where to start - Jetting Gsxr750 Mikuni BST38ss carbs for the GS500

[DanB]

Hi all,
//
Has anyone ever fitted a set of BST38SS Mikuni carbs to the GS500? I happen to have a spare set so have re-racked them using a spare set of gs500 carbs so they now match the GS port spacing, I'm having some tapered flanges CNC turned at my work that will taper down from the 36mm carb to the stock port sizes while I port my spare head out to better suit the new carbs.

The main question i have is where do i start with jetting. stock main jet for the GSXR750 is 122.5

currently the bike has a lunch box, slip-on, 5-deg ignition advance, run on UK 97ron fuel with 150 main jets with the clip 1 notch down on the needle(same as 1 washer).

carb photo for Ref (not my picture)

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/~AIAAOSw7ThUoUqN/$_1.JPG

Any help appreciated.

Thanks
Dan

[Big Rich]

The GSXR carbs wouldn't happen to use the same style jets as the GS500 carbs, would they? That would make things much easier.... if they are different you should be able to find flow rate comparison charts online for different jets (although I think the charts are for Mikuni = Keihin = Dynojet).

From there you'll still need to tune the jetting for your individual intake & exhaust though.

[iamhiding]

ohhhh you have my interest depends on what the carbs are from really but slingy and wc 750s should use the same style mains as the gs, my wc 11 certainly does.

i'm putting a set of 38 blade carbs onto my cbr6 in the near future and its been recommended thats the best way to do it by someone who had already done the swap and he was running them just using the jets he was using in the previous carbs without any hesitation or bogging. although i can't speak for his credibility when it comes to tuning you might just get away with emulating your current setup onto your other carbs as a base tune at least

look forward to hearing how it goes 

[DanB]

it would help if i listed the correct carbs and picture wouldn't it....

there BST38SS not the 36's

they do indeed use the same size jets as the GS, only they also have a power jet, this adds a small amount of fuel at WOT over 10,000rpm so not much use on the GS although this means it could run a bit rich up top.

there basically the same as the 1992 GS carb's but bigger, fortunately all the frame work from the Gs's carbs fits straight on, as does the choke mechanism meaning i can use the gs's bar mounted lever (the gsxr750 choke was a pull knob on the carbs not the bars)

I do have to add a lever extension to actuate the second carb as the carbs are now further apart ,but I can sort that with a bit of fab work.

[Big Rich]

Dan, I just remembered - there was a guy here with the screen name Burning1 that did a good bit of engine work on his GS track bike. You might look around for his posts, or the bike that Cedric built. Cedric used a GSXR top end (slightly modified of course.... haha) to build a monster GS500.

Good luck and keep us posted! I love seeing projects like this.

[DanB]

ahh yes i have seen Burning1's track videos, thanks i will have a look at the others.

Slightly surprised i haven't had the obligatory, buy a bigger bike post that i usually get from other forums.

Just trying not to go overboard with mods like i did on my old SV650.. 

[DanB]

Comparison.

[The Buddha]

The oil cooled Kat's and GSXR's are similar to GS carbs, the parts are interchangeable. Carbs themselves - not really, but even if you did - this is what you'd run into.
You go from a 33 to a 38, you will lose velocity through the venturi.
That means, you will have to fit larger jets than the stock GS jets for the same setup.
You may be able to use the GSXR jets, if the swept volumes of the 2 bikes are close ... I mean you're filling 250cc but a max of 10K and the 750 fills 187 cc to 13.5k. You may be able to get good velocity in the 38mm to get good carburetion but @ what rpm is the question.

A larger carb will let it rev up faster, but not neccesarily run better at either the lower or higher rpm. For example, you start @ 2k an rev to 8k. A 38 vs 33, a 38 will get from 2k to 8k in a lot less time if the motor can rev up that quick. but it may not burn or make more power @ 2k or 8k than the 33.

You're likely to need a larger carb if the motor can rev higher than your current carb can supply - as in you cant rev to redline under light load.
Or your motor can rev quicker than your carb can. As in your motor can rev 2K-8K in 4 seconds but that carb makes it take 6 seconds.

I think the GS motor cant rev any faster, nor any higher than the 33's will allow.

Cool.
Buddha.

[DanB]

Hi Buddha,

Thanks for the reply, so without supporting mods its fairly pointless.

The other mods I'm looking into are decking the head and porting the Inlet and exhaust tracts. I have heard rumours that the only thing stopping the GS from revving higher is the valve springs, this of cause does not mean the GS would benefit in any way from revving higher as the power drops off before the redline as standard, unless the larger carbs start to come into play and extend the power beyond the standard RPM range.

I'm not sure what effect having tapered inlet runners from carb - head will do to the flow characteristics as well, obviously length is a big factor.

Thanks
Dan

[yanome peoples]

ahh yes i have seen Burning1's track videos, thanks i will have a look at the others.

Slightly surprised i haven't had the obligatory, buy a bigger bike post that i usually get from other forums.

Just trying not to go overboard with mods like i did on my old SV650.. 

wow looks killer !! i bet it performs very well too .. i love the seat & exhaust mods ... good luck with completions of your project