78 or 80 carbs

[Guy&2goldens]

Hello all,
have a question for the seasoned Xs people. I checked the site and couldn’t find any info so figured I would post.

I have a 78e I’m working on restoring. I also had a parts bike of an 80sg. Is the carbs on either model any better? I read there is some slight differences between them but couldn’t find if one year performed better than the other year.

Any input is appreciated

[DiverRay]

The set that is in better condition is the one to use. The '78 you can tune easier if all the parts are good, such as the rubber at the top of the slides. The 80's have the plastic floats and less tune function, but still work well when they are set for the engine and synced.

[Dan Hodges]

The 80 and up flow more air and Dyno Jet kits are still available for 80 and up models. A Dyno Jet kit in the late carburetors are the cat's meow and you can still get new slides for the late ones. If you have a pristine set of the earlier one's use them.

[Guy&2goldens]

Thanks Ray & Dan. Both are in decent shape. The previous owner re-jetted the 78 as he was going to run pods an a 4 into 1. So I will need to re-jet. The 80 set is all stock, but I haven’t gone through them yet. Kits for the 78 seem a bit harder to find than the 80. I may as well go through both while I’m at it.

cheers,

[oldyam80sg]

Cleaned, tuned, and synced carbs are the only way to go. My 80 SG has a 4 into 1 Kerker, APE filters, and carbs have been rejetted with a Dyno Jet kit. When they are cleaned, can be an absolute joy to ride the bike.

[bikerphil]

The 80 and up flow more air

That's interesting, I would imagine this is due to larger diameter of the slides?

[DEEBS11]

Didn't the later 1100s get bigger valves in the heads? That might account for more air flow. Are the venturi sizes on the BS34s the same for each year?

[gtem]

The slide diameter is 1mm larger on the 80+ carbs from what I've observed messing with a set of '80 and '78 carbs side by side. I suppose that would support a bit more air flow. IMO if they're complete and original, stick with the '78 carbs.

[Dan Hodges]

That's interesting, I would imagine this is due to larger diameter of the slides?

The venturi on the late carbs are 30 mm and the throttle body is 34 mm and the bell is 50 mm. I measured the slide diameter and it's 30 mm. I don't know what the slide diameter is on the 78-79, I've never measured them. Back in 2005 I researched the carb stuff when I was building my big motor and used the late carbs because Dyno Jet didn't make a jet kit for the early stuff. Simply stated, the late carbs are a better piece if you can find them. FJ 1100-1200 carbs will also work nicely with a big motor but the choke is cable operated however they will fetch 3-4 hp on a big motor with a jet kit and K& N Pods and the other pods are junk.

[bikerphil]

The venturi on the late carbs are 30 mm and the throttle body is 34 mm and the bell is 50 mm. I measured the slide diameter and it's 30 mm. I don't know what the slide diameter is on the 78-79, I've never measured them

On the early type, the slide diameter is 28 mm, throttle body 34, and bell 50, so 2 mm smaller on the slides.