All text & images © The Flying Banana, 2007.

GSX-R Carbies for the Katana...

Something all motorcyclists know is that Mikuni make a fine carburettor. More to the point, this famous company makes whole banks of carburettors. I don't know about you, but whenever I pull a bank of Mikunis from one of my bikes, I usually end up gazing and tinkering with them for hours. Masterpieces of design and intricate casting, there is much to be learnt pondering their innards.

One day I will write a tome on 'Zen and the Art of Carburettor Comprehension', but in the meantime I have more learning to do, for a stray bank of 36mm '89-'90 GSX-R 1100 carbs has found its way (August 2007) onto my workbench. Well it's not quite as ethereal as that; I got them from Arnout (cheers again mate) who has moved on to mucking around with RS flatslides and the like for his two-wheeled beasties.

Now the OE Mikunis on the Katana are quite up to the job of coping with all the engine mods you can throw at the ultra-tuneable GSX donk. All you have to do is re-jet. But as time marched on, so did the innovative chaps at Mikuni. A decade after the big Kats hit the streets, Mikuni carbs had gone through a couple more stages of development. One thing that did not change, however, was the distance between the carbies — which means it's a dead sinch to slot later sets of carbies onto our ageing warhorses.

But why would you want to do that? Well, it's not like we really need the extra horsepower... Oi! what tripe am I spouting?! OF COURSE we always need more horsepower and stuffing a bank of later carbs onto these machines is a good way to flush extra ponies out of the metalwork. How many more ponies, I hear you ask? Look, I dunno... but somewhere around another 10HP is my guesstimate. Into the bargain, these later GSX-R CV carbs will take pod filters without the (alleged) tuning difficulties of the earlier CV carbs, so that means we get better breathing — but also, Dear Reader, the joys of induction roar.

Well it's not like we have much of a choice with the pods, anyhow. The fact is that the original airbox set-up can't be coaxed to fit onto the very-much larger intakes of the GSX-R carbies. So, pods it is. However, your standard pods (which fit GS1000's and the like) are too small as well. Thankfully, K&N make a 'dual flange oval' filter to fit the GSX-R carbs — the RU-2922. The trouble is, it's fairly expensive for a pair of the suckers.

Enter eBay, one of the current banes of my marriage, but nevertheless it is very handy for tracking down good deals and saving lots of dough when you're on the never-ending quest for parts, as I am. After systematically trawling through all the listings for 'K&N' I finally found them: a pair of the filters I needed, going for US$5. That's right, five bucks! Naturally postage to Western Australia blew the price out a bit, but by the time all was said and done all I had parted with was about AU$32. You beauty!

The elation was short-lived. When the filters arrived, the rotten things simply didn't fit. The inside diameter of the flanges was spot-on, but alas there was about 5mm too much space between the flanges. So what model of K&N filter did I have? I spent a fair bit of time trawling through the K&N spec. pages, but I was unable to find my filters listed. So I could only conclude that I had landed some obsolete product. Rats.

That meant there was only one thing to do: modify them. After pondering my options, it was time to get brave and take a sharp knife to the filters. I sliced out a 5mm section from the centre-line of each pod, and glued them back together with my favourite snot-coloured goop, 'Selley's Gel Grip'. To make sure the things didn't split apart where I'd joined them (as no glue I had would 'take' to the rubber completely), I screwed on thin plates of aluminium sheeting to the backs of the pods, and also between the flanges. After that, they slipped straight on.

The next thing to sort out before slotting them into the Katana, was replacing the needle jets (aka 'emulsion tubes') in each carbie. Arnout warned me that the wretched things had been chopped out by the needles — a fate all needle jets suffer if used for long enough (see further musings below). So I took a peek at them and sure enough, the orifices were worn into an oval shape — a state of affairs that leads to too much fuel getting through and things running way too rich. The cost of new needle jets from Suzuki was mildly crippling, so I had no choice but to fork out.

After completely dismantling and cleaning out the carbies with the usual amazing-to-sniff chemicals and compressed air, I slotted them in. Which you would think would be a piece of cake... but no, the usual unforseen rigmarol of having to shorten throttle cables, etc. prevailed. But finally they were in.

Then it was time to start the bike, and take her for a quick strop around the block. First impressions? Sheer amazement! A bit sluggish below 4000rpm, but above that maaaaaate the old girl fairly flew! So it was back to the garage to get the pilot mixtures adjusted, balance the carbies with the mercury vacuum gauges, and hook up the AFR meter and see how things were going. I ended up putting the pilot mixture screws 1 3/4 turns out, and the needle clips at position #4 (counting from the top of the needle). Balanced the carbies again, and that was that.

And now, the bike absolutely FLIES. It is difficult to exaggerate the difference, folks; but before, you really had to wring the old girl's neck to get the front wheel anywhere near coming off the deck. I mean, it's one old, long, heavy motorcycle, so lofting the wheel was always going to be a challenge. But now it's much easier to do... The bike just takes off, heaps more grunt. And before, it seemed to run out of puff at higher rpm/ speeds, but now there is no sign of that: just relentless gobs of power all the way to redline! Amazing!!! I, for one, would never have imagined that getting a set of carbs that were a mere 2mm bigger in the throat (36mm vs. the original 34mm) could make such a gobsmacking difference. But there you go.

Actually, when you crunch the numbers, the 2mm increase in throat diameter mightn't sound like much, but it works out at about a 12.5% increase in theoretical fuel mixture delivery. Tell you what, it's got me thinking about a 38mm set of GSX-R carbs, or even a set of the Mikuni RS flatslides. I wonder...

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Psssst... wanna good run-down on how to dismantle, clean, fix and set your GSX-R carbies? Go to this thread at and you'll get an excellent and comprehensive article by a few blokes who know their stuff. Lots of excellent, clear photos along the way — you can't go wrong!

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But if you want to keep your airbox...

Well, having gone down the 'pods' route, I sold off the old airboxes on eBay. They were bought by Katana Australia forum member 'Mega' — who has also chosen to go down the 'Slingshot' carbie route with his Katana. But he wanted to keep the stock appearance as much as possible, which meant retaining the airbox. But as we know, the old airbox simply won't fit onto the much larger intakes of the GSX-R carbs. Or will they?! Here's what Mega did with my Kat's old airboxes, in his own words from the forum:

This project has been 2 1/2 years in the making. Anyway, what I was planning was to add 1988 GSX-R 750 carburetors, the oval ones, I just did not want to add the pods as it would look like it was a 'highly' modified Kat. I wanted to keep the stock appearance as much as possible, so I had to find a way to adapt the stock plenum and filter to these carbies, and thus also not having the problems when pods are used.

I finally got some reasonable parts for the 36mm jobbies I had bought years ago of E*ay, but the slides were shot, emulsifier tubes no good etc. etc. BUT, they were Dynojetted 136!!

To modify the stock plenum, I first cut away the stock velocity stacks and trimmed a bit more to fit the velocity stacks from the 1988 GSX-R 750 carburetors. Then I trimmed the outside to fit into the plenum I had trimmed, then I mounted it all. The space that was between the plenum and the stacks (that were mounted on the carbies) I filled in with some of that quick setting plastic weld stuff — that gave me the distance and alignment that was needed so that I could get the plenum mated to the stacks by a plastic welder. Unfortunately, they could not be welded as the materials are different, so 'Sikaflex' was used.

I finally fitted the fuel lines and then started it. After a few minutes it settled to a nice idle of around 1100 rpm. I thought I set everything up before I mounted them; the air screws are set to 2 1/2 turns out and the idle adjustment was a pure fluke, so far so good. The filter box and plenum is from TFB (thanks Mike, the lid is a treat, others may want to do the same after seeing this pic). The throttle cable is too long and I'll have to shorten it, I'm thinking that maybe a solderless nipple may fit or perhaps I'll silversolder another nipple on it, we'll see...

Here are some pics of the job, it might look a bit messy, but it is proof of concept.

Update July 13 2009:
I've been a bit busy the last couple of days, so been a little slow on it, however, I put it all back together and did what I could with the throttle cable, its got a LOT of freeplay but I've got a couple of ideas I want to try first before cutting a perfectly good cable.

Well, I took it for a spin today, only a short one though and I can report that the difference is amazing, the mid range is REALLY strong, it pulls from 1500 hard and gets to 5000 real easy, I did try it to 9000 and it pulled hard and evenly all the way to it (not on a public road of course), there are no flatspots either, it just pulls hard whatever revs you are doing, I think a dyno is in order to check what the output is now.

It is a bit rich (judging from my nose) at idle, I think I will try screwing in the air jet from 2 1/2 turns out to 1 1/2 turns out and see where that goes.

One thing that is missing now is that fabulous Kat Krackle on the over-run, might have something to do with being a bit rich too...

I'm leaning towards a "Sleeper" (sleeping?) Kat, or a wolf in sheeps clothing...

Here is a photo of how it looks now, I wonder how many people will think that is how they came originally????

A good-looking bit of work there; thanks for sharing mate!

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A few thoughts on needle jets (aka 'emulsion tubes')...

As I mentioned above, the needle jets on the GSX-R 'Slingshot' carbies are prone to wear. Very prone, in fact. With the needle sliding up and down on one side of the jet (they are 'biased' this way, to prevent the needle rattling around and causing fuel atomisation problems), it's only a matter of time until the orifice is worn, and allows too much fuel through, making your mid-range too rich. This is what it looks like:

As you can see, the side of the orifice (red arrow) has been worn away by the needle. Sadly, this can happen very quickly; the jet in this photo had only about 8000kms of use — mate, I get a better run out of a set of tyres. And the symptoms? Well, the bike had started bogging-down whenever I took off from the lights; and the last straw was when it got so bad that it started stalling. "Surely the needle jets can't be worn already?!" I thought — but upon whipping them out for a quick inspection, the oval-ness of the hole in the jets was easily visible to the naked eye. Blast!

The FactoryPro website has a run-down about this Achilles Heel of the Slingshot carbs, here. As you can see, they also sell nickel-plated emulsion tubes — the idea being that the plating will protect the brass from wear. So, how much longer do these needle-jets last? Dunno, but I'll let you know how they go. Stay tuned!

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