hi im looking to maybe get anyother vfr 400 nc30 and making it purely a track bike.
i feel confident working on the nc30 as my project in the summer took me a while and i dug into the engine a bit and done a bit of work. also got a new project zxr400 l3 where i have also done a major service clutch, cam chain, valves, shims and generator etc.

but the one thing that is stumping me is the carbs!
i know carbs are a mysterious and magical things since like i said i set up the nc30 and zxr 400 carbs which both have full race systems and k&n airfilters and i have owned a few 2 stroke motorcrossers.

BUT...
what differences are there between the carbs from the nc21,24,30 and 35?
i know the specs of the 30 but since i havent worked on any of the others i dont know too much about the specs or setup? in the aim to go hrc with track bike so carbs tray, carb setup so emulisfying jets, main jet, pilot, slides and springs? (from the carb point of view)

thanks webby

I'm only getting back to playing around with my '24 again after about 2 years hiatus and I've no doubt some of the racers on here will be able to elaborate further on the following points but the following is how things stand as I understand them.

Carbs in the '21, '24 & '30, contrary to popular belief, are the same size - 32mm. Although they all differ in jetting, slides and needles for the most part.

Those in the know will tell you that the bodies of the '30 carbs are the best (I've no idea of the why's of this - apparently it's something to do with the internal ducting...?...) but the '24 carbs come, as stock, with the supposed HRC slides - the ones with the concave base rather than flat. These slides are interchangable and if I'm correctly informed do not require any modification.

The '35 on the other hand, has 30mm carbs as standard - this was as a result of laws in Japan at the time (can't recall if it was speed or power that was the issue) leading to Honda wanting to improve the mid range torque at the expense of outright power. The narrower carbs deliver air at higher velocity and along with adaptations to the airbox lead to a tuned air charge in the mid-upper rev range.

The wider, 32mm carbs however, pass a higher volume of air in the upper range, so as usual, it's a balancing act between outright power, and power delivery.

'30 carbs have been installed on '35's and vice versa and those who go either way will have their own justifications for the decision to go whichever way depending on what they were looking for from the switch. I don't have any figures for the rewards/trade offs associated with such switches but I'm sure there are some on here who will.

I thought that allegedly the 35 carbs flowed better as despite being a little smaller, the inlets and velocity stacks were of a designed that allowed for a smoother air flow?

speedy231278 wrote:I thought that allegedly the 35 carbs flowed better as despite being a little smaller, the inlets and velocity stacks were of a designed that allowed for a smoother air flow?

There is a pic gloating about on tinterweb of the venturi flow paths, show the 30 at a cranked angle and the rvf perfectly straight I'll see if I can find it unless someone else has it.

Ok.

The NC21 uses 30mm carbs also.

The outer-bore is the same size on all the carbs though so you can fit whatever.

Lets start with the NC21/24/30 carbs.

They are all very similar. The main differences are in the air-cicuitary.

You can think of the carbs being revised, each model is generally smoother than the last.

The RVF carbs had a smaller bore to increase the speed of the air, combined with straight velocity stacks to provide better air-flow.

The general concensus amongst most riders seems to be for street use the RVF carbs are slightly smoother but for all out racing the 30 carbs are better.

I've not really noticed much difference between any of them

porndoguk wrote:

speedy231278 wrote:I thought that allegedly the 35 carbs flowed better as despite being a little smaller, the inlets and velocity stacks were of a designed that allowed for a smoother air flow?

There is a pic gloating about on tinterweb of the venturi flow paths, show the 30 at a cranked angle and the rvf perfectly straight I'll see if I can find it unless someone else has it.

That's partly what I was referring to. Whether or not the straight path on the 35 somehow compensates for the smaller diameter is of course open to question!

In my opinion the nc30 is 'over-carbed', that is the carb (venturi) size is nearly too big for the demands of the engine over the entire rev range. They have trouble fuelling at certain points, particularly the transition from needle to main jet. With wide open throttle (wot) and on the main jet (higher revs) this is not a problem and they perform well.

The smaller bore size of the rvf carbs (and other changes) ease this problem and perform quite well over the entire rev range.

For outright peak power I believe nc30 carbs are best, but rvf carbs will better for the rest of us not looking for that. RLR's IoM winning rvf used nc30 carbs bored out to 34mm(!).

I use rvf carbs on my road nc30 and prefer them for thi application. Both types of carbs can be made to work equally well on track for the majority of riders, nc30 carbs are less forgiving when it comes to getting them set up right for all types of induction setups, rvf carbs are far easier to setup. For expert racers (not me!) I reckon nc30 carbs are the go.

What difference does using the nc24 springs make?