Carburetor sizes - recommendations please (for the 1000th time probably)

[slabsideB12Kent]

I know this has probably been asked a 1000 times on this forum but each situation is different. What's better for the road, 36mm or 38mm flatsides? I have a GSXR 750 Slabside with a 1200 bandit lump in it. It runs out of top end at times and I've always thought that a Gsxr 1100 engine would suit the chassis more. I've found myself a Gsxr 1100 engine and rather than recondition the 40mm CV's I'm thinking of putting 36mm or 38mm flatslide carburettors on it. I'm hoping flatsides will help the engine breathe so I can get more power but I'd like the bike to work in situations like traffic. I enjoy the torque of the 1200 lump riding the bike around A roads, so I don't necessarily need to have 40mm flatsides. 

Edited August 19, 2021 by slabsideB12Kent

[Joseph]

On my GSXR i'm running what was initially a race engine 1216 GSXR with 38 mikunis and it is really excellent and rideable in all conditions.

In your case I would leave the B12 engine in and fit 34 flatslides to it if you're looking for lowrange power and torque.

Edited August 19, 2021 by Joseph

[Gixer1460]

IMHO a bone stock GSXR11 or a Bindit 12 will work ok with 36mm Flatslides but not optimally. Throw a bit of tuning / headwork / compression / capacity / cams and the 36mm flatties work brilliant on the road and probably the 38's will add a bit more to the top end numbers whilst losing a bit off the bottom! Got to ask yourself - where to you ride most, low to mid or mid to top? 40's are for racebikes IMO or really big capacity engines which can make them work in the midrange.

[wraith]

Not quite the same but, my kat with a 1127 motor fitted. I was running 38mm CV carbs on it till a couple of weeks ago (kept fouling plugs and not the same one) anyway put some 36mm CV carbs on instead (just to see if it's the carbs).

The bike wheeled on the throttle in 1st and 2nd with no problems with the 38mm carbs and the revs etc just kept going and my arms ran out before the bike stopped pulling. With the 36mm carbs it's much harder to get the wheel up on the throttle and it definitely seems to run out of steam way before the 38mm did, I would say the response with the 36mm carbs is a bit quicker at lower revs.

I will be putting the 38mm back on, as the bike is much more fun to ride with them ever in town or country.

Hope this helps a bit.

Ps, I've got a b12 motor in my et with a ported DOT head, gsxr cams and gsxr 38mm CV carbs and it is great all round lots of grunt good top end (not on the highway officer) pulls like a train and is doing 70mph at 4000-4.500 rpm  sprockets are 16/48 before you ask. 

[Damienga15de]

Im running my 7/11 using the 38s from the 750 and a factory pro kit , works great iv been tuning it myself and it runs great but I’d bet a proper dyno tune would see better power 

[Gixer1460]

Need to clarify whether the OP is talking std Mikuni CV type carbs or RS type FLAT slides - different carbs, different operation and different response to throttle inputs!

[Fazz711]

19 hours ago, Gixer1460 said:

Need to clarify whether the OP is talking std Mikuni CV type carbs or RS type FLAT slides - different carbs, different operation and different response to throttle inputs!

In original post OP asked about flatslide 

[Gixer1460]

3 hours ago, Fazz711 said:

In original post OP asked about flatslide 

And some people mistakenly refer to CV's as flatslides because the slide is flat faced! And the GSXR 750's 38mm's refer to above were definately CV's - so you see the confusion!

[slabsideB12Kent]

So I see a CV carburetor as the standard item with the diaphragm acting slide. I like the idea of smooth bore flatside Mikuni's as I will be able to meter the fuel. Slightly concerned about fuel economy regarding the accelerator jump but I find CV carbs jerky and laggy. I wish for abit more power at the top end but mid range torque is where I spend most time riding. For the exhaust I'm going to be using a Micron 4-2-1. I think the 4-2-1 gives you a bit more bottom end? Would be great to have a bike with 38mm carbs and tell everyone it makes 130+bhp. But. Sounds like a set of 34mm or 36's could be a good idea? 

[vizman]

Sounds like you need to make your mind up…..

[KATANAMANGLER]

RS36 will work fine out of the box on a B12 motor with a 130 main and a push pull throttle. Fit velocity stacks and socks to get the induction right.

You can turn accelerator pumps off if you like but if you adjust them to squirt as it starts to pick up off the cam around 5-K it's a nice kick in the pants.

You'll do more for low down torque by fitting adjustable cam sprockets and spreading the lobe centres slightly or getting some porting done.

You don't need 38s on a standard B12 motor IMHO

[Gixer1460]

A M8 has a 1216 kitted Bindit - it's had some work internally with a ridiculously lightened GSXR crank that was a 'steal it price' so rude not to, but std cams and an Ackro exhaust. Initially had 41mm Keihin FCR's and it made 165+hp but just didn't feel nice to ride so he swapped to RS36mm Flatslides and now makes 155hp with loads of torque, lifts the front in 3rd on the throttle, it's a hoot to ride and gets loads more tank range compared to stock ! ! ! with 50 more HP - how's that happen LOL! So I know what my choice would be!

[Dezza]

1 hour ago, Gixer1460 said:

A M8 has a 1216 kitted Bindit - it's had some work internally with a ridiculously lightened GSXR crank that was a 'steal it price' so rude not to, but std cams and an Ackro exhaust. Initially had 41mm Keihin FCR's and it made 165+hp but just didn't feel nice to ride so he swapped to RS36mm Flatslides and now makes 155hp with loads of torque, lifts the front in 3rd on the throttle, it's a hoot to ride and gets loads more tank range compared to stock ! ! ! with 50 more HP - how's that happen LOL! So I know what my choice would be!

Sounds good but the OP is BST v RS, rather than FCR v RS. I wonder how your mate's bike would perform with either BST36s or BST38s.

[Gixer1460]

4 hours ago, Dezza said:

Sounds good but the OP is BST v RS, rather than FCR v RS. I wonder how your mate's bike would perform with either BST36s or BST38s.

Initially after the work, it was using the stock 36mm CV's and was I believe in the 135-138hp range which isn't bad.

[slabsideB12Kent]

Also. Which ever carburetors I buy will be fitted to a GSXR 1100 M motor, The Bandit motor will be saved for spares. I won't be messing with the engine internally as I'll be happy with the power it produces. Because the Mikuni's are all the same price it makes it more difficult to make a decision. I know that the 40mm's will be overkill. 

[Joseph]

On 8/21/2021 at 11:04 AM, Gixer1460 said:

it's a hoot to ride and gets loads more tank range compared to stock ! ! ! with 50 more HP - how's that happen LOL! So I know what my choice would be!

My 1216 with massaged head, cams and RS38 does 48.5 mpg doing fast riding on long distance main road travel with no attention to fuel economy as far as throttle use is concerned. Haven't dynoed it but i'm guessing similar figures can be expected.

[dupersunc]

Stock gsxr1100m motor, RS36 will be best all round performance. Throttle will be heavy to operated though due to the return springs in the RS carbs.

Gsxr motor will have more top end than the bandit motor.

Look at your gearing too. If you are running stock 750 sprockets it will run out of puff as you are  geared for 185mph.

15/48 works really well. Good for a real 155mph

[Sandman]

11 hours ago, dupersunc said:

Stock gsxr1100m motor, RS36 will be best all round performance. Throttle will be heavy to operated though due to the return springs in the RS carbs.

Gsxr motor will have more top end than the bandit motor.

Look at your gearing too. If you are running stock 750 sprockets it will run out of puff as you are  geared for 185mph.

15/48 works really well. Good for a real 155mph

Totaly agree, the main drawback with Mikuni RS is the heavy return springs. Go for Keihin FCR carbs.

[Gixer1460]

11 hours ago, Sandman said:

Totaly agree, the main drawback with Mikuni RS is the heavy return springs. Go for Keihin FCR carbs.

Maybe - but with a choice of 39 or 41mm - they are slightly / much to large for a stocker size engine! As said, RS spring pressure can be reduced!

[no class]

The FCR's are also available in a 37mm size..... to fit "H" and "D"  applications .

[Gixer1460]

4 hours ago, no class said:

. to fit "H" and "D"  applications .

So not bolt on then . . . . . re-spacing nitemare! 'D' applications as in the Italian 'D' ?

[no class]

D as in downdraft . fcr’s are easy to respace.

[no class]

…H designates horizontal

[Sandman]

23 hours ago, Gixer1460 said:

Maybe - but with a choice of 39 or 41mm - they are slightly / much to large for a stocker size engine! As said, RS spring pressure can be reduced!

But still way to heavy. 

[Gixer1460]

Why be so cryptic LOL! Honestly never seen 37's for sale in the UK but I guess they must be ! Just had a look on the Keihin WWW and the FCR35 and 37's are listed for GSXR600's and RF600's so the manufacturer thinks they may be a bit weee for a bigger engine possibly? Mind you they also recommend 41's on GSX's LOL!