GS550 carbs on GS650 head?

[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.

[Chuck78]

Mike, what jetting and needles are you running now???? I wonder if I got a set of 33's, if the jetting would still be pretty spot-on if I started out with your combination? I also wondered if I could squeeze the BST31's into the BS32 intake boots???? Maybe worth it for performance to go bigger rather than smaller and just buy some GSXR BST33 or BST34 intake boots

[boontonmike]

I'm using 110 Mains, stock idle jets, idle mix screws at 2 turns out. I have a 4 into 1 and K&N pods as well as dyna coils and dyna iginition. I have the dyna stage 3 needles at the 2nd clip form the bottom. I still need to do some tweaking when riding season starts. MY AFRs are looking with a half point of where I want them and the bike starts up in 1-2 kicks and idles like a champ.

As for boots I kept buying different katana and GSXR boots, I'm not sure which I ended up with. But i did use 2 of my stock BS32 boots aluminum plates as spacers. I want to buy some more because I am almost positive I could make 4 work if I had the time.

Check out my build thread:

http://www.thegsresources.com/_forum/showthread.php?t=183707

[Chuck78]

Well I am watching some carbs on ebay now. I see rebuilt/reman sets for $400-$500, but used sets needing rebuilt starting around $50-$150. I think I will probably pick up a set and deal with delving into the unknown and try your jetting and intake boot suggestions. If all else fails, I can rebuild them and re-ebay them.

The local vintage japanese mc junkyard has a whole giant drawer full of intake boots, so maybe I could come up with something. I think in your build thread, you may have listed exactly what models you had to combine boots from to get the wider carbs to fit the narrower spacing on the 650 head. I thought I reposted that info somewhere here but maybe it didn't go through.

[Chuck78]

http://www.ebay.com/itm/200856086897

That is a KILLER deal as it includes $60 worth of Emgo pods that I'd need and $60 for the carb set and free Katana throttle grips and cable etc. They are BST31's though. I was hoping for 33's.

What do you guys think, Mike in particular? BST slingshots flow better than the original GS650 BS32's, so maybe going with the 31's would still be an improvement? The BST33's are cheaper than the 34's, and they are the same casting, so same boot and filter sizes. Not sure about the 31's on that note. So I am leaning towards the 33's.



I can get the flat oval pods for $60 a set, or what would be really beneficial to tuning and cross breezes at speed would be K&N dual oval filters so that you have more filter area and you have two carbs sucking from one filter plenum that won't suffer from carbs 1&4 running rich due to cross breezes, and more dirt ad moisture accumulating on cy's 1&4 pods. I'm doing this style on my VM26'd GS750.

[cowboyup3371]

The Emgo pods are crap. If you are going to use them, go to the K&Ns

[Chuck78]

I had heard that they don't filter well, and have heard other people just saying they weren't as good in general.

The intake boots needed to fit the wider spacing GSXR carbs is still somewhat of a mystery, but Mike had enough info on his build thread that I think his parts matching can be duplicated. I am most likely going to buy the GSXR BST33SSS carbs, although I might be able to find a set off of an older GPZ, as I had read that the older they are, the less plastic parts there are in them and the more aluminum. Not sure if that goes for the 90-97 GSXR models I'm looking at, as they may be the same through the whole run.

I can snatch up the carbs that came off the head that I bought, 81 GS650G BS32's, for $150, but I am really leaning towards getting these slingshots and trying to one up everybody else (except boontonmike's sick build) and make my wife a really happy girl, as she always wants to go faster faster faster, speed demon... She is short (5'6") with shorter legs, so the GS550 fits her perfectly after we lowered it an inch, so going bigger to a GS750 or 1000 for more speed is not an option, making this the raddest fastest and best braking GS550 based bike IS the only option!

[Chuck78]

I had accidentally reposted this on Mike's build thread when I meant to copy and paste it to here, oops:

From your build thread, thanks again Mike! Between you and 49er, our two bike builds here 920cc GS750 with VM26's, and 673cc GS550 with hopefully BST33SS flatslide CV's will both end up being nearly dead on with the first set of jets and needles we put in them!

I got these 88 GSXR 750 intake boots which I though would line up with the two 92 GSXR750 ones I already had. Turn out 2 from each set did line up spacing wise and in a nice horizontal line. Only problem was the 88's were 1/4" shorter then the 92's.



Luckily I had the stock 650 boots which are an aluminum base moulded into rubber. An hour of cutting filing and drilling on 2 of the 650 boots turned into perfect spacers (well within 1/16th" which should do just fine). The final step is to port match the head inlet to the carb boots (as seen on other members 650 head projects).





I think I finally narrowed the carbs down to BST33SS (or 32 ) Mikuni flatslide CVs off of late 80s GSXRs.

[Chuck78]

Well, I picked up two complete racks of 1990 BST33SS Carbs off of a Suzuki GSX600 for $75 off ebay shipped, and two dual oval K&N air filters like I pictured above for $50, now I just need to do some searching for intake boots until I find at least what boontonmike has, if not a set that can be adapted that doesn't need the 1/4" or so extra spacer. The spacing on these carbs is wider than the GS650, and I suspect that it would be tough to find any compatible carbs other than the BS32 setup with the spacing to fit a GS650 head.

Any other tips on intake boots that will make this fit as good or easier than Mike did?

[Chuck78]

Several years later, I realized KZ1000 late 1970's VM28 carbs have the exact same spacing as the ports on the gs650 heads! Same spacing as GS750/1000/Kz750-4/Z1/Z900. The boots for those models are straight, no offset. Only problem is the heads for those applications have the intake port extended as a spigot beyond the head a bit, the gs650 intake boot surface is flush with the head and the boots are taller or longer to get the carbs past the cam chain tunnel.

The gs400/425 2v models have a spacer between the head and intake boots that would solve most of this problem. The vm28 kz1000 boots have a bit shorter distance between the mounting bolts, but I think I could slot the ends of the boot bolt holes and use a couple of larger washers to distribute the torque inwards more. Then we have manual slide carbs that have a diameter that matches up really well to the bs32 intake ports!

I'll be trying k&n rc2222 dual oval filters to see how they fit in the compact gs550 frame as well. Not much room for even standard size pods on the 550 frames.

Also we can run these more kean in the cruising throttle range as Kawasaki did on the late 70's for EPA/Emissions standards, as they added an accelerator pump to eliminate the throttle roll on stumbling that was a byproduct of the leaning out the mixture

[tkent02]

I think if I rode mostly at one elevation or a narrow range of elevations like on the eastern half of the US I'd go with VM26 from a 750, 850, or 1000. Yep, it might take some doing but I'd make them fit. Better throttle response, easy to tune, the larger size would flow plenty of air for a 700.

Where I typically ride I could go from 2,000' to over 14,000' in a day, and sometimes up and down 6,000' or so many times a day, the CVs work much much better for that. I'm happy with them, at least I will be once I get them dialed in.