GS550 carbs on GS650 head?

[Chuck78]

I have a spare GS550 engine with worn rings sitting on my workbench, and just pulled a nice GS650G shafty top end jugs-pistons-head-cams to do a complete overhaul and 673cc 9.4:1 conversion to the engine to probably swap into my wife's 77 550B. I was told many things about the carbs. Lots say do the BS32's, they will run better on the street, lots say those carbs are really hard to get tuned properly, best of luck, run the VM carbs.
I have also heard that the VM's won't fit, that they will swap right on with the 550 intake boots, that they require intake boot customization to make work. What's the deal??? I have a real nice set of VM's that I rebuilt. They wanted more for the BS32's than I paid for almost the entire rest of the engine, but they still have them at the junkyard.

I will have my hand at rebuilding my first CV's, some BS32's on my buddy' 82 KZ650CSR soon, so I'll see how that goes first I guess. Looks as if the big differences are that the needle is raised based on engine vacuum through the rubber diaphragm and spring setup, and that the mains are larger on a CV style because the pilot/idle circuits also flow through the main I had read.

To save money, I'd really like to keep the VM22's and run them for now at least if possible easily. I have an extra set of carbs but no extra boots to easily mock it up on the 650 head.

Also, I was wondering if a VM22 was too small of a bore for the 125cc's extra? We will be running K&N pods.


OPINIONS???? I've heard lots of them so far, but I want to hear a full blown discussion on this topic. Thanks!

 

[Steve]

The BS32s are not that hard to tune properly, especially if you are keeping the bike on the street.

I think it would be much harder to adapt the smaller VM22 carbs to the 650 head, and the smaller carbs might offset any gains that you might see with the pods.

.

 

[Chuck78]

So on the BS32 cv style carbs, the high engine vacuum at idle must pull the vacuum diaphragm and jet needle downward, with the spring pushing upward with lesser force than the vacuum force resisting it? Then when engine vacuum drops drastically as you open the throttle, the spring force pushing up on the needle/diaphragm is greater than the vacuum force sucking the diaphragm and needle downward, therefor raising the needle and giving much more fuel to the engine??? This seems to have an effect of an accelerator pump as well in my mind, as a drastic throttle opening will momentarily lose all engine vacuum, but then it may settle back just a slight bit with more vacuum after the sudden opening, which would have the effect of raising the needle slightly higher for a momentary burst of air/fuel mixture, and then backing off just a tad once settled out??????? I often wondered about the lack of accelerator pumps on bike carbs vs musclecar carbs. The other night when looking at a rebuild pdf file manual on this site was the first time I ever really looked at the internals of the CV's.

I can see how having no vacuum leaks and having the carbs synched properly will be more crucial with these carbs, or "fussy" or "sensitive" rather

I'd still like to know if the VM22's will bolt up to the 650 head with the 550 boots. Maybe I can scrounge up a spare set of boots and mock it up. We don';t have an airbox for the 550, but have two sets of pods. I have heard no good tales of running pods on constant velocity carbs, is there any hope? she loves the way the pods look as well.

I may be welding in a cro-moly steel tubular X brace in the frame where my airbox goes through, so I think I may switch to dual oval K&N RC-2222 filters, like pods but better as they have a shared plenum for the left 2 cylinders into one filter, and the right 2 cylinders into a separate filter assembly. This helps even out the inside vs outside carb airflow flaws of pods due to dirt and moisture and also cross breezes at highway speeds. No such filter is available for the 550's as far as I know unless they have the same carb spacing as a GS750/GS1000/kz900

 

[Chuck78]

Another dinger for me is that we were searching for BS32 parts the other night for my buddy's KZ, and there are NO rebuild kits available! You have to piece together what parts you need, and his needle and seats are shot, need new gaskets, and skeptical about the vacuum diaphragms. If you need a vacuum diaphragm, Bike Bandit wants $148 for a single diaphragm!!!!!!! WTF!?!??! I can get a nice used bank of 4 carbs for that!!!!

 

[mike_of_bbg]

Chuck, I'll just throw in my $.02, I'm sure someone might come along to tell me I'm full of it but anyway...

First, there are lots of us with CV carbs. New OEM slides are very expensive indeed, but those diaphragms also very durable. If you need a new slide, I'd just post on here for a good used one. Aftermarket rebuild kits are available (search Z1 Enterprise's site), though the jets therein are usually considered to be junk. You'll get some of the O-rings you need and needles and seats that seem to have a better reputation than the jets. It's about the only way to get a new idle screw any more if you break the tip off of one. A "rebuild kit" for BS32's is usually just an O-ring kit and maybe new gaskets and pilot plugs. New needle valves and seats are still available OEM from Suzuki at least as of earlier this year. BikeBandit is about the last place I'd go for OEM parts, try gssuzuki.com instead.

In any thread where there is discussion about swapping GS550 VM and CV heads, invariably the answer is that it cannot be done (short of the old catch-all "with enough time and money anything can be done"). The CV carbs don't fit the VM boots and vice versa. The bolt patterns are not the same on the heads, and the ports are different sizes. You're probably looking at fabricating boots or at least adapters. AFAICT there was never a 650 with VM carbs, so even if you could make it fit without leaks you're on your own in the jetting department.

The same BS32 carbs were used on the 550 up through the 850s IIRC, but with different jets and different spacing. I've been told that the 550s and 650s had identical spacing, though it gets wider above that. So you ought to be able to get any 80-82 550 or 81-83 650 carbs and use them if you can find them for less than your salvage yard wants. If you're going pods with CVs you're probably going to want to rejet anyway, and Dynojet has a stage 3 kit for the 650.

I'd get the CVs and DJ kit. At least it will start you off very close to where you need to be.

 

[Chuck78]

The mikuni jets are so inexpensive & the Dynojet kits are so costly! I would rather just find from other peoples experiences what the proper jet sizes would be for intake and exhaust mods. I can get a pair of 550 boots to double check the diameter without dismantling the good running bike. Maybe I can talk the junk yard down, $150 Is what they wanted for a set of VM's off a good running bike. Maybe $100 Since I already bought half of the parts of of that 650! Dual disc forks, top half of engine, triple clamp, and handlebars, possibly fender and ignition pickups...

 

[Chuck78]

We checked with the 1 enterprises for the 82 KZ 650 bs32's & they said they had no such rebuild kit for those carbs but they did have the needle and seats

 

[cowboyup3371]

You don't need rebuild kits...just get new orings from www.cycleorings.com, a can of Berryman's, a new bowl gasket, and clean the carbs properly according to the carb rebuild guide that Nessism put together.

 

[tkent02]

So on the BS32 cv style carbs, the high engine vacuum at idle must pull the vacuum diaphragm and jet needle downward, with the spring pushing upward with lesser force than the vacuum force resisting it?

You have this reversed. The spring pushes the slide down. Vacuum is routed above the diaphragm and lifts the slide up.
There are plenty of descriptions about how CV carburetors work.

 

[Chuck78]

we needed the needle and seats, his airbox was filling full of gasoline from the fuel bowl vent tubes being routed into there!!! Already got him a shopping list of the o-ring kit, gaskets, and needle and seat. Need to look into KZ650 jetting to order the right jets.

You have this reversed. The spring pushes the slide down. Vacuum is routed above the diaphragm and lifts the slide up.
There are plenty of descriptions about how CV carburetors work.

That's the way it looked like they worked, but I figured it had to be the other way since measuring engine vacuum, it drops drastically as you open the throttle. I got some extra GS550 boots, results in a minute

 

[tkent02]

When you open the throttle quickly, the slides do drop, and the engine has to start taking in more air through the Venturi to make the vacuum come back up. When it does, the slide comes up, perfectly metering the fuel flow. There is a slight lag when you whack the throttle quickly, especially on these older CVs. If you move the throttle a little slower, there is no lag.

 

[Chuck78]

Well... I got a spare set of 550 boots, and they DEFINITELY will not fit the 650 heads! I suppose one could machine down the throats to open them up to match the 650 ports, but the bolt pattern on the heads is much larger on the 650's and would need a drill and tap to the head to make fit.

Not sure what the spacing and mounting flange diameter on the GS750 VM26 carbs is (I can measure the ones on my GS750 if needed), but that could be an option if someone absolutely did not want to use the CV's. Here is a picture of the major difference between the 550 VM boot and the 650 CV boot. The 550 fits INSIDE the throat of the 650! Crap, so much for re-using the good running VM22's that are probably already jetted good for the larger engine (I did them too rich upon recommendation from another 550 owner).

Off to the junkyard again...

On the good side of things, I realize that these bikes could stand to have some serious port matching done by looking at the almost 1mm offset on the 650 head to intake boot relationship. Wow.

 

[boontonmike]

upgrade to BS34s

 

[Chuck78]

What models did BS34's come on? I am wondering if VM29's would be the same spacing and diameter to fit onto GS650 intake boots? I believe from what 49er was telling me that the VM29's were more of a race carb. a GS750 and early GS1000 only came with VM26's, later GS1000's I THINK took a larger carb than the later GS750's.

I am a little leery of running BS32/34's originally because of the amount of people saying how they were much more difficult to tune than a VM manual slide carb, but now I am more concerned with the tendency for the needle to drop (momentarily too lean) when you really crank on the throttle grip rapidly. That is the exact opposite of an accelerator pump! She is less concerned with mileage than she is a bike that REALLY GOES (IMMEDIATELY) when you want it to really go!

 

[Chuck78]

I think the GS750 VM26's are smaller diameter still than the BS32's, but it was too darn hard to measure the carbs' outlet while installed on the bike and in the dark under the cover holding a flashlight. If they would clamp straight to the GS650's CV boots, even with a slight bit of modification, I would say SOLD if I saw a set of VM26's! Keep it looking like a stock 77 GS550B engine with the 550 top cover fitting the early 650 heads, and VM carbs still (larger however!).

The spacing on the inside two carbs between the CV 650 boots and a spare rack of VM22 550 carbs did not seem the same, but the outside carbs seemed the same distance center to center. 650 intake ports are MUCH MUCH LARGER than the 550's. I don't want to mess around with superior engineering of the later engine to retrofit older stuff, so if I can't make a VM29 fit (good luck finding a rack of those), I will probably suck it up and buy the BS32's that were on that same bike I got the rest of the parts from. I am tempted to look into making intake boot spacers to fit some VM26's, but afraid of the greater potential for vacuum leaks. Rice Paddy (local japanese mc junkyard) has a huge drawer full of various intake boots however. Maybe I should take the head if I can get those darn #3 phillips screws out of the boots to try and mix/match.

 

[Chuck78]

650 intake ports are MUCH MUCH LARGER than the 550's. I don't want to mess around with superior engineering of the later engine to retrofit older stuff

I say this because (in (muscle)car engines at least)larger intake ports will take longer to fill (not as crisp low end throttle response), but will be fire-breathing monsters at higher rpm's, smaller intake runners will give crisper throttle response down low but run out of breath up high. hogging out the 550 boots to the 650 ports and then drilling and tapping to mount the 550 boots would be experimenting into the unknown as far as engine running characteristics that I mentioned above.

 

[boontonmike]

The 34s were used on the larger bored engine, I know kz1000s used them. I made boots and adapters from oil cooled gsxrs work on my gsxr style flatslide cv carbs. They are BST34ss carb, basically a flat slide 34mm version of the stock 650 bs carbs. I really had very little trouble tuning them. I used a stage 3 dyna kit and within a couple of needle and jetting changes haave them working flawlessly. The throttle response is great and I have not had one single regret. I did however use a wideband 02 sensor and guage made by AEM to get my baseline tune in check using AFRs. This was all experimental and it want way better than expected. I also did some port matching to the manifolds that fit the carbs.

I originally thought I would lose some low end torque / throttle response with this setup, but it seems that the CVs handle everything extremely well. You can crack the throttle and lunge the bike forward right off idle and after 5/6,000 rpm the bike really takes off as its gulping a ton of air.

To be honest I too was treking down the VM 26 route and it just didn't really add up in the end for the effort comparted to the BST34's in my opinion. On top of that the weight difference between the two sets is pretty dramatic and I was going for the most lightweight components every where I could. The BST carbs weigh about half of the VMs judging by holding them.

 

[Chuck78]

BS34's have the bonus of the added availability of $20/each vacuum diaphragms from I think it's called JB something or other, found a link here. THey deal with mostly yamaha stuff (intake boots) but BS's are also used on the rest of the big 4 as well.

BS32's were used on 550's and 650's, BS34's on GS1000's, pretty wide range of useage! I wonder if it would decrease velocity much going with the BS34's on this 673cc GS550 build? Or if it would work the same as yours?

I am going to the junkyard tomorrow, I may check and see if they have any of those BST34ss carbs. Do you know what models they came on? Having reports from someone knowledgeable like yourself as to how this worked out and how it was to tune is invaluable, so I may explore this route as well!

 

[Chuck78]

Did you measure like my site shows? I think the Mikuni BS34 is the same diaphragm. Don James JBM Industries 3761 Morley Drive Kent, OHIO 44240 USA 330-678-9537 http://www.jbmindustries.com/

I emailed them, another member on here also thought the diaphragms were the same between bs32 & 34. This is definitely good news for my wifes gs550-673cc & my best bud's poorly running 82 kz650csr whose carbs we are rebuilding soon! Still gonna look at the vm26/bs32 comparisons though, & check out those bst34ss carbs. Thanks for the tip!

 

[Chuck78]

I made boots and adapters from oil cooled gsxrs work on my gsxr style flatslide cv carbs. They are BST34ss carb, basically a flat slide 34mm version of the stock 650 bs carbs. I really had very little trouble tuning them. I used a stage 3 dyna kit and within a couple of needle and jetting changes haave them working flawlessly. The throttle response is great and I have not had one single regret.

I also did some port matching to the manifolds that fit the carbs.

So did you have to do anything other than port match the boots to the head in order to get them to bolt up? Is the GS650 BS32 spacing the same as the bigger bikes?

http://kzrider.com/archive/viewthread.php?fid=12&tid=5907&action=printable

I am really thinking of going the same route on the GS550/673 engine project on my workbench, although undecided on BST33SS, BST34SS, or BST31SS. I think with the CV operation, the vacuum slides will compensate if you have a larger carb bore than needed. The flatslides work better because the slides are lighter than the BS32 round slides, and therefore are more sensitive to the engine's immediate needs and thus respond faster. I've done a fair amount of reading since last night on this topic.