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 '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.
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...
The carbs look impossible to remove due to lack of space, but in fact are fairly easy.
- Remove fuel tank, battery and battery cage
- Remove the airfilter cover and airfilter element
- Slacken the Jubilee clips holding the airbox stubs onto the carb inlets
- 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.
- Slacken the Jubilee clips holding the cylinder stubs onto the carb bodies
- 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)
- Remove the breather pipe that runs from the cylinder head, across the middle of the carb bank and connects to the airbox
- 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.
- On the SP models only, disconnect the Throttle Position Sensor by pulling apart the white triangular connector block.
- 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.
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.
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
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.
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
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.