NC30 carby jetting - DIY guide

[Cammo]

It might be an idea to just get it dynoed to see how the air/fuel ratios are and then sort it yourself.

The big cost of dyno tuning is the labour time when changing jets, you could save yourself the pennies and do this yourself.

[juggernaut]

Standard jet sizes are: 1989 Jap model (4 x 110), all other year Jap models (2 x 115f, 2x 118r), all UK bikes (2 x 120f, 2 x 122r). 1989 Jap bikes run very lean from the factory, best to rejet as a precaution. Other year Jap bikes will need rejetting if you run any non-standard parts (pipe etc).

Main jet sizes: If you run a nice sounding aftermarket exhaust and/or pipe, then the common consensus is to run 118 size front and 120 size rear (often referred to as 118/120). If you run a very high flowing/loud exhaust or full exhaust system then you may wish to consider running 120f/122r size main jets.

Note: If you have a modified induction system (aftermarket filter, removed rubber flaps, modified snorkel etc) then this jet size guide is NOT applicable to you. NC30 jet size hugely depends on your induction/filter setup. I strongly suggest only running the standard induction setup on a road bike, there are little gains and more rideability losses to be had by running a different setup! I return all road NC30’s that I work on to standard induction setups, most often with better results. Modify at own risk…


You can buy main jets from any Keihin dealer/supplier, most dyno tuners (google one near you) or NC30 parts stockists. The part numbers you need to order are 99101-393-###0 (e.g. for a 118 size jet: 99101-393-1180). Simple. These are exactly the same jets that Honda uses in their carbied road bikes.

Maybe a daft question but ill ask it anyway, if UK NC30's run 120/122 due to altitude and ambient temp of the country, why don't we rejet all nc30's to this and work from there instead of going up one size from what we have in the carbs as standard?
I've got a 1990 import which will be running much leaner than this, surely this would be the logical place to start than its standard sizes whether they are 110 or 115/118..... Unless there is something different about the carbs for different markets?

Apologies if this has been covered but couldn't find anything myself
Cheers
Luke

[Drunkn Munky]

That would be the logical answer wouldnt it although i know the UK carbs are slightly different with regards to the slides and the UK bikes are also rumoured to have a less restrictive exhaust. Saying that i just rejetted my stock jap spec bike to 115/117.5 and it runs lovely, it already had the slides drilled to 2.5mm so couldnt do nothing about that but it had no flat spots at all right through the rev range.

[juggernaut]

Thanks for the quick response, I see now, going to try and get stuck in to mine today !


Cheers
Luke

[Jac-al]

Hi guys, MeRRY Christmas to all.
I need some info. After rejetting the bike, i try starting, it wants to take but just wont start, the mechanic said the carbs need to be bled as theres an airlock. Can anyone explain to me how its done? as they wantalot of cash to do it and im broke after christmas. Thanks

[vfrman]

Calling bullshit on that one! The bowls are gravity fed and will fill on their own as long as fuel is flowing to them. Things to check are the vacuum line going to the tap and ensuring the fuel line isn't kinked.

[Jac-al]

U reckon they tryna rip me off? The thing is ive checked everything, the vacuum seem a little weak, what could be the reason for that? And when i took the carbs off, 2 floatbowls were full and 2 empty, why is this?

[speedy231278]

Check the float valves are working properly, and make sure the float heights are set right.

[CMSMJ1]

Make sure the vacuum hos eis also solid and not knackered/holed/weak etc

air lock= blag

[Jac-al]

hi, took carbs off again and checked the float valves. They were blocked with allsorts of crap. I thought that would do the trick but it didnt, its doin the same. It started a couple times for a few seconds then died like theres no fuel. Could my jets be too small, very confused cause it started fine a few days ago