Help with carb decision

[franklin270h]

I've just gotten to tearing down the carbs on my 1977 model xs650.

I've identified the carbs as being from a 1978 model, except the previous owner had put slides from a 74-75 model (4n8 needle in it), it was jetted as if it were a 77, sync was blatantly off, thing is just a mess.

Anyway, just wondering given that 78-79 bs38 slides are getting pretty hard to find and my current slide/diaphragm are fine, is there anything about the 78 carb design that would prevent me from just putting a needle jet/needle and main and pilot jetting to 77 spec and be done with it? Or do I need to go on a slide/spring/plastic needle retainer hunt?

 

[5twins]

I would try going to the '77 specs. It would be difficult, if not impossible, to go with the '74-'75 specs because the short style Z-6 needle jet is not available. Also, that carb set was tuned using BS30/96 pilots. '76 to '79 sets used the VM22/210 type pilot. I'm not sure how you would match flow rates between the two types. I suppose it's possible but I couldn't tell you what size of one matches what size of the other as far as the flow rate goes. Make sure the bowls you have are correct for the carb set.

 

[franklin270h]

I would try going to the '77 specs. It would be difficult, if not impossible, to go the the '74-'75 specs because the short style Z-6 needle jet is not available. Also, that carb set was tuned using BS30/96 pilots. '76 to '79 sets used the VM22/210 type pilot. I'm not sure how you would match flow rates between the two types. I suppose it's possible but I couldn't tell you what size of one matches what size of the other as far as the flow rate goes. Make sure the bowls you have are correct for the carb set.

I probably should have clarified more, I did intend to (with 77 spec) change the needle jet to Z8 and needle to 4M1 as well as 77 jet settings, just wanted to make sure nothing about the 78 carbs were all that different from 77 as far as affecting airflow relative to the jets. I know the float bowl overflow and such were different. And bowls are correct for 78, it seems they just changed out the slide or at least pulled the 78 needle/jet in favor of an earlier setup.

I have no clue how the previous owner (or whoever went through the carbs) arrived on that combo, but with older bikes it's best not to even bother wondering the intent. I'll be off ordering 77 jets and needles and such, thank you very much for reply

 

[5twins]

I understood what you were going to try. In fact, I considered it myself since '78-'79 slide/diaphragm assemblies are so hard to come by. I don't think there's anything in the design or flow characteristics that would prevent doing this. The '78-'79 slide was just an updated version. It introduced the spring-loaded needle which meters more consistently. I think this was an E.P.A. thing as most newer carbs use it. That's why the '78-'79 needle is longer. It needs to be to sit on top of the spring but still hang out the bottom of the slide about the same as the earlier ones.