I am currently fettling with my project NC30 in the shed and am trying to do as many HRC mods to the engine as I can. I have already done the HRC fuel tap mod as instructed by the HRC Manual and i am now looking at modifying the Carbs.


The HRC manual states that standard carb slides have a single Air Bleed hole in the bottom of them next to the Needle hole. The HRC Suggested Mod for standard slides is to drill this hole out to 2.5mm to increase responsiveness and help the engine pick up faster? This is what I want to do to my carbs. however...
On removing the slides, i noticed they have TWO holes in the bottom next to the needle hole, (See Picture). So I am a little confused. Are these some odd aftermarket slides, have they been modified already form standard? Can I still drill out the hole/holes and if so which ones?


Also, the HRC slides are curved at the bottom so that when the slide is fully open it does not obstruct the flow through the Carb. Is it possible to get a large drill bit and very carefully do the same thing to standard slides? Albeit just a small amount? Will it make any difference to the engine and will I need to change the Needle etc? Also, will it work happily with the 2.5mm Air Bleed holes modification?



Sorry if its a lot to ask. LOL I just want to press on and do something to this bike! LOL

erm, are they not UK spec slides you have there? are they metal and weighted ? look the same as the slides i have in my uk carbs. i wouldnt go drilling them just yet mate

They are from a UK bike yes... but that doesnt mean they still can't be drilled for better responsiveness does it? I hope not, otherwise I will be buying a new set of Carbs to fettle with instead. LOL

I have asked a company what is included in the HRC Carb kit that they can apparently supply but they havent got back to me.. Grrr.

get a spare set of carbs and play with those. leave the uk ones well alone, slides and needles are different. if the bikes a UK one, then keep all the standard bits intact otherwise it will be another non standard uk spec bike when you come to sell it on and worth no more than a jap one.
if its a bucket then drill away, but you will get better results from a set of RVF carbs or jap spec NC ones i think

Suppose I should mention, these technically are my spare set of Carbs as i have two NC30's. One Jap Spec (VFR400R3-R) and a UK Spec one. (VFR400R3-N)

My Jap spec one is my everyday run around whilst I do up my UK spec one. Would the UK spec Carbs work ok on the Jap spec bike then, so that I can have a fettle with the Jap Carbs instead? I didnt really want to do anything to my jap one other than keep it in good original, low mileage condition, but if it is the best option for my project bike I will interchange parts. (I've already swiped loads of parts of my UK bike for the jap one. LOL! Going to need a whole new bike at this rate! Ha Ha!)

And you mention about the RVF Carbs.... I wasn't sure if they would fit properly? I thought the inlets were slightly different? otherwise I will definetley look into RVF carbs as the HRC parts are more available for them than the VFR ones which are discontinued!
I found a HRC CDI Unit on ebay the other day, they wanted £400 for it buy it now? is that really how much im going to have to spend? And this was for an official programmable HRC CDI unit, not the SP Add-On to the Standard ECU. (The SP unit simply advances the igniton if i am thinking correctly whereas the CDI unit can be mapped to change the Ignition adn timing for various Fuel/Carborettor setups?)

ref the CDI

The HRC unit that Tanuki is selling (on this forum) increases advance and rev limit. It is not programmable and with the ignitech programmable CDIs at £150 I think you would have ot be minted to spend more than twice that on the HRC one.

The SP unit removees the 112mph limit..that is it.

Carbs wise - stick the UK carbs on the Jap bike and fiddle with some other carbs.

RVF carbs fit prefectly to NC30 - I have some on mine.. They also are very good too - makes a difference that you can feel but the big power chasers use the NC30 carbs for the outright flow potential they have.

thanks for the replies, ill swap the carbs on my bikes then at a later date and leave the fiddling until then.


but Im still unclear as to which is better? The RVF Carbs or the VFR ones?
Your saying the VFR ones flow better so presumably they are the best? but Im not going completely OTT on the tuning side of things, and the RVF HRC setup uses an air filter so would that be the best option here?

And also, what about the Concave cut-out in the bottom of the slides. will that have much of an affect?

If you are looking for the concave then you need some NC24 slides..

For a road bike then RVF carbs, if you have them, are great.

I found that the bike was more responsive on the throttle with the RVF carbs.

The which one is better question is a how long is a piece of string kind of affair..

If you have Jap NC30 carbs then use them - you can use a filter anyways and if you do think to use an HRC setup on the road it will be crap probably as not suitable for the day to day riding.

I know the urge is to get power but you want to get a rideable bike firstly.

+1 been tried and tested to death over the years with some saying they prefer vfr carbs and vice versa. i too have RVF carbs on my NC30 with a 3/4 yamamoto pipe and it works.
chris at dynopro ran mine in several permutations with pipes and carbs and will concur that the RVF carbs work better overall and are just as strong as vfr ones up top but flow better in the midrange - where you need it certainly as a road bike.

The Hamster wrote: but Im still unclear as to which is better? The RVF Carbs or the VFR ones?
Your saying the VFR ones flow better so presumably they are the best? but Im not going completely OTT on the tuning side of things,

I think you'll find rvf carbs have better rideability, vfr carbs will give more outright power especially on highly tuned engines.

The Hamster wrote: and the RVF HRC setup uses an air filter

No it doesn't, it uses an airbox. Neither of the hrc setups use filters.

The Hamster wrote: And also, what about the Concave cut-out in the bottom of the slides. will that have much of an affect?

NC24 slides have the concave bottom, they fit straight into nc30 carbs (I use them). It won't make a difference to peak power, the nc30 straigfht bottom slides don't obstruct any air flow at full open. It does affect low-midrange fuelling.