I'm having a little trouble jetting my NC for a ram air/open airbox.

Currently running NC30 carbs:
150 mains
35 pilots
RVF HRC needles (was way too rich with the RO NC30 needles)
Slides drilled
5 hole emulsion tubes

Spec of the bike is:
NC35 motor
NC30 pickups
Ignitech cdi running a HRC map
Gas flowed & skimmed heads
Two Brothers exhaust system
Fuel pump off a CBR600

Motor was tuned & built by Slick Bass in the IoM, and was refreshed recently.

This is one of my dyno charts from this week:


The red line is with a 1mm air corrector jet, and the blue is with a 2mm air corrector.

From reading what other people jet their bikes at with open/ram airbox, using the air corrector jets enables IRO 135 mains, but I'm on 150 and still looking lean even with a tiny air corrector?

That lean spike at 10-12k, what could be causing that? Slides opening too soon & the main jet is unable to keep up?


I'd appreciate any input, its had some weird jetting since I've bought it but was producing a healthy 66hp before I started fiddling with the airbox, but even then the fuelling was far from perfect!

Still struggling! Running 155 mains & 1mmR/2mmF air correctors. But it still looks a little lean. That spike is there still too.

Making 65hp now.

Check this out... http://www.factorypro.com/tech/tech_tun ... gines.html

From looking at your dyno, you are go from rich to lean at 10k, which is weird because you are on the main jet after 6k. It looks like the first thing to do is flatten that lean condition at 11k so the entire time on the main is rich then lean it all out.

I don't even know what to say about what is going on under 5k.

up yours slow jets first, too lean down low... it will have added benefit of proviging a bit more fuel

the stock f3 kit nc30 (78hp) ran open tray carbs with 150 ish? jets and that sometimes had 38s if i remember rightly.. i think the nc24 kit even used 40 slows depending on setup..

looks like yours could do with it though based on how lean it is. (unless of course your pilot screws are all closed off?

is your dyno testing the ram air too? it could be some weird air turblance going on at that kind of speed... even the stock airbox setup is easily messed up with air intake troubles

I tried a different set of carbs yesterday with some unknown adjustable needles (that were already fitted) & 150 mains. The small air correctors were completly blocked off too.

It went massively rich at the midrange, so bad it barely pulled through 7k! Up top was quite rich too, so I opted for the next size down that i had (136 mains) and it went too lean. Stock needles went in, and the midrange was almost perfect. That spike at 10k was still there.

I'm going to try some different needles this afternoon & try and get a set of 140 mains to put in. I'm tempted to run with the airbox top off to see what revs the slides are fully open at. Might require some filming to study though!

Not too fussed about right down the bottom at the moment, it's a race bike so barely sees much below 8k!

Updates to follow this evening, but interested that a different set of carbs has changed things..

Made some progress!

Looks like it was the needles causing the problem, silly me. Ran with stock needles + .5mm washer, and 140 Dynojet mains (so around 136) and it was a lot better. Not perfect, but will wait until I get a better jet selection before continuing!

The red line is with the stock airbox back on


Still a bit down on power, but guessing the heat of the day has something to do with that!

I'd say leave that stock airbox on. It actually looks pretty close to me, just need to solve the low rpm lean condition and the rich dip as it crosses from pilot to main and you are set.