fairly certain (never actually seen one personally) is the late model carb and the left one is the regular early model carb. They don't mix well and you need to jet the different model carbs.differently. you would need to find a replacement carb body to match the other 3 for that carb in that position on the rack.

[tcoop]

Looks like you have an 80 or newer carb body bolted in to the 78 carb rack there. I would look to ebay to get a good complete rack.

[crazy steve]

The carb on the right is a '80-81 type, while the other is original '78-79. You'll need to find a replacement, as these two types jet and tune quite differently. All one type or the other is OK, but mixed is a no-no...

[Edisto]

Thanks guys! I have another set of carbs I can put on this bike to at least let me see if there is anything else wrong with her before I spend $$ on more parts.

[DGXSER]

Yep you decisively have a mix and match set of carb bodies there. The number one is from an 80-81 set. the other three are 78-79 models. The screw hole off to the side of the float needle seat hole is for the retainer that holds that seat in place on the later style. The older ones, that seat screws in.

I'm not sure why it would not work like that if jetted properly to be honest since they are individual to each cylinder, and you can put the ful bank of 80-81 carbs on any year XS and they will run fine. I've heard others state the combo does not work out well though.

The 80-81 carbs do get jetted substantially different though.

[Edisto]

I thought the mechanic that did the work screwed the pooch. The fuel lines were a mess, the inside of the tank was poorly done, and now this. Now, I really feel bad for the guy I bought it from.

[crazy steve]

I'm not sure why it would not work like that if jetted properly to be honest since they are individual to each cylinder, and you can put the ful bank of 80-81 carbs on any year XS and they will run fine. I've heard others state the combo does not work out well though.

I think the big problem is the slide size difference. The later carbs have larger slides and different slide springs, so don't 'react' the same as the early units. This is fine as long as all carbs match, but with a oddball in there I would think that getting fuel curves to match at various rpms/loads would be extremely hard. The float bowl venting is another difference (I wonder how that was addressed with this combo?).

Whatever you do, it's important that parts match. While you can swap some parts between the early/late carbs (the later plastic floats into the early carbs is a common swap), for any individual part they should all be the same.

[cywelchjr]

I'll have to agree with Steve. The characteristics of the carbs once off idle are different, and they tuned up differently. As was said, you can change the whole bank to either one, but you can't mix them. From what I have seen with them on other models, the mixture and airflow characteristics in the midrange are quite a bit different.