Carb question for Mikuni experts

[Jhurt]

Picked up at 1978 GS750E project (barn find basically, hasn’t run in 10 years). The carbs were simply mangled, brass screws loctited in, slots hogged out, screw heads chewed up...so I also picked up an entire second carb rack, gangplate, linkages and everything that came off (supposedly) and 1977 GS750 in much better condition, the idea being I’ll combine the best of both sets into one. Carbs look identical on the outside and for the most part on the inside as well EXCEPT...

The needle rides on a spring on the carbs that came on the ‘78 and there is no spring on the other set. Looking at a parts fiche you’re offered 2 choices of carbs to order parts for and i see the same thing, one slide has a provision for a spring for the needle to ride on and one doesn’t.

Do any of you carb gurus know why this is and more importantly is there an advantage to one or the other? Since I have both I suppose I’ll use whichever is better IF one is better. The slides appear interchangeable between carb bodies as well.

[Jhurt]

https://imgur.com/a/NsRcyX4

https://imgur.com/a/NsRcyX4

edit..,that was my attempt at posting up an image in a thread...bleeping photobucket now I gotta figure out how to use something else. If you click on that you can also see they have different needles as well

[OldVet66]

(http://members.dslextreme.com/users/bikecliff/) Get the 750 8 valve service manual center column and the VM carburetor rebuild guide in the column to the right of the picture of his bike. It has a detailed break down of the VM carburetors.

[Nessism]

Correct me if I'm wrong but with the spring version the needle can float a little more in the slide. This allows it to conform and align easily with the Jet Needle in the carb body thus reduce possible needle drag and wear. I'd use those.

[Jhurt]

(http://members.dslextreme.com/users/bikecliff/) Get the 750 8 valve service manual center column and the VM carburetor rebuild guide in the column to the right of the picture of his bike. It has a detailed break down of the VM carburetors.

I’ve actually read both of those extensively trying to sort this out. The tutorial on bikecliff’s website shows the slide/needle with spring...the factory service manual indicates only that the correct needle is “5F21-3” which happens to be the one installed into the slide without a spring....Clymer manual makes no mention regarding this. The slides are both marked with the Mikuni logo and “1.5”. It appears that both are correct depending on which a particular bike came with. Nessim’s logic regarding needle centering and float will nudge me towards using the slide/needle with spring and see how it runs.

[OldVet66]

My '78 has the springs. They are pretty much the same between the 750 and 1000 except the removable parts inside and some of them might be the same.

[hannibal]

Very interesting. I never noticed the spring in the VM tutorial on BikeCliff's site.

My 77 doesn't have the springs.

[Jhurt]

After a bit more research I found an article from Cycle World 1979...it seems the GS carbs (and probably others) underwent a slight redesign for the 1978-1979 model years due to emissions (of course). The article cited tighter tolerances, 1 size increase in main jet to 102.5, diffferent needle jet and jet needle all were changed after 1977. I bet this information is on the site here somewhere but i wasn’t able to find it, or is it reflected in any manual that I have at my disposal. In case anyone is interested here are the particulars from the Cycle World article:

1977: #100 main jet
0-6 needle jet
5F21-3 jet needle

1978-1979:
#102.5 main jet
0-4 needle jet
5DL36-2 jet needle

I addition to the above I measured/observed on my own carbs several additional changes:
-Slide is recessed for a spring to ride on for 1978-1979
-slide wall thickness is lesser for 1978-1979
-the 3-hole plate inside the slide is not there for the 1977 carbs
-the armature that connects the throttle arm was ever so slightly longer in 1977
-needle has plastic shims for 1978-1979, only a clip for 1977


Slide height and profile appear/measure identical otherwise. I’ll try once more to upload an image but you may have to click on it to see...sorry. Like I said this info is probably here somewhere already but I couldn’t find it. Maybe someone rebuilding or cleaning a set of carbs will find it useful.

1978 on the left, 1977 on the right:

[UKAndy]

When I stripped down my GS750E carbs I noticed the spring on the needle. I have rebuilt a lot of VM carbs mainly from 2 strokes but never seen a spring on a needle before. I also read the cycle world article which details the changes from the ‘77 to ‘78 models. The changes made as you stated were for improved emissions, I believe this is only at slow speed. The spring function is probably to help the needle keep centered in the needle jet. According to the article the ‘78 changes make the jetting leaner on small throttle openings. Referring to my Mikuni jet information the ‘78 needle is a twin taper needle and the 04 needle jet is slightly smaller diameter than the 06 used in the ‘77.

I don’t think there are big differences in either set up so you can use either the ‘77 or ‘78 set up but don’t mix and match the jetting. Also remember the float height changed for the ‘78 set up which will affect how the carb performs.