The pilot screw o-rings are 2.70 * 1.18. They are exactly the same ID as the OEM part.
See Mikuni reference N133.037 at http://www.sudco.com/carharori.html
cheers

Thanks for the link, Blair. I've got a pack of ten 2.5 mm I.D. x 1.00 mm x sect. o-rings on order from a vendor just a few miles up the road from me in Dundee after reading Tony Sleep's GSX-R 400 carb saga on the web ..... again!

'Kin 'ell!! Has it been that long since I last posted an update on this seemingly never ending saga?! My 3XV's motor lunching a piston ring, piston & (plated) cylinder late last Sept. hasn't helped as that distracted me for a while but I eventually got back to working on my baby Gixer. Unfortunately I still haven't quite cracked getting the carb's balanced yet!

Well ... saying that, I could get them balanced spot on at 2,000 - 2,500 thou RPM, but on winding the rev's back down to idle at around 1,300 rpm it was as lumpy as hell ! When balanced at idle rpm they were way out at higher rpm !!
I had the carb's off, dismantled, thoroughly checked over & thoroughly cleaned out wi' good quality carb cleaner three times to no avail ... so I gave up in disgust! ( I sprayed ALL the fuel, air & vacuum passageways through in both directions, there were no blockages in any of the carb's, partially or fully ).

Then one day I had a "light bulb moment". Maybe it's the ignition leads & plug caps causing the lumpy idle? The plug caps are the original OEM ones so are at least 20 years old now & it's highly likely the ignition leads are just as old!
So, I did a bit of web research & found that it's recommended to replace copper core ignition leads every 5-6 years & carbon core leads every 2-3 years!! (Their resistance starts to increase as the insulation/materials break down with age, heat etc. Plug caps do the same thing apparently). So, armed with the above knowledge, I was straight on to Fleabay to order 2 metres of 7 mm O.D. copper core ignition lead & four NGK plug caps. That was back in early November. Due to work & my Christmas/New Year holidays, ( I had a whole month off. Yeehaa!! ), it's taken 'til now to finally get the leads & caps fitted. (Drain some coolant & remove thermostat housing for access! ).
I've also replaced the L/H coil as the threaded terminal for No. 4 cyl's lead was badly corroded compared to the other 3 terminals. Every little helps as they say nowadays.


This is the caps I fitted.


I've also got round to fitting a NOS choke cable I bought a while back. (The outer on the pattern cable fitted was looking a bit rough in places).



Next on my "to do" list was turning this crustiness...


... into this ...


... with the aid of new OEM chain adjuster plates & a set of GSX-R stainless adjuster nuts & circlips. The plates were in the spares stash that came with the bike when I bought it & the stainless adjuster nuts & circlips were bought from an Ebay vendor. (They fit all capacity's of early '90's GSX-R models). So that's another bit of the bike tidied up.
(Sorry Si, I wasn't gonna fit the new plates you bought, but when I found the stainless adjuster nuts I just had to fit them. ).

I've also swapped the fuel filler cap & fuel tap over to a replacement fuel tank that was in a MUCH better condition externally than the original. The replacement was bought last spring via Ebay for a bargain "Buy It Now" price of £70.00. MUCH cheaper than a respray ...


... AND it came complete with the OEM black rubber trim around the bottom seam & a natty looking tank pad. Braw.


Now all I need is for all this stormy winter weather Scotland's been battered with lately to bugger off so I can balance the carb's ... yet again !! If new ignition leads, spark plugs, plug caps & a coil swap don't help to cure the lumpy idle problem I'll be officially stumped!
( And yes, I've checked & cleaned all the ignition system electrical connections ).

Anyhoo, ch'o the noo.

Wullie.

P.S. Belated Merry Christmas & Happy New Year folks.

Obsessing about carb balance is basically chasing your tail. When using vacuum gauges to synchronise throttle opening you should wind the idle up to about 2000 rpm, NOT at idle. I usually go higher than that myself.

Running poorly at, and just above idle, is most often due to the pilot circuit. The mixture screw has a recommended number of turns but it is a guide not an absolute. Ideally the idle mixture should be set by ear. That is not very easy. So often the best compromise is to set all four screws at the same number of turns out and experiment by changing that value by 1/4 turn increments. If it is too lean the idle will tend to stay too high and won't settle. If it is too rich it will start easily when cold and run very poorly when hot or stall. You have to ride the bike to get it right, not just sit in the shed.

Putting the wrong size "Tony Sleep" o-rings may also give you a different value for your pilot mixture screws. Given that they are 0.5mm pitch and your o-rings are 0.2mm too thin you have already lost .4 of a turn of your mixture screw.
cheers
Blair

Thanks for your reply, Blair. Some more info for you. I've got the mixture screws set at 2 turns out & I did experiment with adjusting them a quarter of a turn either way after finding a post elsewhere online on balancing GK76a carbs's, but it didn't improve things enough for me. I would have replaced the old OEM leads & plug caps a lot earlier but I stupidly ordered 8 mm O.D. ignition lead which wouldn't fit in to the lead recesses in the coils!!

But ... I did fit slightly longer old leads (from a spare set of coils) to the R/H coil (No.2 & 3 cyl's coil) after trimming the ends to get fresh copper. (The original ones were a tad too short & were being strained). Stupidly I didn't trim the ends of the leads on the L/H coil (for cyl's 1 & 4) & they're the cylinders that are running poorly at idle, but ARE balanced with the others at 2,000 - 2,500 rpm, hence why I had my "light bulb" moment about the old leads & caps maybe affecting things at idle rpm due to increased resistance due to their age/corrosion etc. As I said above, the OEM leads & caps are at least 20 years old now! Nothing lasts forever.
If the weather here improves enough next week I'll set the mixture screws to 2.5 turns out, do another balance run & see what happens. ( The bike's at my work 30 miles away in a warm dry hangar. ).

Maybe I'm too much of a perfectionist and, as you rightly point out, just need to ride the thing & take it from there. Thanks again Blair.

Kind regards.

Wullie.

Sounds like you are almost there.
Try setting the mixture screws by ear. If it is that bad at idle then you should be able to improve it. If you can hear the air box at the same time by having the tank off it will also help. You rotate the mixture screw until you hear that cylinder falter or the rpm slows. Then you go in the opposite direction until the same happens. The put the screw half way between those points. Repeat on the other three cylinders. If you are on the right track the idle should have climbed because it is running better. Then you lower the idle and repeat the whole process. If you can get down to a 1/4 to 1/2 turn either way and that cylinder slows then you have nailed it. These are mixture (fuel & air) screws not air screws so the whole "turns out" thing is a load of bunkum actually.
Maybe too difficult to get a/f ratio on each cylinder but in this day and age I am sure you could use some digital means to monitor the rpm or the sound and here is one guy using an infrared thermometer http://www.xjrider.com/viewtopic.php?t=673
cheers

Thanks for the procedure above, Blair. I'll give it a go next time & hopefully I get a smoother idle that I'm finally happy with. I just want it to purr like the proverbial kitten. Not too much to ask for is it? I've been using a (flushed out) old coolant expansion bottle as a fuel tank during the balance runs so listening to the air box will be no problem.

Just so you're aware, Blair, this is only my 2nd four stroke in 31 years of biking. The first was a pre owned GPz 750 A3 (the last of the Kawa. air cooled GPz's / ZX's) that I bought from a dealer & only owned for approx 4 months way back in 1987, (traded it in for a new TZR 250 ), so never did any maintenance/servicing on it, so, apart from the carb's saga, this has been an enjoyable learning experience servicing a four stroke bike engine & its ancilliaries. I've got years of experience maintaining & overhauling 2 stroke engines but fancied a trip to "the dark side", hence why I bought 4hundread's GK76
Yes, it 's been frustrating lately, (especially when I see it every day at work looking all shiny, clean & tidy).
Soooo near ... but yet so far ... !!

Bye for now.

Wullie.

Well you've come this far I would agree that you have to finish it off well.
I had a look at those infrared laser thermometers on eBay, they cost nothing. Definitely worth tying.

Nice write up and the bike is looking great, will look mint when finished.
I read you got a K&N filter plus aftermarket exhaust so you will need to bring the mixture screws out much further than the manual says, think I ran around 3 turns last K&N I fitted.
My bandit 4's running a K&N and its around 3 1/4 if I remember.

Thanks for the info above, VVS.
Yes, to summarise I have the GSX-R K & N filter kit (Pt.No. SU-7592) with the large intake rubber fitted, stock 100 main jets replaced with 117.5 main jets, stock needles with the clip in the 2nd from bottom slot ( needles raised from stock middle slot) & the stock 47.5 pilot jets replaced with 52.5's. The stainless down pipes were bought from Sandy Bike Spares via Ebay, the link pipe is from an R6 & the end can is an old Scorpion round can.

More info for you.
One of the checks I did on the initial carb strip & service was to check the top of the stock O-5 needle jets for ovality after reading posts elsewhere in here. I inserted the chuck end of a suitable sized twist drill into the upper end of the jets & checked for ovality with a X10 magnifier & was relieved to find negligible wear. The needles looked servicable too.

Thanks again for all your input, gents, it's much appreciated.

Cheers.

Wullie.