Mikuni BDS26 fitted to all US models.
Mikuni BDST28 fitted to UK models 1992-95 & Canada.
Mikuni BDS28 fitted to UK models 1996 onwards.
An excellent article covering strip down, cleaning & rebuilding the BDS26 (BDS28's are much the same) by Radare can be found here: viewtopic.php?f=8&t=7991&p=78670&hilit= ... ild#p78670
Although similar the BDST28 differs somewhat as it used plastic chokes, plastic tops, flat sliders, plastic sleeves, different jet arrangements, floats, connecting rails & a oil heating system on the bowls. Another excellent article covering this type by Lite Tek (who supply all the seals for these & other carbs) can be found here: https://litetek.co/Guide_MikuniBDST_Rebuild.html
My bike being a 1992 UK model is fitted with the BDST28's running a modified airbox, original downpipes & Black Widow Exhausts. Prior to the airbox mods the bike was running rich with a flat spot around the 6K mark. I had previously removed the float bowls & cleaned out the carbs which were very cruddy inside & out. After modding the airbox things improved a little on cold damp days, but remained similar on hot dry days. My initial idea was to lower the needles, but I was also beginning to think that it may be a choke problem. The carbs were removed, stripped & ultrasonicly cleaned. Lo & behold look what we found:
The tips of the chokes (AKA starter set) were almost completely worn away, probably causing rich running. This diagram shows the workings:
Further Googling revealed that this is a not uncommon problem on these carbs. No problem, see if we can order some new ones. WHAT! £68 each! That's £272 + p&p! I can buy a whole bike for that! You thieving &$£*! After being discharged from A&E for a suspected heart attack, I spent many a late night glued to the computer, researching anything I could find out about these carbs - which is not a great deal incidentally. The choke (part no. 3LN-1410A-10) was only used on Yamaha XJ600's & FZX250 however an almost identical choke (part no. 3EN-1410A-01) was used on early Yamaha FZR, TDM, TRX, XTZ &YZF series & Ducati Monsters, which are available but at £41.60 each still quite expensive. The choke assembly pulls apart easily & all you need is the plunger/piston:
I bought a set from here: https://www.stein-dinse.biz/product_inf ... ts_id=3085 At a shade under £36 delivered, a result, or so I thought. It was not to be, it appears that these items are slightly larger & used on the 38mm vesrion of these carbs:
These being the only plungers available I had no option but to take a chance on some second hand items from fleabay. Although very slightly worn these items were a lot better than the originals & didn't cost an arm & leg. The wear problem appears to arise from the choke intakes, for some unknown reason instead of using nice filtered air from the airbox the choke circuit is supplied by its own independent intake:
which only has the most rudimentary mesh screen to keep the flies out. A suitable air filter foam was trimmed to size & shoved inside the offending orifice to help alleviate this problem:
The complete Lite Tek seal set was purchased at a cost of £95 delivered - sounds a lot but the OEM seals would set you back over £300 & are not ethanol compatible. These promptly arrived within 3 weeks from Thailand & were fitted without incident. One diaphragm had to be replaced, luckily a reasonably priced item was found on fleabay. Reassembled, balanced by eye, refitted & all up & running:
I will break out the vac gauges & get these properly balanced soon. Hopefully a test ride will be in the offing once the weather cheers up.