Differences between carbs

[Clumzi]

I feel like I've maybe got some conflicting information about carbs, or perhaps I was just confused in my own brain. With the bikes that use the "same" carbs, what are the differences between them as you go between different displacement bikes? Are there fundamental differences that cannot be adjusted or are the differences just in things that can be adjusted, like the float heights and such?

Also, what are all the various things that can be adjusted in the carbs? Is it only float height and the air screw (and is it an air screw or a mixture screw or what? I see different things) or are there other things? -nick

[Steve]

OK, presuming we are talking about the Mikuni BS series carbs found on your 1980 550?

They are BS32 carbs, meaning the venturi is 32 mm in diameter. These same carbs are used up to 850 cc bikes in the Suzuki GS line. The differences are the main jet, the needle jet, the jet needle, the pilot fuel jet and the pilot air jet. If you look in the first post in this forum, you will see, in various places, a listing of the stock sizes for these items. Another difference will be the length of the fuel and vent tubes between the carbs as the spacing might be different (I don't know for sure).

Things that can be adjusted? Any of the above-mentioned parts and the idle mixture adjustment screw.
Things that should be adjusted? That depends on what you have changed.

If you have removed the stock airbox and installed a header, you will need to change the main jets for sure. You will probably have to shim the jet needle height, and, in extreme cases, might have to change the pilot fuel jet, as well. For the ultimate in fine-tuning, you might also have to adjust the pilot air jet, but that is not very often.

.

[Clumzi]

So am I to understand that if these parts aren't replaced the bike won't run with carbs off a 650? I just pulled one of the four carbs apart and they look identical to the bits and pieces in the carbs that are currently on the 550 (except the needle seat, that was different for some reason), but I'm certainly not saying your wrong. Even though they look identical, there are minute differences between all the pieces you mentioned that'll keep the bike from working with the 650 carbs? -nick

[Nessism]

My understanding is that the carb bodies are the same but the various bits inside are tuned for each application. Check the carb sticky on the top of the forum and compare your carbs to the 650 version. There also may be difference in the linkage but I don't think so.

To get the 650 carbs to work on a 550 you will have to move over all the jets from your 550 carbs and install them over.

Good luck

[Clumzi]

Alright, thanks folks. I'm just gonna finish rebuilding the 650s, plug them in and see what happens. There is a chance the carbs are in fact off a 550 because the forks I got that came of the same bike are set up for a single disk which implies the bike wasn't a 650 so... I have no idea what these things are -nick

[Nessism]

The jets have numbers on them, cross reference with the carb sticky to find out what the carbs are off of. And no use sticking them on unless you have the proper jets, bike won't run right.

[Clumzi]

The bike always ran like ****, if it runs at all I'm thrilled . When I first got the thing, I rode it for a month or two with crossed ignition wires and needles and seats off another bike and didn't even know it. My expectations really aren't all that high for this thing. -nick

[Steve]

There is a chance the carbs are in fact off a 550 because the forks I got that came of the same bike are set up for a single disk which implies the bike wasn't a 650 so...

Don't use that as a determination what the carbs came off of.

Check out this picture:


Note the single disk on the front wheel? As far as I know, it's stock. 1982 GS650GL. That's right, a 650 with a single disk.

Although the internal parts in the carbs might look similar, check the numbers with a magnifying glass. Checking with the 'carb sticky' at the top of the forum, post #1 shows the main jet for a 550 is a 92.5. Post #13 shows that the main jets for a 650 are 110. If you use the 110 jets in your 550, it won't run very well, because it will be too rich. I did not bother to compare the rest of the jets, but I am sure you will soon see the differences there, too.

Go ahead and use the 650 carb bodies, but use the 550's jets.

.

[Clumzi]

I remember you posting that before, Steve . That is what made me check the numbers. Although they are tough as hell to read, they definitely point towards a 650. I'm not sure what exactly the plan is now, but I pulled one of the carbs completely apart, dipped it, reassembled it and now feel a good bit more confident about the process. It was a good practice run on something that, if I did bust something, wouldn't have screwed me.

So now I'll either keep going on these 650 carbs, get all of them good and cleaned up and see if anybody here wants them for a couple bucks or go pull the 550 carbs and do those first and then come back to the 650 carbs later. Six of one / half dozen of the other. -nick

[Steve]

Pull both set of carbs apart. Dip the 650 bodies and the 550 jets, re-assemble as needed.
Keep the 550 bodies and 650 jets as spares to help somebody else.

[first timer]

Like Steve said use the 650 bodies if your 550 bodies are shot. Then compare the numbers of all the little bits inside the carbs, these numbers are posted in the carb sticky at the top. if the 650 and 550 share the same numbers per bit then use teh better bit. Other wise you will need to use the 550 bits. the 650 is a bigger engine and requires more fuel so if you use 650 bits specifcaly for a 650 your bike will run like poop because it will be getting to much fuel. Also rember you can not get a good idea how these bike run unless you have your air box and filter installed. These motors are very sensitive to fuel changes and with no filter and air box it will not be happy.

After you get everything running good and are please then go to the next step of rejetting and removing the airbox/filter if you so desire.

There is no magic bullet with these carbs and chances are very very high that you will need to dip, clean and re-o-ring any carb you have found before you get the bike to run well. Don't waste your time doing it half assed just clean everything like the carb clean up sez.

[waterman]

I am going to go against the grain here, but you can try the 650 carbs in the 550 and probably get it to run okay. I have both bikes and my 550 didn't run well with the stock setup. Pretty sure it was nearly stock with aftermarket main jets. Since I had an extra set of 650e carbs sitting on shelf, I decided to play around. First was replacing 650 carbs with 550 parts. Ran about the same, still pretty long warm up needed, excessive bogging until fully warmed. Then I went the other direction with 650e bits. No longer required the 2 minute warm up and seems to give a little more total zip. I haven't ran it more than about 15 minutes, but still doesn't run nearly as nice as my 650g at all speeds.

I will tell you that the 550 carb parts; needles, slide springs, are much different from 650s. The 650 shaft drive models also seems to be tuned a lot richer than 650 chain drive on main jets (110 vs 97.5). Conversely the main jets between 550 and 650e is similar 92.5 vs 97.5.

I find these changes interesting since there is only a 123cc difference in motor displacement.

One thing you may not consider with the carb body swap is that there are non replaceable air jets that are different between models. Likewise, the throttle butterflies are numbered different.

Here is compiled data from top link of tech section
GS 650 GZ/GLZ (82)

idle r/min 1100-+/-100r/min
carb mik bs32ss (4 seprate carbs)
id no 34300
bore size 32 (1.26)
float height 22.4.+/-1.0 (0.88 +/-0.04)
fuel level 5.0+/-0.5 (0.20 +/-0.04)
main jet #110
main air jet 2.0
jet needle 5C45
needle jet Y-7
pilot jet #42.5
throttle valve #135
by pass 0.8, 0.7, 0.8
pilot outlet 0.7
valve seat 2.0
starter jet #45
pilot screw pre-set (1.5 turns back (mixture screw))
pilot air jet #160
throttle cable play 0.5-1.0 (0.02-0.04)
choke cable play 0.5-1.0 (0.02-0.04)

GS550 (80-82) the specs are for the CV's

idle r/min 1100-+/-100r/min
carb mik bs32ss (4 seprate carbs)
id no 47160 (us) 47170 (can)
bore size 32 (1.26)
float height 22.4.+/-1.0 (0.88 +/-0.04)
fuel level 5.0+/-0.5 (0.20 +/-0.04)
main jet #92.5
main air jet 1.6
jet needle (4bel2 us model) (5f42-3rd notch canadian adjusable needle)
needle jet x-6
pilot jet #40
throttle valve #135
by pass (0.9, 0.7, 0.7 us model) (1.0,0.8,0.8 canadian)
pilot outlet 0.7
valve seat 2.0
starter jet #35
pilot screw 3.5 turns back (mixture screw)
pilot air jet (us model #150) (#120 canadian)

I can't copy 650e info, this link has it;

http://www.thegsresources.com/_forum/showthread.php?t=73723&page=5

Clean them up and give them a go, I would bet it won't run that badly.

[first timer]

(removed for being dumb)

I have a 550 as well and if you clean the carbs very good and make sure the boots are sealing, the bike will start very quicky and can be ridden immedatily. I find the 550 engine is very sensitive to changes and your carbs need to be spotless and dipped, the littlest air leak or crude will mess it all up.

[Steve]

I am pretty sure the 650 block is acutally a 673cc block and the 550 is a 449cc block so the differacne would be 224cc's that pretty big, but i could be wrong. Nerbro, skeemer want to chime in?

Any idea why they might call a bike a 550 when it only displaced 449cc?

Maybe it really displaced 549cc?

That would make the difference between the 550 and 650 engines only ONE hundred twenty-four cc, right in line with waterman's version of 123cc.

.

[first timer]

opps my bad yeah..... I knew that