The great GSXR400 carburettor saga

Once upon a time we bought a 1993 GSXR400SP. My wife had recently passed her test and being something of a shortarse it was one of very few bikes she could cope with. An eBay bike, I collected it from near Southampton in November 2003. Being an SP, it had a ludicrously tall first gear and was awkward in traffic but it looked gorgeous, the chassis felt very 'planted' and stable and the brakes were superb. The run back up the M3 went pretty well. It loped along steady as a rock at 80-90mph but didn't want to go any quicker. Still, it was a 10 year old bike and I expected a few 'issues' that needed attention, but basically it was sound.

the little bitch

The 'issues' turned out to be two basic problems. First that it was impossible to engage neutral with the engine warmed up. That is a very common problem with these bikes and a new set of clutch plates fixed it completely.

The second was that even after I'd balanced the carbs, changed the plugs, given it a good service and fitted an MMax derestrictor, it didn't want to rev out. Still my wife didn't want to go fast so it hardly mattered. Eventually she took it out one day and it suddenly developed a loud top end rattle and ran on 3 cylinders only. She limped it home and it was immediately obvious a valve spring had broken.

I stripped the head and sure enough one inlet spring was broken in 3 pieces. One break was evidently old, the other new. The valve was, amazingly, fine. I'm sure that had been the revving out problem all along, the previous owner had thrashed it and broken a spring, then sold it to us. The joys of eBay.

We had a long wait for parts, which took 6 weeks to get from Japan. Eventually I got it all back together and it ran fine, no more missing top end. But then it began to display carb problems that had never previously existed. It would bog down and stumble just off idle, making it almost unrideable. It was clearly running rich and needed no choke for cold starts. I hadn't touched the carbs, they had just sat in the shed for 2m while the bike was apart.

This was the beginning of the worst, most intractable problems I have ever experienced with any vehicle. Singlehandedly, the GSXR400 and its damn carbs pretty much ruined summer 2005 and 2006, as every spare weekend was spent trying to figure out what the hell was wrong. In that time I removed and stripped and rebuilt them at least a dozen times. Information is sparse, even the OE Suzuki manual (in Japanese only) is of little help.

What's more, half the GSXR400 owners in the UK are having, have had, or will soon have similar problems. It is a recurrent issue at the GSXR400 forum and the GSXR400 workshop section of the 400Greybike forum. If you want to know more of the grisly story the 'FAQ'ing carbs thread is a good place to start. But here I'm just going to summarise what I've learned, and what it has taken to fix the carbs, hopefully it will save someone 2 years of frustrated sodding about.

carb balance

These bikes are very sensitive to correctly balanced carbs. Don't expect it ever to run right at low revs unless they are spot on. If you have rough tickover start with balancing the carbs! I use a Morgan Carbtune kit, which is far the best I've used. Mixture screw settings are 1.5 turns out for all GK76a models except the 1991 SP, which the book gives as 1.8x. These settings should be close enough to get it running and balanced. However, if you find you need to adjust mixtures from that to get OK idle and pickup, read on...

carb removal

The carbs from hell

The carbs look impossible to remove due to lack of space, but in fact are fairly easy.

  1. Remove fuel tank, battery and battery cage
  2. Remove the airfilter cover and airfilter element
  3. Slacken the Jubilee clips holding the airbox stubs onto the carb inlets
  4. Withdraw the large wire 'U' clips (one each side of the airbox) that secure the airbox stubs to the airbox body, and push the stubs back off the carbs and into the airbox.
  5. Slacken the Jubilee clips holding the cylinder stubs onto the carb bodies
  6. Undo the choke cable at the carb end, and dismantle the throttle twistgrip to remove the throttle cable from the twistgrip end (removing it at the carb end is a fiddle, so don't)
  7. Remove the breather pipe that runs from the cylinder head, across the middle of the carb bank and connects to the airbox
  8. Take a look at the various hoses that attach to the carbs - some are breathers, some are vacuum and fuel supply pipes from the petrol tank. Do not disconnect any from the carbs, it's far easier to leave them all in place, just free them off at the other end.
  9. On the SP models only, disconnect the Throttle Position Sensor by pulling apart the white triangular connector block.
  10. The carbs should now be free to be removed as a bank, compete with hoses. Pull the carbs backwards off the inlet stubs and down a little, so you can remove the whole lot to the offside (RHS of the bike).

That's it. Replacement is the reverse. Once you've done it 10 or so times, it'll take 10mins max :-) You can now set about servicing them.

A couple of things to note there. First, don't be tempted to dismantle the bank of 4 carbs unless you absolutely have to. All you need to do is remove the float bowls and diaphragm caps to clean and service the interiors. Unfortunately that may not be easy, as the mild steel Philips head screws tend to seize in the carb bodies. They can be very difficult to unscrew without mashing the heads. I strongly suggest you gently use an impact driver to remove them, then throw them away and replace with M5x12 stainless socket head screws, coated with a little Copaslip.

Also, buy a can or two of proper carb cleaner aerosol. The deposits of gum that form inside carbs are impossible to remove using almost any other solvent, including petrol, meths, white spirit, WD40, PlusGas, water or gin & tonic. At a pinch, brake cleaner will do, as it's very similar stuff based on toluene or similar noxious chemicals. But keep either away from gaskets and O rings, it causes them to swell and distort.

the bleedin' o-rings

These Mikuni carbs each contain 4 different O rings, and each of them causes big trouble. After several years they harden and leak, and - to cut a very long story short - it's these which are the fundamental problem.

Ours began playing up after the carbs dried out in storage whilst I fixed the broken valve spring, but the problem is now well known on the 400Greybike forum in particular - which I would recommend to anyone who is afflicted by a misbehaving baby Gixxer.

GK76a O rings

If #1 or #2 leak, you will get constant overfuelling, and if it's allowed to persist, premature and rapid bore and piston wear. Both are immersed in warm fuel. If #1 leaks fuel will be drawn up around the main jet as well as through it. If #2 leaks, the float valve will be bypassed by fuel leaking into the bowl.

Either can and will cause the motor to run very rich, which is a common GSXR400 fault. A good clue is the temperature gauge seldom showing even mid-range temperatures even in warm weather after hard use. The over-rich mixture burns cool and leads a lot of people to suspect the gauge is broken. If your bike is like this, if the fan never comes on, investigate the mixture ASAP.

Another indication is if your engine doesn't need choke to start from cold, or is producing sooty exhaust or plugs.

DO NOT IGNORE RICH RUNNING. A rebore or new engine costs an awful lot more than a handful of O rings. Excess fuel will wash the oil off the bores and cause rapid wear, as  a distressing number of GSXR400 owners have found to their cost.

If #3 leaks, it will allow fuel and/or air to be drawn up around the edge of the slide holder.

#4, the tiny O rings that seal the idle mixture adjustment screws, are the big surprise. I gave myself 3 months of needless grief by ignoring them. Ours seemed in good condition, just a little hardened. But I couldn't get the idle mixture consistent or stable - I would fiddle with the mixture screws until just right, then go and test the bike and the thing would become utterly unrideable as it heated up. I replaced them in the end simply because I had run out of other explanations for the misbehaviour. The result was a complete surprise and a total transformation : with idle mixtures set to 1.5x turns out, as per the manual, it now ticks over properly and pulls from tickover with no bogs, stumbles or any hint of the previous evil temperament. For the first time in over 2 years, it is running right.

The easy source for all these O rings is of course Suzuki, but they charge 10x what anybody else does. Mikuni sell no parts for OEM carbs, which these are. Unfortunately I haven't been able to find a single source for all 4 sizes. Wherever you get them, obtain Viton rather than Nitrile rubber at least for the perch and mainjet O rings. Viton is petrol and heat proof, nitrile rubber isn't up to immersion.

Altecweb are a UK site who sell 2 out of the 4 types. [EDIT: see the comments below for more info about where to get O-rings].

Altecweb quickorder codes for Viton O rings
ORV-BS010 mainjet holder
ORV-BS011 float perch

These are Imperial sizes and a bit snugger and fatter than the OE metric, which is a good thing - especially in the case of the float perch, where the OE O-ring is not a terribly convincing seal in the carb body even when new.

You will also need 4off 10mm ID x 1mm for the slide holders. Altec don't sell this size in Viton. Nitrile (standard material for O-rings) seems OK in this location as they aren't immersed in petrol. Halfords and B&Q sell mixed packets of O rings, one of which contains some 10mm x 1mm. Unfortunately I can't now remember which shop - B&Q medium size assorted O rings, I think. They're cheap enough, a quid or so per pack. Alternatively Altec's quick order code for these is

The tiny 4.5mm x 1 O rings used for the idle screws are even harder to find. In the end I got a kit which included them and many others on eBay from PJ Tooling supplies shop on eBay. Do a search on 'O-ring' there; the kits is described as 'New 200pcs O-ring set / Milling / Washer / Lathe'. A couple of quid plus P&P.

NB: If you are searching elsewhere for these small O-rings, some vendors list by internal diameter (ID) instead of outside diameter (OD). 4.5mmOD x 1 are a tight fit round the 3mm pilot screws, which is probably a good thing, and 2.5mmID x 1 5mm are exactly the same size (2.5+1+1=4.5). 

I used the above but 5mmOD x 1 should be OK too, a little looser but compression from the spring should mean they seal OK. 3mmID x1 are the same size. 

DO NOT be tempted to use any of the other nitrile O-rings in the pack that might fit the mainjet and float perch, Nitrile is NOT good for constant immersion in hot petrol and they'll deteriorate rapidly and you'll end up doing them again in a few months. However if you need to replace the O-ring under the carb cap vacuum outlets, the kit gives you about 20 of the right size which will be fine.



float heights

GK76a float height

The manual is extremely evasive on the subject of float heights. Which is unfortunate as they are critical. The only figure anyone has been able to find is '2-4mm' measured (apparently - I can't read Japanese) as the difference between the metal bridge on the float and the jet holder. It's a weird place to measure and varies with viewpoint.

Fortunately I measured our float heights the normal way before I started messing with them in order to try and cure the fuelling problems. I tried various heights from 18mm to 25mm (a few people thought 25mm was correct - I think it was on early Bandits). All of these experiments made matters worse, and eventually I set them back to the 20mm I had measured at the outset.

Some months later and getting desperate to cure the poor running I decided I was unhappy with the weak springs and slightly notched viton tips in the float valve plungers on 3 of the 4 carbs and bought new replacement float assemblies from Suzuki - at £36 per carb! They arrived set to 20mm and I have left them at that. They're fine, but didn't ultimately cure the problem. The problem never was the floats in the first place!

It's much easier to set the heights with the carbs angled at about 45degrees to take the weight of the float off the plunger. But I suggest you set them carefully and then leave them alone. It's too easy to try and fix problems elsewhere by fiddling with float heights, and just end up even more confused.

things not to do

Carb + silicone = bad idea

There's quite a list of things not to do with these carbs. Usually people tear half the bike apart, unbolting the airbox and so on - completely unnecessary - trying to figure out how to remove them. I've even seen a dealer do this. Anyhow, with the scarcity of information that exists in UK I think all of us who have had these problems have made plenty of mistakes.

One of my dumbest was this 'economy' idea of packing up the old, compressed and leaking float bowl gaskets with Silicone RTV. It lasted about 5 minutes before frilling and disintegrating messily, blocking jets and entailing yet another strip and clean. Incidentally, I also tried Blue Hylomar and that was just as bad. New gaskets are readily available from Suzuki for about £4.50 each. I gave up and bought them.

There are, inevitably, other faults with the GK76a that can seem similar to the carb problems. They are all pretty old now, having been discontinued in Japan in 1997-8. Coils and CDI pickups occasionally give trouble, as do ageing HT leads and plug caps (OE Suzuki caps are £18 each! - most people adapt the straight NGK type whch cost about £2). The sidestand cut-out is another source of grief, and there is a small rectangular diode tucked away in the loom on the nearside just below the seat rail which is part of the sidestand/clutch cut-out circuit. The diode terminals often oxidise; intermittency with ignition and/or neutral light point to this circuit. Valve clearances tend to be neglected too, because it's a fairly fiddly job that involves draining the cooling system, but wrong gaps can cause anything from poor running to a dropped or bent valve. But, by far the commonest bit of resident evil is the carbs and their damn O rings. So now you know.

With acknowledgement and thanks to Eddie v Twin, gsxr400 racer, stripes1976, Mr Clark, SuperGuy and all the other long-suffering victims of Mikuni O ring syndrome at the 400Greybike forum who have contributed to the sum of human knowledge regarding the sodding things.



Carb hose attachment

On a GK76a you should have 2 large fuel supply pipes. These are 3/8" dia. and should have metal springs around the outside to prevent crush. They come off the fuel rails between carbs #1 & #2, and between #3 and #4 and attach to the tap front side and rear side. These both feed all the time, whether from res or main tank is just determined by the tap position.

The smaller diameter hose off the back of the tap is a vacuum pipe, and connects to the inlet vacuum. I can't now remember where it attaches to the carbs or inlet on this bike (it's been 3 years since I had to touch the carbs :-), but there will be a fairly obvious spigot somewhere on a carb or the inlet manifold that the pipe pushes on to. The fuel tap is vacuum operated and no fuel will flow unless this is connected. Consequently the tap has no 'Off' position - it's either On (down), Reserve (back), or Prime (forward). I think that's right ... it's marked on the tap anyhow.

Prime bypasses the vacuum mechanism and should only be used to fill the carbs after maintenance or after the bike has been standing long enough for the fuel to have evaporated from the carbs. Once they're filled, turn the tap back to 'On'. Don't leave it on Prime, else if the bike falls over all the fuel will fall out creating a fire hazard, and if stored, say over winter, the carbs will gum themselves up as the entire contents of the tank slowly evaporates through them.

Regards, Tony Sleep

gsxr 400 r

hi there i have purchased 2 400,s in boxes. i have nearly completed the build on one but i am not sure about the fuel pipes to the carbs. on the fuel tap there is the 2 outlets 1 being of average size and the other being smaller wich will be the reserve.doesw the reserve pipe go to the top inlet on the carbs and main fuel line to the bottom inlet on the carbs ? can anyone help out here ?

GK73a carbs

You have a few clues here

  • you say you just checked the O rings. You can't tell by inspection whether they're still any good. They go hard with age, heat and fuel and stop sealing properly if you try and reuse them. They have to be soft and squidgy.
  • that it runs better when hot is a good sign that it's not running rich, which suggests the O rings inside the carbs are actually OK
  • your huge flat spot suggests exactly the same trouble I had with the tiny O rings that seal the pilot screws (assuming GK73a carbs are similar to GK76a). I wasted months ignoring those because they looked fine. New ones, when I finally ran out of other ideas, fixed the flat spot entirely. Before you do anything else, change them for new ones and see if it fixes the problem.

These Mikunis have no accelerator pump, they rely on high vacuum (as you open the throttle) to suck extra fuel through the progression jets. These poxy little O rings are absolutely critical : if hardened they can then leak enough air into the carb to destroy the enrichment needed and give the flat spot, but the rest of the time they don't matter so much. It's just the first quarter of the throttle where they are critical.

One way to check whether pilot screw O-rings really are the problem would be to seal them temporarily so air can't be sucked in. A small lump of BluTack or silicone sealant over each of the pilot screws should do it. Clean the area with a spray of carb cleaner first, so it sticks well. If your flatspot then goes away, you know it's the O-rings.

There are other possible explanations for flatspots (eg exhaust leaks, especially header to cyl. head but that's pretty obvious, noise etc; a cam replaced 1 tooth out). However I'd bet on those O-rings. They only cost a few pence and can be changed (bit fiddly) without taking the carbs off. You may need a mirror and a torch and something like an unbent paperclip to fish out the old ones.

These small O-rings can be a bit tricky to find. On eBay look out for the Marksman brand kit I bought (NOT the far more common Rolson/Sealey/various other names, which only has much larger seals). The smallest size in the Marksman kit, 3x1, are the ones you want. Alternatively see the comment that says supply all O-rings. I think Altecweb now do, too.

I went through all this about 3 years ago. The bike is still running sweetly now, with no more trouble (well, except my dear wife blew the head gasket last year by neglecting to check coolant levels that fell due to a leaky radiator cap).


Regards, Tony Sleep

Gsxr400 Gk73a

Hi there this site is awsome !! Hoping for some advise ,my gixxer 400 gk73a fires up and runs sweet on choke and ticks over fine , however when blipping the throttle or when taking up drive I get massive engine hesitation around 4-5000revs ,it feels like a mass flooding ,I have stripped carbs and checked o rings and all presentand appear ok , i then adjusted mixture screw to 1 1/2 turns out and still ng ,floats checked and also adjusted float cut off by 1mm just in Case fuel is leaking into the carbs ,and now appears to be worse than ever ,I balanced carbs and still ng ! Plugs also changed ! One last thing to mention is when it warms up it appears to be a lot better ! At the end of my tether and considering selling the bike .. Any suggestions or advice greatly appreciated ...

OMG!!!!!!!!!!!!!!!!! I had a

OMG!!!!!!!!!!!!!!!!! I had a show room condition GK76A SP about 10 years ago that out of the blue started to run badly. After 6 months of trying everything that I and a few local mechanics could think of we never did get it running right and hence I sold it stupidly cheap. I can not believe the solutions was so simple and was not something we ever thought about, Im kicking myself as I loved that bike :( shame the interweb wasnt what it is now and ebay was not known otherwise I could have saved myself the head and heartache of trying to solve a never ending problem!!!!!! Very good right up may just check out the classifieds for another 76A now :)

Grey Bike parts

Hi Tony, Good write up on how to cure a hair pulling headache ! Just to let you know I source JDM parts along with any other parts available in Japan as I live here Smile OEM parts are ordered from factory and usually take 2 days for Suzuki , if I then use 3 day express to send on you could be looking at under a week from payment to receiving. I only have access to Japanese parts manuals - many parts are interchangeable but if you have the Euro part code I can order that too. Companies aren't obliged to keep manufacturing spares for bikes over 10yrs old , Honda & Suzuki seem to be the best at stocking for older machines like NSR's , RG's , Katana's etc.... All the best, Mori!

Contact : Mori's site :

Valve springs

I got valve springs from Japan via BAT
Spares: 01959 577145 10-2 Wed-Sat

Franco's Motorcycles (formerly Spares Direct) are also very good and I now tend to go there first as they're fairly local to me
Spares: 020 8960 1000

Robinsons should also be able to supply
Spares: 01227 454366

All do mail order.

Regards, Tony Sleep

broken valve springs ???where to get em?

hey,,,love your site ,,saved me loads ov aggro getting carbs out,(even though its not the problem lol)mine sounds exactly like you,ve described running on 3 and all ov a sudden has a tappy sound like an old cortina i used 2 own,where can i get the springs from???

gsxr400 o rings!!!

hey mate ur page has been a godsend!!!! my new pride n joy(1st bike ;) is sweet as but just so u can let ppl know sell every o ring including the idle scew ones!!!cost me 23.00 all in pkts of 20!!! altecweb are now £12.50 a pkt!!!and as u know only sell x2 ;( i got x2 needed in viton other x2 in nitrile from polymax!tel no 01420 474 123 ref no/ordr no bs010v75/mainjet holder. bs011v75/float perch (viton) 10x1n70 /slide holder . 3.5x1n70/idle screws (nitrile)

                                                 hope this helps as u have helped me- joe :)

hi there does anyone know

hi there does anyone know where i can get a carb kit for a gsxr 400 gk71f thanks.

1989 gk73a gsxrr400 grey

i ahve a prob with it .started ride well one day,next won't start,fuel ok,spark ok, any ideas would be good,love to talk to people about this cheers chris

89 gsxr 400

my bike 89 gsxr 400 seems rare and cant find enough articles about its mechanics.. i live in jordan (middle east) and its very hard to find motorcycle parts here since they have only been legal since this year 2010.. iv had mine illegally and unrigestered but i only take it for top speed test runs away from the eye of the law.. my bike runs rich .. it has high high idle and if i give it gas it just chokes ..its been burning oil i suppose because it had alot alot of smoke .. like u said the temp stayed barely mid or even lower ... it does start without the choke ..and with the smoke "it didnt seem blue or maybe i couldnt tell" but it bleeds oil to the exhaust immediatly after i started it and i turned it off and didnt start it since .. im scared i need an engine rebuild.. another question.. does my bike fit a bigger engine ? and are there turbo kits i can install on it? thank you very very much for your time and very useful article.. couldnt find this info anywhere else

GSXR 400 issues, Please help!!!


My bike has a flat spot just of idle at about 2000rpm & at about 8000rpm. My mechanic has stripped & cleaned them out many times so far trying to find the problem but he's stumped. The riding experience is as follows; when taking off it limps & does not want to rev until it gets pass the 1st flat spot & then it runs hard & smooth all the way to 8000rpm & stalls again until you pass & then revs nice to redline. Secondly my clutch/gearbox is giving me a hassle. When the bike is stationery & started its difficult to impossible to find neutral as the gearbox feels very tight & skips from 1-2 & 2-1 without seeing neutral however if you switch the bike off everything loosens up & neutral can be found easily. When riding the gear lever feels a bit tough to engage but there is no clutch slip on take off or when riding. When the bike is stationery & 1st gear is selected a bit of clutch slip can be felt as resistance when moving the bike backwards. My mechanic has made use of the adjustment on both sides & now wants to change the clutch. Please help guys, respond to my email Thanks in advance

Mixture screws

The mixture screws are located on the underside of the carbs and are difficult to see or adjust without a mirror. They also need a very short screwdriver. Look at the float bowl along the edge facing the engine. Where there is a recess, a tube protrudes downwards from the carb body - which you can see on the left hand side of my photo showing float adjustment. The adjustment screw is located inside the end of that tube, slightly recessed.

BUT I very much doubt your problem will be fixed by adjusting them. Excess richness on a GK76a is a sure pointer that you have leaky O-rings inside the carb and need to strip the carbs and replace them as described in this article. Messing with mixture screws is likely only to make things worse and get you more confused. Once O-rings etc are replaced the mixture screws should be fine on factory settings of 2 turns out.

EDIT: Oh, and anyway, you don't set carb balance by adjusting mixture screws. The carbs are balanced via the three adjustment screws in the carb linkage, visible from the top of the carbs with the tank off.

Regards, Tony Sleep

carb balancing

hi.. i have a GK76A and am having problems with unbalanced carbs, its running way to rich and is cutting out when it gets hot. i have a friend who has kit but im unsure where the mixture screws are to make the adjustments.. do you know where on the carb these are??? i need to get this sorted quickly as i need the bike for work so if anyone can help id be very grateful Smile

Depends on the carbs

It depends on the carburettor design. Most CV Mikunis used by Suzuki in the past 30 years are basically similar designs that use O-rings in certain key locations - although not all have all the O-rings. For example Mikuni BS carbs fitted to late '80's and early '90's GSXR 750 and 1100's can suffer some of the same problems. They have O rings on the pilot screw and some have O rings sealing the float jet, although they don't have the plastic float perch of the 400's. DR350's and DR200's ditto. Smaller engines seem to suffer more.

I don't know what carb the RG125 has, but a carb strip and clean, replacing O rings and checking jet condition etc seems like a good idea. However there are plenty of other possible causes of bad running with 2 strokes, including choked exhausts and carbon build up, worn rings and pistons, leaky reed blocks or disc valves, and worn crankshaft seals messing up primary compression, and of course sparkplug condition is critical.

I guess that means the answer is "maybe".


Regards, Tony Sleep

Suzuki bikes and o-rings

Having read through your article about the o-rings and the high revs to keep the bike running, i would just like to know if this could happen through all suzukis. i'm not mechanically minded altho i do try. i have a 1988 suzuki RG125 gamma and currently have this problem. i would love to be able to fix this problem without spending a fortune on a mechanic. if you can give me any advise that would be great. Kind Regards.

Float Bowl Kits

Just as a note if anyone needs to know what to get in kit form, I have found KEYSTER Float Bowl repair kits. They come with Bowl Crush Gasket, Jet Holder Gasket, the 4 O-Rings and the Float Needle Valve. They cost me R145 each form a distributor (aprox 10GBP) Each, The local importer here in RSA has them on his website the Original kit number is K-585

GSXR manuals

Unfortunately there is no English language workshop manual for the GSXR400, either by Suzuki nor any of the independents, because the bikes were only ever originally sold in Japan and a few Asian markets. I have never found a copy online.

There is an English manual for the GS400F Bandit, which came to the UK briefly, but quite a lot is different to the GSXR on that bike aside from the basic engine design.

Suzuki's Japanese manual is available but unless you can read Kanji it's of limited use. You will probably still be able to get one through BAT Motorcycles but it will cost abput £60.

Regards, Tony Sleep

Hi there, Im trying to find

Hi there, Im trying to find any sort of workshop manual for GSXR400. Mines a 95 but any year frm 90-95 will do. Just need it for the engine side of things... Can anyone help..? Thanks Matt. T

I doubt it.

Sorry, I haven't come across any other bike with the 35mm BST's. They may exist, but I don't know what or where.

Even if they do exist, just about everything else is likely to be utterly different - jets, slide, needles etc. Even body designs vary depending on the series.

If they cannot be repaired your best bet may be to obtain a 33mm set, fitted to non-SP GSXR400's and also the Bandit GSX400. These do not use a TPS however, so you will either have to rebuild them with some components from your damaged set, to take the TPS, or fit a matching ECU (engine management black box). The ECU is unfortunately completely different on non-TPS models with different loom connections so this may not be easy.

It may be possible to have a 33mm carb body bored out to 35mm by a carb specialist, and that may be a solution. I think the castings are externally near identical. But most jets and other components from your damaged 35 will have to be moved across. Repair of your original, again by a specialist, is likely to be the better choice unless it is smashed to bits. Pot metal - the alloy's proper name is Mazak - is quite easy to repair, and carb damage and wear is common on classic bikes, so there must be people capable of doing it. Ask NRP or Motocarb if they can assist, perhaps. The latter are Mikuni specialists but no spares for OEM Mikunis are available via Mikuni outlets. They may have some ideas though.

And of course keep watching eBay. I've seen at least one set of GSXR400 carbs with a TPS, so they do come up, unfortunately about 2yrs ago. Asking on the mailing lists and at Suzuki owners club may help track down a set. I'm pretty sure gsxr_racer on the (now defunct) Graybike forum had a couple of sets and I think he's now on

Regards, Tony Sleep

Mikuni 35mm Carbs

Thank you for the fantastic information here. I have 35mm Mikuni carbs with TPS but unfortunately the cast is damaged. Do you have any idea if these carbs were used on any other model or if the cast is the same on anything else? I'm not doing very well sourcing a replacement set :( Thanks, J

Pilot screw O-rings

To clarify an emailed question about the O-rings needed for the pilot adjustment screws:-

I have a small question - What is the actual size of the o-ring for the idle mixture screw?As mentioned on your website, it suppose to be 3.5mm x 1mm but you indicated
that they can be brought at ebay. I followed the link and found the 200pcs of mix o-ring but it didn't show any size which fit your description.

It's the right kit.

O-ring sizing is specified by the OD, which is pretty unhelpful. Also I just can't measure the damn things accurately with a digital vernier. 3.5mm was an attempt to measure the hardened and stretched OE ones, clearly inaccurate.

The diameter of the pilot screw is 2.98mm, and the O-rings you want from that set are the 4.5 x 1 which are a perfect tight fit around the pilot screw, and into the carb body. It's pretty obvious which ones are correct, when you get the eBay kit - the largest 1mm supplied are the ones.

We still haven't had any further trouble with the carbs since I did all this around 2.5 years ago :)

Regards, Tony Sleep

GSXR ills

Hi Tom,

Sorry to hear you're still having trouble with your gsxr400... It still sounds like carbs to me. Specifically slow run issues.

The prime suspect would still be the tiny O rings. Going lean as air leaks past them is what causes the stumble, just as the motor needs extra rich mixture through the progression jets.

Have you tried slowing down tickover (back off the big adjuster thumbscrew between the #2 & #3 carb)? If it's set too fast the progression jets are already part uncovered, which ruins pickup and might be the whole of your problem. With well-balanced carbs and correct mixture it should tickover fine at @1,100rpm. I assume they are well-balanced... it matters!

Are you sure you have fished out the old pilot screw O-rings, and then installed new ones backed with the plain washer and spring? Are the mixture adjustment screws set to 1.5 turns out? A degraded inlet rubber won't help, but you'll find it hard to balance the carbs if that's the problem. An old trick is to spray some WD40 onto the rubber and see if it causes the revs to fluctuate. But I doubt that's the problem.


Tony Sleep

Still not running well! What havn't I tried?!

Hi Tony, firstly thanks for such a great site again (I bought an undamaged top of carbs off you a while back). I have very clean carbs, all o rings have been replaced, float valves at 20mm, yet still cant get the thing to rev above 2k. It usually sits around 1300-1500rpm. If I open the throttle it bogs down and stalls. What havnt I covered!? Dont think its a fuelling issue. One thing I noticed is that on carb 2, where it attaches to the rubber manifold to the engine, the rubber has slightly deteriorated, ive heard this could be an issue? Any help would be much appreciated!

Carb balancing

Yes, the little nipple on the carb top is the pick-up for carb balancing. There should be a neoprene cover over each secured with a spring-clip, which has to be removed for balancing.

If you don't get any reading on idle this can mean one of 3 things. Either

  1. Someone has put the carb tops on in the wrong position, so the nipples don't align with the airway through the carb body.
  2. There should be (yet another) O-ring between the base of the nipple (inside the cap) and the airway. If there isn't, the vacuum will leak giving no or wrong readings
  3. The nipple is blocked. If a previous owner lost the neoprene covers for the nipples they may have jammed something into the nipples to prevent air being drawn in and messing up the running.

Fixing any of these should be straightforward..

One thing to note : carb vacuum is much lower on these bikes than on most, at least on the SP it is, because of the large carb diameter and small swept volume of 100cc cylinders. Conventional balancing guages should be OK but I found that with the Morgan Carbtune I used, it was necessary to use the Carbtune upside down to get a sensible reading. This is suggested in the instructions as necessary on some bikes and works fine. Right way up they barely registered a vacuum.


Regards, Tony Sleep

I have been trying to

I have been trying to balance my carbs on my gk76a do i fit the vacuum gauges to the carb top on to the little nipple if not where. the info you have given me is the best piece of info i have found thanks i thought i was an expert on the mikuni carbs having taken them out about fifteen times but i have learnt something new.

GK73a exhaust

I think there may be an aftermarket can made by ART. Probably the best bet is to ask on the forum (and look in the 'for sale' list at the same site).

Apart from that, they do come up quite often on eBay, and breakers yards are worth trying, perhaps through Partsfinder.

Lack of baffles will mess up the jetting, for sure, with at least larger mainjets being needed, and quite likely full setup on a dyno.

Regards, Tony Sleep

Hi there. first of all

Hi there. first of all excellent advice. i've just bought a GK73A and am experiencing some with the fuelling also. I think my problem may be the exhaust as someone removed the baffles from the exhaust silencer. so my question is do you know of anyone who makes a silencer to fit my bike or where to get a original exhaust system from?

many thanks


@Dave Cole

Assuming the choke mechanism is working correctly, the trhottle cable has a bit of slack and the slow run adjustment screws are set to 1.5 turns out, I think you will probably find that someone has wound the throttle adjustment screw too far in to try and get it to tickover with the carbs buggered up (as they were and now hopefully are not).

Try backing off the throttle stop (the big thumbwheel pointing downwards between carb 2 & 3), a few turns and see if it improves things. Keep going and I suspect it will work properly.

If the carbs were off the bike you could visually confirm whether this is the case. For correct idle speed the butterflies are almost completely shut, and only one or two of the progression jets (minute drillings into the carb body) are not blocked by the edge of the butterfly. Most of the idle mixture is supplied via the slowrun bypass circuit. I can tell you from messing about that too much opening of the butterfly has exactly the result you're seeing, it'll run (not very well) at a high idle speed but won't pickup. That thumbscrew needs to be as closed as possible to get an idle of around 1300-1400rpm. Any faster and it all goes to ratshit.

Explanation : If the throttle stop is set too far in, the butterflies are too far open and the progression jets are exposed, which means the engine is getting a rich mixture and too much of it. Worse, the transient richer mixture necessary for the RPM to increase is not available so it'll just bog and fail to pickup. The backfiring through the carb is consistent with this too.

I doubt very much that CDI or coils or anything else are involved. These carbs are weird and delicate things but work fine when set up correctly. And you are going to have to get a new carb top in order to balance them, which is essential - unbalanced the thing won't run right at all. Don't pay any attention to no vacuum reading to speak of when you crank the engine, you'd only expect a sensible reading when running.

I have the remains of a bank of carbs I bought for spares, and I'll sell you an undamaged top for a few quid if you like.

Regards, Tony Sleep

gsxr400 wont rev up

hi, your article is awesome !! I have just bought a clapped out gsxr400 76a and it is very difficult to start and when evetually running will only run at 2000rpm no more and no less! I have stripped blew out , cleaned and replaced the 4 o rings in each carb but still no good, please help !!!! Could it be a vacuum problem within the carbs?? The tank and ap seem to be working fine , it backfires when trying to start , but when eventually runs sounds ok at 2000rpm with no knocks ,or weird noises . One of the plastic stub nipples on plastic diaphram covers is broken off but has been sealed with a screw and sealant. I have tried pulling off one nipple cover and attaching a vac pump but doesnt pump up and when engine cranking just flicks needle on guage. Could it be anything else but carbs, cdi pickup or coils?? Any help would be massively appreciated!!! Cheers dave cole

It's staying put

Hi Gary,

Yes I remember you from the forums, though I've not visited any for a while as the bike is still running perfectly (2+ years without carb issues, a miracle) and in use most days.

The page isn't going anywhere :-)

Regards, Tony Sleep

Tony, this is such a useful

Tony, this is such a useful page of info thanks so much for putting it up!
Although I don't actually run a 76 (mine's a 71F) I have directed a large number of people from the gsxr400 forum (I'm a mod there) and the gixerboyz forum to here.
Please, whatever you do, don't take this page down as it's absolutely invaluable!!


The nipple for balancing

The nipple to which you attach the balance gauge tubes can just about be seen in the top picture. It's the small black thing sticking up, just on the front of the screw that secures the diaphragm cover. By 'front' I mean in between the carb and engine block. These nipples have a neoprene cover in place when not in use, secured by a circlip, and that's what you can see sticking up in the photo.

Beware that it's quite possible to put the diaphragm covers back onto the carbs in 4 different positions, and only one allows the nipples to align with the airway through the carb body. Yet another O-ring makes the seal there, and of course if it's missing or damaged, carb balance will be impossible.

Regards, Tony Sleep

bloody carbs

I have been trying to balance my carbs on my gk76a do i fit the vacuum gauges to the carb top on to the little nipple if not where. the info you have given me is the best piece of info i have found thanks i thought i was an expert on the mikuni carbs having taken them out about fifteen times but i have learnt something new.

Thank you

Tony, this is the most useful, informative, clearly laid out and welcome web page I have come across this year! (You can guess, I too have been fighting with carbsCry) Just wanted to say thank you for sharing your experience, it has probably saved me another 3 months of work! All the best, Matt

Sorry, no

Sorry, I don't.

BAT Motorcycles will probably have the most complete parts lists in UK, and will be able to get any tubes and needles that are still in production, but I tried for our '93 SP and only needles were still available in Japan. The tubes are discontinued.

It's worth asking at the Greybike forum, as several members are knowledgeable about the carbs and you may be able to piece info together.

Regards, Tony Sleep

Hi do you know where i can

Hi do you know where i can find a listing of the different suzuki 400 emulsion tube and needle numbers. All the 400 bandits seem to have different numbers. Thanks colin

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