Mikuni Two-Barrel BS30 carburetors

[Thomas Kenny]

I have been looking for a book or articles on these carburetors that are installed on my 1984 GS550ES. These carbs are unique and sort of goofy from the rest the the GS carb family. There is not much info that I can find except for jet kits. In addition there is virtually no information on tuning these carbs in the Clymer manual. There are no idle adjustments referenced in the manual or that I can find on the carbs. I have been looking all over the internet for some info on these carbs and have found nothing. Any one know of a source of information on them? Thanks.

[distraction628]

carbs

The haynes manual #1133 about how to tune the carbs. I live in Portland I would be glad to help you with them. I loved my 550 till it came back to me in lets say a bit here and there from the damn theives!
The carbs are great once you learn a few tricks. making spacers for the float bowls so the will hold more fuel. I play'd with lots of jets instead of the DYNO kit . PM me if you want to chat or if you want me to come down sometime to help with the carbs.

[oldschoolorange]

Interesting..... I was just thinking the other day that a larger float bowl would help to eliminate some problems I have been having recently with my bike beeing jetted fat because it is so cold out , have a 4-1 on it with a low restriction muffler ( no baffles) and UNI foam air filter.

Do you make the spacers yourself?
And can you put in a larger needle and seat?

[Nerobro]

BSW30's Not BS30's. :-) That's an IMPORTANT "w" There are "related" carbs from several other bikes. The GSX-R 250 and GSF250's used BSW28's. And the KZ1300 used BSW32's

distraction628: care to share your carb tuning experience with us? My household has two running 1983 GSX550's that could use the advice. What do the spacers really do?

[distraction628]

The spacers give you just enough more fuel to keep it from foaming under hard running and in the coners it keeps enough fuel to keep a constant fuel flow. I have seen many of the old timers well now they are old timers LOL, do this because of the fuel starvation under there race conditions, they said that it would help on the street as well. It made a big difference on mine and yes I have tried on a KZ also for my friend and He swears he gets a better all around performance as well as better mileage.
I think Mikuni just fell a little short on making the carbs great and once you have tuned solex/mikuni carbs on a car you look at these and say peace of cake to tune some differences but the idea is the same and when I started messing with bikes is when I was racing Datsuns and I was Nissan trained so I just tried things that worked on my race cars to bikes.
Ok need to goto work so I will try to write more about all the things we did with the carbs what worked and what really didn't work!
Take care all
Kevin

[Thomas Kenny]

Thanks Neboro, BSW30 it is. Kevin has been giving me some good information on these carbs. I drilled out/uncapped and found the mixture screws on top in front. Got a Haynes manual coming and I understand that Haynes goes into more detail on these carbs than Clymer.

[skreemer]

for the 83 - 86 GS550 bikes the clymers is crap... the Hayne's does a much better job...

In all the messing about what are the settings you all use for these carbs? I have my fuel/air mix screws out at 1 and 1/8th turn and the stock pilots... the mains I'm using the dynojet 114's... but I have no idea how they translate to real jet sizes...

the intake has the K&N "OMFG!" sized pods on it and the exhaust is a Vance&Hines unit that has no baffles just packing... it's nice and throaty with OK low end power when the packing is in place... with no packing it's loud has hell and the bottom end goes away and the top end of the RPMs is hang on and pray...

[distraction628]

DYNO JETs

Ok they have some that are strait across as Mikuni and some that aren't even close. I learned a lot about jets from some good people along the way. Some aftermarket jets can vary by a whole size, so it's best to get a set of jet drill bit's, yes they are out there, but you can do the same with jewler drill bit's be carefull if you choose to drill out jets, I use them to measure my jets. Nice thing about where I work we have them so i don't have to spend my cash on something I only need once in awhile.
There where many things we tried with the carbs from intake spacers to cooling tubes on the float bowls to say the least it wasn't worth are time on those things. I would say the best improvement was shim'd needles and the FB spacers and then jet sizes depending on exhaust and intake setup. But on the jets we saw on the A/F meter that 2&3 were way on the lean side above 2200 rpm's on the dyno and that 1&2 got lean at about 5500 so if you wanted the safe way to go with stock exhaust and intake we move the outside jets to the middle and the next size to replace outside but we still thought it could be better so we put same size all across( 1size bigger) and things where a little rich to about 4600 and lean at 10000 rpm's so we tried the next half size and it was way to rich under 4000 so we shimmed the needles( I know it should have gotten richer down on bottom but something is a little different with thes carbs, someone tell me why this worked this way please, Yes i know dirt could have been or a stiff slide spring or something) with 2 shims and what would you know we had it running very good. The idel air screws were always between 1 1/2 to 2 1/4 Then the after market exhaust went on and we where back to square one. I will go into that another time. Need to get for awhile. hope I'm helping all of us GSR members
Take care all
Kevin

[Nerobro]

I"m waiting with baited breath. What carburation settings did you end up with? or do you not have those notes still?

[distraction628]

final main jet sizes

For all of you asking what size we ended up with, stock exhaust and stock air box we found 105 all 4 best to 5000 feet and 102.5 above 5000 feet.
Stock air box k&n filter Motad 4 into 1 performance exhaust 110 in all four.
Stock air box with 4 inch holes k&n motad exhaust 112.5 but we liked the 115 better for top but would load up if running under 3500rpm's for to long. All setting where with one size bigger pilots and 2 shim's under the needles even in stock form with the bigger pilots we found the idle was better and the 1/4 to half throttle was very good. I hope all that try this only use this as a guide and not a perfect answer to all your jetting, as there are many things that can change what size you need. Portland is almost sea level and all tunning was done on a dyno frist then on the track at PIR and I only did the drag part. now stock I was around 13.3 and after all done I was getting 12.0 best and 12.4 average. I weighed in at 225 lbs during testing and my friend was 185 and it seem to make .2 sec difference. This was all done in 1988 with a bike that had 10thou on the clock and a barnett clutch. Wheeleeeeeeessssssss came up with ease.
Stay safe have fun
Kevin

[Nerobro]

what did you use for shims? :-)

[distraction628]

shims

Stainless metric washers got them at TrueValue Hardware stores.
Take one of your neeles with you to find the size that will slide all the way up the needle and that they are the same size as the top of the needle.It's always a good idea to have extra, plus you will see that some are thicker/thinner try to find matching ones. We started with one worked our way upto three and ended up with 2 your results may vary.

[txironhead]

Vance&Hines unit that has no baffles just packing...

How do you set up the exhaust that way? In other words, how do you pack the pipe without a baffle?

[Nerobro]

Call it what you will. You might call it a baffle. But there's a cage you wrap the fiberglass around in the V&H pipe.

Not all pipes have facilites for packing. If you need a famous example of a "packing-less" pipe, look at dynomax mufflers. Or if you need examples more close to home, the stock pipes are all about baffles, not about packing to absorb sound.

These are some good tips. I wish I had an O2 sensor, if I did I'd be much closer to having my 83 running in tip top shape. ;-)

FirstTimer jinxed me. he said I shoudl clean my pilots. Then the bike wouldn't start. I parked the bike last night, and when I came to it in the morning, there was a puddle around the kickstand. Skreemer had mentioned that I was leaking oil.

It wasn't oil. Looks like one of the float valves in my carb isn't sealing quite right. So all the gas that's in the line from the tank to the carb dribbles out onto the ground. *shakes head* And the reason why it looked like oil? It washed the oil off the base of the back of the crankcase.

SO... Dem carbs need to come apart, the mixture screw covers drilled out, and I need to order some jets. :-) (not many.. I happen to have most of these.... *shakes head*)

[txironhead]

Sigh.....looks like I'm going to be buying a baffle after all. Might just try to fabricate something. That's the problem with lots of time and little money.

But I have metal shears and a drill!