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Interchangeability of carbs and carb rubbers for oil-cooled engines

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Here is the third, final version. A summary table has been included as a pic, and the pics added by peeps support the text:)

On this forum threads in the oil-cooled section often arise asking: ‘What carbs fit what engine?’ And then the discussion begins and progresses with the same answers: again and again, like some sort of cross between a ‘Suzuki oil-cooled carbs’ specialist-subject-on-Mastermind and Groundhog Day. I therefore thought it would help to collate the relevant information on this subject for an article. The oil-cooled engines people here are most interested in are the larger capacity units, i.e. 1052 cc and bigger. The 750s are also popular and often their carbs are fitted to the larger engines. The smaller 600 Blandits and Teapots are not covered here. I present some data on the stock carbs and rubbers for different models and also for Mikuni RSs, which we all know give the most pub-bragging rights for those of us without deep pockets (FCRs) or have had luck on the horses/lottery (TMRs).

 

First, it needs to be made clear that the inlet rubbers for all oil-cooled models are interchangeable and as far as I know the o’rings that fit between each rubber and the head are the same diameter. All models (except for MK2 1200 Blandits, see below) have the same carb spacings, 77-93-77. In short, the individual rubbers will bolt onto any oil-cooled head (but see below for information regarding carb rubber-inlet port misalignment). However, the carbs, and hence the carb rubbers, vary in size among the different models, hence the regular questions asked on the forum. I present details on much of this variation below.

 

Second, the size measurements of carbs and carb rubbers are taken from internal diameters (id) (see pics in subsequent posts). I want to make this clear as this often appears to cause confusion with some people, especially when they are trying to source a set of rubbers to match a set of carbs. For carbs, the size measurement is taken as the internal diameter of the stub that goes into the carb rubber, i.e. the inlet port side of the throttle butterfly on a CV carb. For carb rubbers, the size is taken as the internal diameter where the outside diameter of the carb inlet stub goes into the rubber. This will obviously be larger than the ‘size’ of the carb that fits because of the thickness of the metal that makes up the wall of the inlet stub of the carb. For BST carbs the wall thickness of the inlet stubs is approximately 2mm (but this must vary a little, see below). So as a rough example, a 36 mm carb fits a rubber of approximately 40 mm internal diameter. For Mikuni RSs, things are a little different because some use the same carb bodies for different sizes. For RS 34, 36 and 38s the outside diameters of the inlet stubs are the same (42mm) so the wall thicknesses must vary; for RS 40s, the od of the stubs is 44mm.

 

This size variation is quite straightforward because almost all models were originally supplied with Mikuni BST CV carbs (that have flat slides) of either 34, 36, 38 or 40mm. Variation in carb sizes among models is thus also almost matched by the same size variation for the carb rubbers among the different models. I say ‘almost’ because (just to add confusion here!) I know of different models with different sized BST carbs as stock use the same rubbers, presumably because there is some ‘give’ in the flexible rubber and there is a little variation in the wall thicknesses, and hence the outer diameter sizes, of the inlet stubs of some different sized carbs. There is also variation in the carb rubbers fitted to different models with the same sized BST carbs, especially BST 36s, such as the length (i.e. how far the carb bank sits relative to the cylinder head).

 

If the carbs and the rubbers from a donor model are all obtained, then these will simply bolt-on to the recipient oil-cooled engine. The problem is that often a set of carbs is obtained without the corresponding carb rubbers and / or the exact model of the source bike of a set of carbs or rubbers is unknown or has been incorrectly identified / measured (we are talking breakers here, obviously).

 

Carb size variation according to different models.

 

 

GSXR 1100 G, H and J (slabsides): BST 34 ‘tintop’, now mainly sought by turbo users. Note, carbs from the earlier G and H models had unusual ‘plates’ that connected carbs 1 and 2, 2 and 3, and 3 and 4, at the tops, rather than by a more conventional one-piece rail. The J has BST 34s with two conventional rails. All 1100 slabsides have a pull-out knob for the choke operation.

 

GSX1100F (powerscreens): BST 34 ‘tintops.’ Two conventional rails as for the GSXR 1100 J. The choke mechanism is cable operated.

 

GSXR 1100 K, L, M and N (slingshots): all had larger BST carbs (with plastic tops) than the earlier slabsides, and sizes varied among the four slingshot models. The K and L both got BST 36, whereas the later M and N models got BST 40.

 

N.B. To my knowledge no GSXR 1100 had as stock BST 38s.

 

 

GSF 1200 (Blandit): for the 1200 MK1 BST 36. The MK2 1200 Blandit has the BSR 36 carbs, which differ to those on the earlier bikes (e.g. these have a sensor for a TPS). The spacings of these carbs are also different to those of almost all other oil-cooled Suzukis (80-85-80; see also GSX 750F teapots below). These BSRs also have larger inlet stub ods (42mm) than BST36s so they must have thicker inlet stub walls.

 

I have noticed that performance carbs for 1200 Blandits are sold as ‘fits all models’ and when stipulated show spacings as 77-93-77. The underlying assumption to get them onto a MK2, is therefore to use the rubbers of the appropriate size from an earlier OSS oil-cooled machine.

 

GSX1100G (shaft drive semi-tourer thingy): BST 36s

 

GSXR 750 F, G and H (slabside): for non-USA markets, had ‘proper’ VM 29 SS slide carbs as standard. Confusingly, the inlet subs of these carbs have an id of 33.80 mm; the od diameter of the inlet stubs is ~40 mm. These carbs are often modified by being ‘bored’ to 33mm by enlarging the holes in the face plates on the airbox side rather than the whole carb.

 

In the USA, 750 slabbies got BST CVs. I am guessing these are 34s, the same as the 1100s of the time.

 

GSXR 750 J, K, L, M (slingshots): all (non-homologation limited-run bikes) had BST carbs with plastic tops. For J and K models, these had BST 36; for L and M, these had the now highly-desirable BST 38. The BST 38s for the L and M differ slightly; the L carbs are the only ones with large float bowl drain plugs similar to the ones on RS Mikunis that allow the changing of the main jets without float bowl removal.

 

GSX750F (750 teapot): BST 36. Later GSX750Fs (1998 onwards) have the different spaced BSR36s carbs and rubbers as do Blandit 1200 MK2’s, presumably also with a TPS.

 

Carb summary (See also Table 1)

 

 

What is clear is that the most common carb by far is the BST 36. However, BST 36s fitted as stock to different models will vary either internally, externally or both (e.g. in the choke operation, rails, cable bracketry, height of the tops, jetting, needles and so on). The same goes for other BST carbs that are the same size but were OEM for different models. Accounting for this variation to make a particular set of carbs work with an individual engine is usually the hard bit. All of these factors need to be considered for a specific application but this article is only concerned with mounting the carbs onto the engine in the first place!

 

Carb rubbers

 

 

Obviously, it is easiest to obtain a complete set of rubbers from a single source machine for each usage unless it is known EXACTLY the source model of each rubber. This is because the rubbers for each of the four cylinders differ within the set for each model as well as among those for different models (including those models with the same sized BST carbs).

 

Clearly, BST 36s will fit the rubbers from any bike originally fitted with BST 36s. Also, the BST 34s from 1100 slabsides / powerscreens will fit using the rubbers from a bike that has BST 36s as stock (e.g. 1200 Blandit Mk1). As far as I can tell the 1100 slabsides (and probably also powerscreens),1200 Blandits and 750 teapots use the same rubbers. They may differ in part numbers (which may differ if they have a take-off for the fuel tap vacuum or not for example) but essentially, all will enable the mounting of a set of oil-cooled carbs onto an oil cooled head.

 

Another important point is that the rubbers for the 750 slingshots with BST 36s (k,l) are 5mm shorter than those for earlier/larger bikes e.g. 1200 Blandit Mk1. In other words, using the 750 rubbers will bring the carb bank 5mm closer to the head. This could obviously work the other way, and have either positive (more space in front of or behind the carbs) or negative (bugger up the alignment if the stock airbox is to be retained) depending on the intended application. Likewise, the larger rubbers for the L,M also bring the carb bank very close to the head.

 

The MK2 Blandit inlet rubbers (and probably also 1998 onwards GSX 750F teapot, sometimes called the ‘baboon’s arse’), are the 36 mm anomalies here and differ to those from other oil cooled bikes in order to fit the differently-spaced BSR carbs. To fit a set of e.g. MK2 Blandit carbs to any other oil-cooled Suzuki, the corresponding B12Mk2 rubbers will have to be used. To fit a set of carbs meant for any other oil-cooled model to a Blandit 1200 MK2 head, the same mix-and match-principles will apply as for other oil-cooled engines.

For the GSXR 1100 M and N, the inlet stub od and inlet rubber id are ~45mm so these have limited use for the fitting of carbs other than huge aftermarket performance units, e.g. RS 40s.

 

The fitting of BST 38s from a GSXR 750 L or M to either a GSXR1100 (obviously not an M or N model) or a 1200 Blandit is a known effective upgrade. All of these recipient machines as far as I know have the same inlet rubbers (to accommodate BST 36s), which have an internal diameter of just a teensy bit more than 40 mm (I know, I have some!). To fit BST 38s, larger rubbers are thus required, preferably from a 750 L or M, which have an internal diameter of 42 mm (I know, I have some!). The rubbers from an 1100 M or N cannot be used as they are too big (id ~45mm). Alternatively, rubbers from a bike that takes BST 36s as stock could be enlarged with a drum sander. (You have to ask the rhetorical question: how were RS 36, 38 and 40s fitted to GSXR1100s before the GSXR 750 L was released in 1990, and hence when rubbers of the ‘perfect’ size were unavailable?)

 

The obvious way to fit BST 40s from an 1100 M or N to another model (which would by default have had as stock smaller carbs) would be to use the rubbers from an 1100 M or N. Alternatively, a lot of sanding of smaller rubbers will be required.

 

Mikuni RS 34, 36, and 38s all have an inlet stub od of 42mm so these can be fitted using the rubbers from the 750 L or M (I know as I have done this!) or by enlarging the id by sanding some from a bike that has as stock BST 36s. The inlet stubs of RS 40s have an od of 44mm so will require the larger rubbers from a GSXR 1100 M or N, or much sanding of the rubbers from another bike.

 

Misalignments between carb rubbers and inlet ports

 

 

Successful mounting of a set of (usually larger) carbs from one oil-cooled Suzuki model onto the cylinder head of another oil-cooled model can result in a misalignment of the internal surface shapes of the carb rubbers and the inlet ports. This may not be so for all possible carb rubber/head combinations or for every cylinder within each carb rubber/head combination (there are clearly many different potential combinations). For example, fitting GSXR 750L rubbers to a Blandit 1200 MK1 cylinder head enables the direct mounting of a set of BST 38s or RS 34, 36 or 38 carbs. However, for cylinders 1 and 4, there is a lip where the alloy of the inlet manifold does not match the internal contours of the carb rubber. I have added a picture further down this thread in order  to make this clear. This can obviously only be rectified by grinding away the now excess metal but is a ‘problem’ that may or may not arise depending on the rubbers and head being used, and may not be a problem at all depending on the intended use of the ‘finished’ bike (like they ever are truly ‘finished’).

 

Oil-cooled carbs onto an air-cooled engine

 

 

I will not here teach grandmothers to suck eggs so this will be brief, as anyone interested has probably already done it decades ago. It is well-known that the carb spacings for Suzuki air- and oil-cooled (and classic air-cooled Kawasaki) engines are the same. The relative abundance of BST 36s seems the obvious way forward here as the only stock air-cooled bike with carbs this big is the 1100 EFE. For those who have never looked, the rubbers on air-cooled bikes differ to those on the oil-cooled bikes and also differ markedly between GSs and GSXs. I am told though that oil cooled (non-BSR) rubbers bolt onto the head of GS 1000s which enables the mounting of BST carbs!

 

Do GSXR 750W (pre-SRAD) BST 38s fit oil-cooled engines?

 

 

In a nutshell, no, because the carb spacings of the first generation water-cooled GSXR engines (85-90-85) are wildly different to those of the oil-cooled engines. Some suggest that these carbs will fit an oil-cooled engine by re-arranging the W carb rubbers but I have never seen any convincing evidence to support this claim. If anyone has done this, please post a thread with a detailed description and pics!

 

Clearly, if Suzuki were more considerate and had the slightest understanding of the market for their bikes 25 years on from their inception (doh), then they would have used the same carb spacings as for a MK2 Blandit, so the 750W BST38s could be fitted to an OSS oil-cooled head using Blandit MK2 rubbers. Alas though, they didn’t so this is not possible.

 

Acknowledgements

 

 

Thanks to all those who also contributed to this article with the supply of information, data, pictures and / or simply ideas: captain chaos; baldrick; Paulm; FBOB; martyngsxr; wraith; GSX1100dreamn; Sibag; bluedog59; Wee Man; dupersunc; Lachie04; Ted M; Svsam. If I have missed anyone, sorry.

Table 1.jpg

Edited by Dezza
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Loving the .info Dezza. I'm going to pin this for a while to see if we can get those red areas confirmed. 

 

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I’ll work on some images to go with it....great to see site users developing and sharing the .info

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8 hours ago, Dezza said:

the M carbs are the only ones with large float bowl drain plugs similar to the ones on RS Mikunis that allow the changing of the main jets without float bowl removal but I stand to be corrected.

my 750L (Dutch market) has the large float bowl drain plugs.

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9 hours ago, Captain Chaos said:

my 750L (Dutch market) has the large float bowl drain plugs.

and the 750m ones don't have the drain plug, 750L does, and both l and m 750s  do have the powerjet tube between float bowl and top of carb body at intake

 

1100L has 36s not 40s. the M and N have 40s tho.

great article though, thanks for taking the time. this is long overdue, and will be very helpful.

Edited by baldrick
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Ok, it looks like I have my 750 Ls and Ms arse about face so this can be updated on my master copy on Word.

 

Keep the updates coming! I especially need info on 750 slabsides, 1200 Blandits (I have it in my head that the MK2 rubbers may be different to the MK1 so MK2 rubbers and carbs have to be transplanted as a unit but this may be my imagination) and 750 teapots anyone? OSS calling FBOB.......

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5 hours ago, Dezza said:

Ok, it looks like I have my 750 Ls and Ms arse about face so this can be updated on my master copy on Word.

 

Keep the updates coming! I especially need info on 750 slabsides, 1200 Blandits (I have it in my head that the MK2 rubbers may be different to the MK1 so MK2 rubbers and carbs have to be transplanted as a unit but this may be my imagination) and 750 teapots anyone? OSS calling FBOB.......

Yes Mk2 1200 Blandits have different carbs and rubbers to Mk11200’s.

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40 minutes ago, Paulm said:

Yes Mk2 1200 Blandits have different carbs and rubbers to Mk11200’s.

Ok, thanks Paul. Do you have any measurements or additional info? I am guessing that MK2 Blandit carbs and rubbers can be transplanted to all other oil-cooled heads if done together, and that the reverse is also possible but any sort of mix and match will not work. Presumably, (most likely) the MK2 carbs have different spacings to those from other oil cooled bikes so to enable fitment to the 'standard' spaced oil cooled head of a MK2 different rubbers are needed. If we can confirm this, then this will be very useful info to have in the finished article:)

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You can fit Mk1 carbs and rubbers on a Mk2 so presumably the reverse is ok.Which in turn works for a lot of the other oil cooled motors.

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15 minutes ago, Paulm said:

You can fit Mk1 carbs and rubbers on a Mk2 so presumably the reverse is ok.Which in turn works for a lot of the other oil cooled motors.

What Chubster said, the mk 2 bandit have very different carbs, with different spacing, BSR36 instead of BST 36's.. :)

all inter changeable as long as the rubbers stay with the carbs, Ive got a MK1 engine at home with MK2 rubbers on it, ill measure them later.:tu

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14 hours ago, fatblokeonbandit said:

What Chubster said, the mk 2 bandit have very different carbs, with different spacing, BSR36 instead of BST 36's.. :)

all inter changeable as long as the rubbers stay with the carbs, Ive got a MK1 engine at home with MK2 rubbers on it, ill measure them later.:tu

Thanks FB!

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i second the l/m access in the float bowls ive got both l and m and its the l that has the access panels in the float bowls 

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Very good info (y)

Just to add my bit bandit 1200 MK1 carbs fit the GS1000 CV head using b12 MK1 rubbers.

Got this set up on my gs1000 :D

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Please use what you can from my setup.

I used new GSX 1100 head to carby rubbers to to fit over the larger 38's and 38 carby rubber clamps.

For the airbox rubbers to fit over the ( these have the ROUND not the oval type) carby inlet I removed the metal spring ring from the rubbers.

 

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Cheers Andrew.

Edited by GSX1100dreamn
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Anyone with a set of stock 750 slabside carbs? Normal id measurements and od of the inlet stubs are needed:) Are they really only 29mm id? I know these are often bored out so they may have relatively thick inlet stub walls.

Edited by Dezza

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Great little article very useful. (y)

If I'm been pedantic I would say change the font. (But that's just me.)

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Vm29ss (Slabside) measure up at 33.8mm I/d, 39.25 o/d and 78/90/78 spacing. They are 33mm on the inlet side side as standard, it's only the face plates on the airbox side you bore out to make them 33mm carbs.  Spacing does match up against a set of stock B12 MK1 carbs.

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13 hours ago, SiBag said:

Great little article very useful. (y)

If I'm been pedantic I would say change the font. (But that's just me.)

No, you are not being pedantic. I intend to change the font type and size so it matches that of the rest of the site or maybe some of the site moderators can do this:)

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12 hours ago, bluedog59 said:

Vm29ss (Slabside) measure up at 33.8mm I/d, 39.25 o/d and 78/90/78 spacing. They are 33mm on the inlet side side as standard, it's only the face plates on the airbox side you bore out to make them 33mm carbs.  Spacing does match up against a set of stock B12 MK1 carbs.

OK, thanks Rob. I can add this info. A set of 750 slabside carbs will therefore fit any oilcooled head using a set of Blandit 1200, GSXR 1100 slabside, K,L rubbers and probably also 750 teapot rubbers (same as any oil cooled 36 mm CVs other than those fitted to a Blandit 1200 MK2). Easy-peasy to work out when the info is all in one place, which is the whole point of the article:)

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Anyone with a set of 750 teapot carbs/rubbers? It would be good to confirm the sizes as being able to use 750 teapot rubbers to mount 36mm CVs from another model as this will widen the available choices for those sourcing 2nd hand parts.

 

Also, does anyone have a set of BST 40s from an GSXR1100 M or N and / or the associated inlet rubbers? Measurements for these will be useful, possibly for those needing rubbers to mount RSs that all have an inlet stub od of 42mm.

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