Big carbs

[MikeyVeitch]

Hi guys, It’s me….  again

just because fuck knows I’m not gonna pay the money for a set of bst40ss carbs, how far could you take a set of 36s or 38s

is it possible to bore them out and would there be any benefit to polishing the bores on them too. In an ideal world, I’d just bang some 40s onto them but I just don’t have the money for that shit.

I also hear of people ‘derestricting’ carbs by drilling the hole bigger on the diaphragm, would this also serve any purpose?

[wraith]

By the time you've bored a set of 36 or 38 carbs it would be cheeper to buy 40 carbs BUT 40's without a air box are renowned to be fuckers to try and jet, so don't bother with 40mm carbs. 

Also if you haven't got the money to buy 40mm carbs you can't afford the rest, sorry to say but tuning is not cheep these days and you'd be looking at £120-£150 for a standard set of 36mm carbs and probably £150-£250 for a set of 38mm if you can find a set. 

Don't know what your putting them on but I find the 38mm gsxr carbs to be a very good all round carbs the next step really is spend the money on a set of flat slide carbs but they are big money.

[Upshotknothole]

CVs can't be bored out. The throttle actuates a butterfly valve. You'd either need the butterflies out of larger carbs or to make up some custom ones, and even then, you'd still have to deal with the jet placements in the bore.

BST38s out of the late gen 750s are good carbs, but the extra circuit on the sides of them is only useful if you're running an airbox. I think you're talking about drilling out the slides that are part of the diaphragm, that's standard with dynojet jet kits. Just part of the tuning process, doesn't really do much to otherwise stock carbs. Don't bother polishing the carbs either, you need a little turbulence to help the fuel mix with the air.

What carbs do you have right now? What's your goal? Honestly by the time you do a full rebuild on a beat up old set of carbs, buy the jet kit, and new K&N filters, you're a good chunk of the way to a set of flat slides.

Also double check that you can actually get the intake boots for the carbs you want. That sticky in this section on carbs and intakes has some huge mistakes regarding what fits and what doesn't.

[DAZ]

6 hours ago, Upshotknothole said:

What carbs do you have right now? What's your goal? Honestly by the time you do a full rebuild on a beat up old set of carbs, buy the jet kit, and new K&N filters, you're a good chunk of the way to a set of flat slides.

Also double check that you can actually get the intake boots for the carbs you want. That sticky in this section on carbs and intakes has some huge mistakes regarding what fits and what doesn't.

Best advice there.  btw 36 mm cv carbs as on gsxr1100 K & L and bandit 1200's are good up to  around 150 rwhp , that's a decent 1216/cams/flowed head build  

[Dezza]

What's wrong with the carbs you have? Pics will help. They may be salvageable because the expensive bits may be OK and serviceable, such as jets, needles, air jets, float valves and seats. You can change all the o'rings for reasonable cost. I have found working with multiple sets of CV carbs that more often than not it's time and effort that is the main cost of restoring carbs but if you DIY these are free. The biggest hurdle I have encountered is seized pilot air screws. These can make or break a carb restoration . Once the carbs are restored then go for a jet kit although re-fitting the OEM airbox is going to be a whole lot cheaper.

 

Maybe message oilyspanner on here. He is the GSXR1100M guru.

Edited August 20, 2023 by Dezza

[MikeyVeitch]

9 hours ago, Upshotknothole said:

CVs can't be bored out. The throttle actuates a butterfly valve. You'd either need the butterflies out of larger carbs or to make up some custom ones, and even then, you'd still have to deal with the jet placements in the bore.

BST38s out of the late gen 750s are good carbs, but the extra circuit on the sides of them is only useful if you're running an airbox. I think you're talking about drilling out the slides that are part of the diaphragm, that's standard with dynojet jet kits. Just part of the tuning process, doesn't really do much to otherwise stock carbs. Don't bother polishing the carbs either, you need a little turbulence to help the fuel mix with the air.

What carbs do you have right now? What's your goal? Honestly by the time you do a full rebuild on a beat up old set of carbs, buy the jet kit, and new K&N filters, you're a good chunk of the way to a set of flat slides.

Also double check that you can actually get the intake boots for the carbs you want. That sticky in this section on carbs and intakes has some huge mistakes regarding what fits and what doesn't.

The bike currently has some gsxr1100k carbs. 36mm. I’m running k&n dual filters and the carbs are jetted to suit,  would there be any benefit to flow if I was to polish them internally 

[wraith]

As above NO.

Take it the engine is a standard gsxr1100k or maybe a standard b12 ? 

Also as above, the 36mm carbs can easily handle 150rwbh but if you want a bit more HP keep a eye open for some gsxr750 38mm carbs.

Your not going to get anymore power out of the carbs you have, it's engine work you need to look at or spend the money on flatslides you can get a second hand set for around £400-600 maybe less if you're lucky.

[Gixer1460]

Bigger and shiney is generally not worth the effort. Bigger carbs mess with where you are 90% of the time and the only polishing worth doing is the cyl. head chamber and even that only lasts for a few hundred miles!

[DAZ]

Unless you are going beyond the 150ish build of the typical street 1216 / flowed head / decent cams  build 36mm are fine or 38's if a bargain turns up ,whether it's cv or flat slides , what do you see is the end product ? Road bike ? Race bike ? If road bike you must think about going too big on carbs, cams and compression ,it's way to easy to go big on everything and loose the big hearted friendly nature of the oilcooled engine by turning it in to a race bike 

[wraith]

You could always get some of them Billet carb tops and polish them, make the air flow over the top of the carbs better and give you more speed 

[Upshotknothole]

6 hours ago, MikeyVeitch said:

The bike currently has some gsxr1100k carbs. 36mm. I’m running k&n dual filters and the carbs are jetted to suit,  would there be any benefit to flow if I was to polish them internally 

No benefit at all to polishing the insides of the carbs, if anything you might lose a little bit of power. If there were cheap hp to be gained there, everyone would be doing it.

If you're tight on money for this build, I'd just stick with those unless something better comes along for a really good price. Save the money and get the bike properly tuned.