Carb Swap

[XSsesive50]

I have a 1975 XS650 and a 1978 XS650 Special. Are these BS38 and BS34's? Can I swap the carbs on my 1978 XS650 to the 1975 XS650? The '78 runs great and the '75 carbs are a pain to try to synch, with the 2 throttle cable connections. I have a second set of carbs from a '78 complete with carb holders. Also, will the needle and jets swap? I went up 2 on the '75 main and pilot and dropped the needle to the 3rd (middle) notch to tune for the pod air filters. And I know the '78 carb has the fixed needle that isn't adjustable.

 

[TwoJugs]

They might both be BS38s? Is the cover on the vacuum diaphram aluminum or chromed steel? You can swap any of the XS carbs as long as you have the correct carb mounts that go with them. Some use a choke cable on the later models tough. Jetting and tuning will be relative to what you have in your motor with some base rules but in the long run you will need to get it right depending on your setup.

 

[gggGary]

Stock the 78-79 had the last (linked) version of the BS38. You can swap the carbs with their holders, the jetting stays with the carbs, start with the correct base jetting for the year of carbs you plan to use, then tune to your engine. To be clear taking the jets out of the 75 bodies and putting them in the 78 bodies doesn't work, the stock sizes settings and needle jet/jet needle are different.

The stock 78 carbs did not have a fixed needle, the 80 and up BS34's had a fixed needle which can be swapped for an adjustable needle.
Like 2jugs says the usual quick tip off is BS34s have the chrome lids, BS38s have aluminum lids. But show us a couple of pics and we can help ID them.

 

[gggGary]

Stock the 78-79 had the last (linked) version of the BS38. You can swap the carbs with their holders, the jetting stays with the carbs, start with the correct base jetting for the year of carbs you plan to use, then tune to your engine. To be clear taking the jets out of the 75 bodies and putting them in the 78 bodies doesn't work, the stock sizes settings and needle jet/jet needle are different.

 

[XSsesive50]

"They might both be BS38s?"

They are both aluminum tops. I do believe they are both 38s. The carbs off of the '75 do not have the angle iron joining bracket and synch screw shaft. And the needles on the '78 carbs are secured with a snap ring, not just sitting in the slide like the '75 carbs. I think I read that the needles are different in size between these years, so I may have to start rejetting all over for the difference in pod air filter mods.

 

[gggGary]

The snap ring comes off then you can remove the needle and get at the clip /position.

 

[XSsesive50]

" To be clear taking the jets out of the 75 bodies and putting them in the 78 bodies doesn't work, the stock sizes settings and needle jet/jet needle are different."

gggGary, That's what I was thinking. So if I went up 2 on the '75 carb pilot and main, and dropped the needle to the 3rd (middle) clip, what is the equivalent for the '78 carb?

 

[gggGary]

I ain't worth a shit on carb tuning, I just listen to what 5twins sez...............
But yeah look at the chart above. for what jets were stock.

Be aware there are 76-77 BS38's that are different also. Positively ID the carbs you have then CHECK the brass to be sure it's all what's supposed to be in there. I have rescued more than one bike with stupid wrong brass in the carbs. Some rebuild kits COME with the wrong brass and they have been shipping them that way for YEARS.

Oh yeah the BS38 is just a general body description, the shape of the venturi and just about every part changed several times through the years.

 

[XSsesive50]

Good point gggGary. I'll check the carb guide and try to positive ID the carbs I have.

 

[5twins]

It sounds like your linked set are '78-'79 carbs. They were the only BS38 set that retained the needle in the slide with a snap ring. You will follow the same basic principles when jetting them, just the jet size numbers will be different. Also, and this is important, your later linked set uses a different type of pilot jet than your '75 set. You must use the proper type as the float bowl is designed for it. It has to do with the air passages that feed air to the pilot jet.

The BS30/96 type pilots in your '75 carbs are fed their air supply to the bottom of the jet and the float bowl is designed to do that. The VM22/210 type pilots in the '78-'79 set get their air fed to the top of the jet .....

 

[XSsesive50]

5twins, I referenced the pilot and mains for the VM22/210 and came up with a larger main of 137.5 and pilot of 30 as equal (the next higher of each per the carb guide) to my current BS30/96 choice. Should I drop the needle to the middle clip also?

 

[5twins]

The middle (#3) clip slot is stock for the '78-'79 carb set so you may need to go up to the #2 slot (one step leaner than stock) when running larger than stock mains. When we talk about Mikuni needle slots, they are numbered from the top down .....

 

[CDNTX650]

If your swapping to a set of 38s from a later year and keeping airbox and exhaust stock tothat bike then dont change jetting. Guarantee it will run fine. The motors never changed really from 74 on. they just used different combos of needle taper, mains and pilots with the same setup thrpugh out the years.

 

[XSsesive50]

If your swapping to a set of 38s from a later year and keeping airbox and exhaust stock tothat bike then dont change jetting. Guarantee it will run fine. The motors never changed really from 74 on. they just used different combos of needle taper, mains and pilots with the same setup thrpugh out the years.

I think it would be much easier if I had the stock air filters, but I am running UNI pod air filters and 15" slant mufflers, so I have to re-jett.