Carb thread is a Companion/Add on thread to
https://www.xs650.com/threads/xs650...orkshop-manuals-tech-information-links.30569/
* Carb Guide PDF
Carb ID
Mikuni BS38's-BS34's
Intro and graph by ........2manyxs1b's, (2M)
As old-school, archaic, obsolete, primitive as they are, carburetors are still delicate/precision instruments, trying to function in a harsh/demanding environment, and should be treated as such.
They come from a time when folks were more tweaky and fiddly with their gizmo's, like fixing wristwatches, shoes, darning socks, fixing vacuum-tube radios, hot-breath warping wings on balsa gliders, adjusting circuits and yoke coils on vacuum-tube TVs, setting ignition points, hand-winding and tweaking rf coils on ham radios, ...etc.
You MUST master the understanding of the carb internals, what each part does, and when, and why, in order to successfully tune the things. Of course, the carb comes fully engineered to perform its function, years of development have taken care of that, but the tuning variables can create a humongous variety of combinations. So, a methodology must be used to isolate and target specific areas of adjustment.
If you find yourself thinking less about the tuning chores and more about the 'bling' and 'intense', like velocity stacks, polishing/painting, custom spigots, etc., there will likely be problems ahead.
2Many
What carbs do I have?
70-73; BS38
74-79; BS38
80-84; BS34
First 2 years there were some changes then from 72-84 the carbs changed every 2 years/
70-75 used twin throttles cables, one for each carb.
76-84 used a single throttle cable...carbs are joined by a plate attached to the tops and an angle iron across the bottom
Any year Carb can be bolted to any year engine. Intake manifolds to match the Carb used
There are changes to the carbs every year or two from 1970 through 1984
It is important to find out exactly which model of carbs you have. Mikuni BS38 is a very general term, it's like saying you have a Chevy at the parts counter, Brass parts, jets, float bowls, and the carb bodies changed between years and models and mostly are NOT interchangeable. BS38s were also used on many other motorcycles with different spacing, brackets throttle shafts etc. Check and confirm that your XS650 carbs have the right set of brass installed. Many after-market "kits" contain wrong parts, even though they are sold for year X they may NOT have the correct brass for the year.
Big change years: Two separate BS38 carbs with 2 throttle cables were used from 1970-75............Bolt together one throttle cable BS38s were used 1976-79.
The next big change in the USA was the introduction of the BS34s in 1980........ The quick tell here is the 1970-1979 BS38s use aluminum caps.........1980-84, BS34s use chrome plated steel caps.
Use the graph below as a quick reference guide .........Then go to the post most likely to be your carbs, then identify
Links.
Throttle shaft seals-Throttle plate Removal-Screw replacements.
http://www.xs650.com/threads/bs-series-throttle-seals-whos-done-em-who-needs-to.11980/
*CARB GUIDE: Required reading for tuning and understanding your CV Carbs.
By Grizld1, (aka Dick Russell, RIP), and 5Twins.
https://www.xs650.com/threads/xs650...orkshop-manuals-tech-information-links.30569/
* Carb Guide PDF
Carb ID
Mikuni BS38's-BS34's
Intro and graph by ........2manyxs1b's, (2M)
As old-school, archaic, obsolete, primitive as they are, carburetors are still delicate/precision instruments, trying to function in a harsh/demanding environment, and should be treated as such.
They come from a time when folks were more tweaky and fiddly with their gizmo's, like fixing wristwatches, shoes, darning socks, fixing vacuum-tube radios, hot-breath warping wings on balsa gliders, adjusting circuits and yoke coils on vacuum-tube TVs, setting ignition points, hand-winding and tweaking rf coils on ham radios, ...etc.
You MUST master the understanding of the carb internals, what each part does, and when, and why, in order to successfully tune the things. Of course, the carb comes fully engineered to perform its function, years of development have taken care of that, but the tuning variables can create a humongous variety of combinations. So, a methodology must be used to isolate and target specific areas of adjustment.
If you find yourself thinking less about the tuning chores and more about the 'bling' and 'intense', like velocity stacks, polishing/painting, custom spigots, etc., there will likely be problems ahead.
2Many
What carbs do I have?
70-73; BS38
74-79; BS38
80-84; BS34
First 2 years there were some changes then from 72-84 the carbs changed every 2 years/
70-75 used twin throttles cables, one for each carb.
76-84 used a single throttle cable...carbs are joined by a plate attached to the tops and an angle iron across the bottom
Any year Carb can be bolted to any year engine. Intake manifolds to match the Carb used
There are changes to the carbs every year or two from 1970 through 1984
It is important to find out exactly which model of carbs you have. Mikuni BS38 is a very general term, it's like saying you have a Chevy at the parts counter, Brass parts, jets, float bowls, and the carb bodies changed between years and models and mostly are NOT interchangeable. BS38s were also used on many other motorcycles with different spacing, brackets throttle shafts etc. Check and confirm that your XS650 carbs have the right set of brass installed. Many after-market "kits" contain wrong parts, even though they are sold for year X they may NOT have the correct brass for the year.
Big change years: Two separate BS38 carbs with 2 throttle cables were used from 1970-75............Bolt together one throttle cable BS38s were used 1976-79.
The next big change in the USA was the introduction of the BS34s in 1980........ The quick tell here is the 1970-1979 BS38s use aluminum caps.........1980-84, BS34s use chrome plated steel caps.
Use the graph below as a quick reference guide .........Then go to the post most likely to be your carbs, then identify
Links.
Throttle shaft seals-Throttle plate Removal-Screw replacements.
http://www.xs650.com/threads/bs-series-throttle-seals-whos-done-em-who-needs-to.11980/
*CARB GUIDE: Required reading for tuning and understanding your CV Carbs.
By Grizld1, (aka Dick Russell, RIP), and 5Twins.
Last edited:
