GS550 with Mikuni Dual Throat Carb, No Info

[Witehorse Motors]

An acquaintance brought me a chopped out custom GS550, needs wiring and revival. It is sporting a set of the Mikuni "dual throat" carbs with 2 carbs running 4 cylinders. I have seen posts online from 15 years ago that attempt to gather specs and info, but none of them go very far. I am trying to learn all I can about these carbs in hopes of overcoming the obvious flaws in this chopper set up such as chopped exhaust and lack of air box.
I would love to know the history of these carbs, what to look out for in tuning them, why they seem to have different main jets on 1 and 4 than they do on 2 and 3?? etc...
They look like they can be saved with a good amount of effort. I have found parts diagrams and my with my first look inside the carbs they seem to need only bowl gaskets, but my goal is to find as much out as I can before attempting to rewire and tune the bike. These units seem like another short lived 80's part quickly orphaned by progress.
ANY help would be great.
Thanks for your time.

 

[Witehorse Motors]

The parts diagram showed it was for a 1983 GS550L BTW.. I have seen references to the bike being a GSX but cant confirm. The internet is not much help for this one.

 

[Nessism]

You can download the factory manual for the bike here...https://gsarchive.bwringer.com/

Different jets is because some cylinders ran hotter than others.

 

[zed1015]

It is sporting a set of the Mikuni "dual throat" carbs with 2 carbs running 4 cylinders. I have seen posts online from 15 years ago that attempt to gather specs and info, but none of them go very far. I am trying to learn all I can about these carbs in hopes of overcoming the obvious flaws in this chopper set up such as chopped exhaust and lack of air box.
I would love to know the history of these carbs, what to look out for in tuning them, why they seem to have different main jets on 1 and 4 than they do on 2 and 3?? etc...

Sounds like a home spun conversion possibly using a pair of Z1300 or Mikuni Solex N40PHH ( Datsun etc ) carbs
Pics would help..
The different jetting is usually to correct the A/F ratio caused by differing air flow rates through unbalanced airbox intake or manifold tracts.

 

[Nessism]

Sounds like a home spun conversion possibly using a pair of Z1300 or Mikuni Solex N40PHH ( Datsun etc ) carbs
Pics would help..
The different jetting is usually to correct the A/F ratio caused by differing air flow rates through unbalanced airbox intake or manifold tracts.

No. The second gen 550, with 16V, came with two, dual throat Mikuni BS30 carbs.


Screenshot 2025-06-12 164914 by nessism, on Flickr

 

[Witehorse Motors]

yes these are the carbs but when I search for these model designations they don't come up in the results. Its like someone wiped them off the interweb. A search for BS30 mikuni shows the 4 individual carb set ups.

 

[Big T]

Yes, it's a GS in North America and GSX everywhere else in the world

And, they're BSW model carbs

 

[Witehorse Motors]

Here they are, No model numbers or letters visible. Pretty cruddy but intact. Considering buying a large ultrasonic cleaner for these (always wanted one anyway) .. look ok but for bowl gaskets. They are on a fella's full custom chopper with lots going against it like exhaust mods and shitty pod filters.
I have to repair the wiring someone hacked apart and get it running like it was few years ago, then see if it can be tuned.

 

[Witehorse Motors]

I noticed no spring above diaphragms?? None on the partzilla diagram also.

 

[Witehorse Motors]

Bowl side

 

[Witehorse Motors]

The jetting is currently Main 105 and 100 and pilots are 35

 

[Witehorse Motors]

Im dumbfounded by the missing slide springs. I cant find a single parts diagram that shows them, but a nice tutorial at Team Ghetto Racing clearly shows them and all CV carbs use them. If anyone can point me to some or get the specs on them I would try and find suitable replacements.
Its a small wonder the bike sat long enough for some knuckle-dragger to start clipping wiring, it's already looking like a pain in the ass to clean up their mess.

 

[1978GS750E]

Find a service manual here.

https://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS550E-ES-L_1983.PDF

It shows those carbs. I've rebuilt a set of them before. No fun finding parts for them!

Good luck!

 

[Witehorse Motors]

I have wasted so many hours already looking for info and now the springs. If i am SOL on them I will have to consider finding a set of 4 BS carbs and intake I suppose. I hate to tell owner I cant do anything for him, but man what an obscure pile of orphan parts.

 

[Witehorse Motors]

Hold the phone.... The carbs in the tutorial from ghetto racing have different slide assemblies with plastic parts and springs. The service manual linked shows the slides I am dealing with that are metal, four screws, NO SPRINGS... I am curious hoe they function to return slide and needle at closed throttle , I am guessing the weight of the assembly helps
.. Man what a challenge these are.

 

[Griffin]

I'm not surprised. The 16 valve '83-'86 GS550E/ES/L models were not made in great numbers, and had a tendency to wipe crankshafts if used for racing for extended periods of time. They had many parts unique to that bike only.

I had an '83 I used for racing in '86 and '87. They were very competitive with the GPZ550 and XJ550 Seca & FJ600. But then the the 600 Ninja came out in late '85, and the air cooled middleweights were swept off the track..

Your customer is best served by parting the bike out and finding a more common bike from the period that hasn't been put under the sawzall. The owners of the few left in good shape could use the parts.

 

[Witehorse Motors]

Well I was away for work but I'm back now looking at the carbs and getting ready for a deep cleaning, there were two hoses just venting to the atmosphere on each of these fittings which seems like on a lot of old Japanese bikes they should be connected to each other

 

[Witehorse Motors]

Also the parts diagram does not specify what this fitting is I would assume it is event for the bowls, it looks like it used to have a hose coming off of it that rotted Off

 

[Witehorse Motors]

Well it may not be easy or fun to work on but this is someone's chopper that they put a lot of effort into so I've got to try my best, up to a point of course

 

[Witehorse Motors]

Hmm.. I think it may be a vacuum petcock fitting actually. This tank is not using one. I will cap it later.