I continue to get emails asking me about carbs and O-ring problems, so here's another summary which I hope includes everything anyone needs to know. In any event, it's just about everything I know.
It being that time of year when a whole new bunch of owners discover their pride and joy is running like a pig, here's a primer...
The Mikuni carbs on GK76a use O ring seals in 3 different locations, any or all of which cause carburation mayhem as they harden with age, heat and fuel immersion. GK73a and GK71f types may or may not be similar, beyond my experience I'm afraid.
Symptoms of leaky O rings and poor carb setup include:-
- cold starting without choke
- rich running/poor MPG/sooty exhaust/spark plugs
Both the above need attention quick. The O rings are almost certainly leaking and WILL allow excess fuel to wash the bores, which WILL result in excessive and rapid piston/bore wear and a totally knackered engine that needs £hundreds spent on a rebore.
- stumbling and jerky pickup from low revs
- hard hot starting
- poor low/mid throttle response, but OK at higher revs
May just be carb balance and slow-run mixture adjustment. But if accompanied by rich running and no need for choke when cold, you're looking at a carb rebuild.
Carb removal and strip:-
This isn't hard. BUT BE AWARE THAT PETROL IS EXPLOSIVE AND WILL KILL YOU GIVEN HALF A CHANCE. You have to get the carbs off which looks an impossible task. Obviously tank, seat, fairing need to come off. Disconnect the battery to avoid spark risks. There's a rats nest of hoses, breathers and cables and sh*t to be disconnected. Everything can be left connected at the carb ends. Take a picture/make a sketch so you know where it all goes back.
- undo jubilee clips between carbs and inlet stubs, and carbs and airbox stubs
- release throttle cable at the handlebar end
- release choke cable at carb end
- unplug TPS (SP only)
- withdraw the 2 large wire clips that retain the airbox stubs, and push the stubs back into the airbox (pushing them back onto the carbs later is easier with the filter removed).
- this will give you enough room to pull the carbs off the inlet stubs and feed them to the offside and remove the entire bank, along with the attached hoses etc.
Unless you need to for other reasons, don't separate the carbs, leave them in a bank. You need to get the float bowls off and also the diaphragm covers. Be warned : these Philips head screws are sods, and apt to corrode in place. At the very least you need a decent, properly-fitting screwdriver, but gentle use of an impact driver is far more likely to work without mashing the heads. Altec also sell caphead M5x12mm stainless steel bolts (code 114-6444, £10 for 50) which you can use to replace the OE Philips rubbish.
The offending O-rings are:-
- around the body of the float perch/shutoff valve holder, the pinky/orange plastic piece. If this leaks, petrol simply bypasses the shutoff valve and overfills the float bowls. IME this is the most problematic of the 3 locations, since the plastic body is a loose fit and the single securing screw allows it to wobble around if the O-ring is knackered.
- around the slide holder base, the large plastic piece that the plastic slide runs in. A leaky O ring here will bugger up differential pressure on the diaphragm and mess up throttle response and mixture progression
- around the main jet, sealing it into the carrier. Another potential disaster, as leaks here will be like having an oversize main jet fitted.
'What O-rings do I need?'
Our bike is a 1994 GK76a SP with Mikuni 35mm carbs with TPS, but all other GK76a's 33mm carbs use the same O-rings. Any other models I DON'T KNOW - you must make your own inquiries.
You can buy OE replacements from Suzuki but they cost a mint and are exactly what caused the problem in the first place.
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.
http://www.altecweb.com are a UK site who sell 2 out of the 3 types
Altecweb quickorder codes for Viton O rings
ORV BS010 mainjet holder
ORV BS011 float perch
These are £4.05 each for a pkt of 50, a lifetime's supply. You only need 4 of each type.
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 a poor 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 or Nitrile.
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.
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 immersion in hot petrol and they'll deteriorate rapidly and you'll end up doing them again in a few months.
Using these O-rings has worked well here. The bike continues to run like a watch and be completely free of carb problems 6m later.
PLEASE NOTE : assuming good engine condition, good plugs and aircleaner, you will need to check the valve clearances AND balance the carbs as accurately as possible AND adjust idle mixture on each carb VERY carefully in order to resolve carb problems. In that order.
Do NOT f$ck with float height settings unless they're wrong. Unfortunately a lot of people try and fix excessive richness (due to leaky O rings most often) by messing with float height. It won't work. It just makes things more confusing. Unfortunately #2, the Jap manual for these bikes (GK76a) contains no intelligible information about float height, so once someone had messed with them there's no way of knowing what they should be. The occasionally-quoted 2-4mm at the perch is impossibly vague. All I can tell you is that the float heights on our GK76a SP are 20.5mm measured in the conventional manner (float bottom to carb flange when shutoff valve just touching seat), and this works fine here. Other models may be different.
These bikes are just amazingly sensitive to carburation. Small adjustments make large differences on these bikes. The patience of a saint, accuracy and perseverance are essential. The difference, once sorted, is remarkable - even the SP is able to pull cleanly from tickover. Skip anything, and you're liable to get nowhere.
I can balance and set the carbs on my GSXR1100J in <10minutes, the same on the 400 took me most of last summer through lack of info and trying to cut corners. I had the carbs on and off more times than I can count - 3 times in one day was my record. Take it from me, it's worth doing it once thoroughly and properly.
Carb balance is tricky to achieve (I use Morgan Carbtunes inverted - excellent kit), but the breakthrough insight is that idle mixture HAS to be set on each carb too. Most bikes don't need that messing with. After experimenting with Colortune I realised it just wasn't accurate enough, and found it much better to just adjust each slow-run screw by ear for the most even running. It's then worth going back and readjusting balance.
If you run a race can and/or aftermarket aircleaner, you will also need the assistance of someone with a dyno to sort out mainjet and needle issues. Unless you do that, it's unlikely you'll be able to get the bike to carburette properly - unless you're sufficiently skilled that you don't need any of the above advice.
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Regards Tony
(snaked from another board from Tony aka halftone from
http://www.400greybike.co.uk/forum/topic.asp?FORUM_ID=20&TOPIC_ID=43615 on may10th, 2006
hope it helps some one out!!
Topic: gsxr400racer Carb Tuning. (Read 23918 times)