BS38 carbs 70-75 vs 76-79

[tunedbytyler]

Hey everyone!

New to the forum, I just recently purchased a 1978 xs650 standard. With the bike i got an extra set of mikuni bs38 carbs, however they are the 70-75 year model. After doing some research i have found some minor differences, but not enough to distinguish one better than the other.

the 70-75 carbs i got looks better externally than the ones currently on the bike. I would like to rebuild them so i have a running bike while i do the rebuild, but want to make sure it would be worth it as opposed to rebuilding the carbs currently on the bike.

 

[gggGary]

there were at least 3 versions between 70 and 75 and the linked 38s came in 2 versions also.
the 78-79 are prolly the best BS38's all around if they are salvageable, the early also are not a direct fit to late adaptors.

 

[nj1639]

'78 and '79 bs38's were the apex for those carbs. I'd stay with them, though I'm no carb expert......I'm just a sponge of info and that's how I've interpretated that info.......

 

[5twins]

Yes, the '78-'79 set are the best of the 38s, but some also really like the '76-'77 set. I've tried both and prefer the later set. They run a bit better and smoother in the low to mid RPMs. There's nothing wrong with the '76-77 set, they're just a little less refined and slightly rougher running at low speeds. They do have a wonderful midrange though, because of the larger needle jets (Z-8 compared to Z-2 in the '78-'79 set).

Personally, I wouldn't even use any of the older unlinked sets unless I had no other choice. You could use them in the interim while refurbing the '78 set, but I would definitely plan to go back to that set. A carb rebuild is like 98% just a thorough cleaning, so refurbing the older set shouldn't cost much more than your time. If you're careful not to damage or break them, the float bowl gaskets can be reused indefinitely. Jets should be fine unless they are all gummed up with varnish or the screw slots in the heads are buggered by some ham fisted P.O.

 

[willis]

So out of curiousity, what would be involved in switching my 75 over to my linked set of carbs off my 78? Obviously I would only need to run a single throttle cable, but do the vacuum boots get swapped over too? Are the air boxes the same?

 

[gggGary]

Yes you need to use late 76-79 intakes, they change the carb position, so the late carbs will not line up with the 75 airboxes.

 

[tunedbytyler]

Awesome! thanks for the advice guys, going to go ahead and just rebuild the carbs currently on the bike.