ZX9R streetfighter airbox hack

Just thought I'd post this up here as well as ASF in case it comes in handy to somebody...

With the fairings off the Minja after Woodsy munted them up taking it motocrossing on our recent trip to brisbane I found the bike simply wouldn't run at freeway speeds. The engine would cut in and out and every gust of wind would cause very uncertain fuelling.

As I anticipate the bike will be like this for some time, I went searching for information to help me solve the problem. Turns out it's because of the float bowl pressurisation tubes no longer being equalised in pressure with the airbox once the ram air system is removed. A simple hack can sort this out nice and easily, so here's what I did:

For reference, this is on a 1994-1997 B model ZX9R:

The two bendy tubes coming from the carbs up and out through the frame holes to the front of the bike are the float bowl pressurisation tubes. Not sure if they're the same on a ZX6, I know the ZX7s are slightly different as are the later model E ZX9Rs. There'll be something though.


Using a $2 copper t-junction from bunnings, I've cut and joined the two float bowl pressurisaton tubes. I've then stuck another straight bit of cut-off tube so it points up through where the airbox will be. I left it long so I could cut it down when I knew how much it should poke out:


Re-fit the airbox and locate roughly where you'll need to drill it, then go for it. I tried to make the hole a bee's dick smaller than the tube diameter, to keep the airbox sealed. I failed. You might succeed. Please note scratch trail where ham-fisted drill user has lost control and wandered off down the airbox:


Drill the filter next - it's very flexy so you can make a fairly small hole. You definately want this bit to be airtight. Fit the airbox with the tube up through it, then fit the filter and poke the tube through. Cut off the excess length so the cover will fit back on, and voila, there you go. The bike should now run without any surging at high speeds - although the jetting will need some adjustment to match the new airbox behaviour.


Bob's yer uncle now, the Minja is running fine although a bit lean if I understand it correctly. At least it can do freeways now. It's also an easy hack to reverse if you want to put the ram air system back on, because all you do is re-route the hoses and plug the holes and you're off.

Further reading, relevant to all kawasakis, here:
http://www.factorypro.com/Prod_Pages/prodk51.html
http://homepage.ntlworld.com/webzxr/zxr ... tbowl.html
http://homepage.ntlworld.com/webzxr/zxr ... carbs.html

Have you noticed any drop in power without the RAM air?

Mate it's running a touch lean and choppy, it needs to be dyno tuned and rejetted, which I'll get around to one of these days when I have the cash.

On the gas it still pulls like a train to redline in 1st, 2nd and 3rd, by which time the lack of fairings start encouraging you to back off a bit. I won't have the chance to do a top speed run for some time. For all legal road purposes it's still a very powerful bike, I can't say I've noticed any lack of power with the throttle open, and I expect it'll be even better after a re-jet lets some more fuel in.

I had race glass on mine but didn't put in the front cowling duct(2000 model has a seperate piece, just to shit ya), thought it wouldn't cause that much of a problem. But down the straight at Oran Park it was causing all sorts of problems. Dodgied it up with some duct tape to funnel the air into the snorkels and it got me through the day. I bought another duct for the race glass, so problem sorted.

Anyway good to hear there is an easy answer for those wanting to nude up the 9.

its a very similar thing when you want to strip most ram air bikes i think, i had the same issues with my thundercat which i fixed by trial and error and then again with the 7 which was pretty easy to work out after the cat. as far as i know, none of this is an issue with injected bikes but it seems most of us streetfighter lovers are poor

i dunno what the 9 is like but both the 7 and the cat felt almost the same (maybe a little light up front ) right up to 150ish where the lack of faring really starts making acceleration difficult. i can tell you now that 230 down at the island is an experience with a nuddie bike, feels like your gunna get lifted off even while hugging the tank

You should have removed the KLEEN while you had apart.
All you do is connect the 2 tubes coming out of the read covers together, remove the KLEEN stuff and block the hole in the air box that it goes into...

Mate, thats the first high performance air filter I have seen that actually looks like it lets less air into your airbox, as the opening is half the size of stock ??

This may have saved me buying a new fuel pump for my standard ZX-9R C1. I've recently had problems with my bike splutteing like it's low on fuel. I changed the fuel filter and drained the float bowls with no effect. Thought I'd have to have the fuel system cleaned or buy a new fuel pump. Then I came across this thread. I checked the float bowl pressurisation pipe and sure enough it had come off somehow just under the instrument panel. Took 10 seconds to reconnect it. Strange there's no clamp to keep it in place but it didn't come off when I took it for a test ride.

You should have removed the KLEEN while you had apart.
All you do is connect the 2 tubes coming out of the read covers together, remove the KLEEN stuff and block the hole in the air box that it goes into...

I've had a bit of a look into this - is there any performance benefit in your opinion?

Jeez Loz, you must be bloody rich mate, I just Streetfightered a ZX7 for my missus & all I did was drill 2 little holes in the bottom of the airbox directly above where the equalisation tubes are on the carbs & stick the tubes up through there (just forward of the ramtubes)
$2 for a T piece ? yeah, that's fine for you wealthy blokes but I couldn't even afford a drill let alone a drill bit - I hadta use Mum's good kitchen knife to make the holes (she'll kill me if she finds out)

Hoffy - You're dead right there, a lot of so called 'Performance' air filters are like that, unfortunately most punters wouldn't know the difference & just blindly shell out for a BMC, K&N, etc because the magazines & all their mates swear by them , I ordered a K&N for my ZXR & when I looked at it in the shop both myself & the spares guy couldn't believe the filter would've effectively halved the cross sectional area of the intake - how the fuck could that improve performance? It would actually strangle the engine's top end.
Needless to say I didn't buy it & went across the road to the Kwak dealer & bought a genuine job.
I would be interested to do back to back runs on a dyno to see how one of the 'high performance' air filters affected the A/F ratio as well as the HP/torque.

On my track bike, when it was owned by Dave, he had had a back to back dyno with a K&N and a factory one with the steel mesh removed, from memory it had 1 horse power more with the stock one in over the K&N. But Dave would be able to confirm this.

$2 for a T piece ? yeah, that's fine for you wealthy blokes