NC21 Carb Specs and Similarities with NC30

[superlite]

Thought I’d add a post to get some discussion going.

I’ve had a look at two sets of NC21 carbs. I believe that they belonged to VFR400Z and VFR400K. The carbs are VD32s – the same as NC30.

They are similar to each other but slightly different to NC30 carbs.

The NC21 carbs both had the rounded HRC style slides (not all NC21s do though). The needles were 8ZC all round in one set and 8ZE all round in the other. Jetting was 98 all round in one set and 112 all round in the other. Pilot jets were 35 all round in both (same as NC30). Emulsion tubes were 4 hole all round in both (same as NC30 front emulsion tubes). So that’s the differences in fuelling.

As for the carb bodies themselves, the main differences were a much more primitive choke linkage system. I think Honda worked out how important the choke is for these little bikes and improved it on the NC30. The throttle cable bracket is slightly different, probably due to the routing of the cables on the naked NC21s (although it would still work on the NC30). An interesting point to note is that the main air jet is 3 mm in diameter compared to 4.5 mm on the NC30.

So what does all this mean? Well, in theory these carbs are easier to HRC in that they already have the restricted MAJ (if you want to cut the velocity stacks down) and they also come with the rounded slides (but not all NC21 models). 8ZC and 8ZE needles are very close in base diameter and profile to 8ZF and 8ZG HRC needles – so close that in fact they would serve the same purpose. So it would only take some jetting to get these carbs performing on the cheap – if you can live with the crappy choke mechanism.

I’ve also measured all needles available to suit VD32 carbs including early and late HRC needles. Watch this space for the specs.

[Cammo]

Interesting.

The smaller main jets will be partly due to the smaller air jets.

[royster81]

and you know if you want to sell a set of those rounded bottom slides i'll be first in line to take a set off your hands

[Cammo]

I have another couple of sets of carbs arriving in a month or so Rhory, If both have rounded slides you're welcome to a set.

[royster81]

I have another couple of sets of carbs arriving in a month or so Rhory, If both have rounded slides you're welcome to a set.

fingercrossed then buddy cam ?

[Cammo]

Yes mate.

1 set coming certainly has them but those ones are spoken for. I bought the other set because they were a steal (yahoo japan), if they have round slides then all yours.

[CRM]

Chaps i am going to actually get involved in a technical thread here as i find it interesting.
In all the years and carbs i have here i have found allsorts in different carbs. i will get some pics and list the needles here.

However some also had what lookedl ike 24 slides which were dished. I have to say in experimentation i found them to perform very very well and i cant find any negatives so far using them. Is that the general feel about them ?

[Cammo]

Chaps i am going to actually get involved in a technical thread here as i find it interesting.

Welcome Paul!

I've been known to dabble with nc21 & 24 carbs myself.




I really don't know what the benefits of the rounded carb slides are, but I run them on my racebike (hrc carb setup) and it performs well. From what i can tell it leans out the mixture a small amount up to about 1/4 throttle, which certainly helps with the mega-rich hrc carb setup (big pilot jets can make it a bit rich down low).

The rounder shape itself might also help with introducing the mixture to the round port shape, but just guessing here.

I've heard off other peeps too that using the round slides with fairly standard carb setup has improved their bikes.

[superlite]

However some also had what lookedl ike 24 slides which were dished. I have to say in experimentation i found them to perform very very well and i cant find any negatives so far using them. Is that the general feel about them ?

I'd agree with that, Paul. I've had them fitted for a fair while (since this thread was started!) and don't have any negative comments (full HRC spec'd carbs). That said though, I haven't noticed any noticeable difference in throttle response. But to be fair, they were probably fitted at the same time as other mods. They certainly don't do any harm.

I've got a load of needle specs lying around. If we pool resources, I'm sure we can unravel some truths here

[CRM]

I think your right. ok i reckon this is worthy of a sticky in the tech section that can grow with pics
I will start tonight with pictures

other thing, it might be worth putting a guide together for cleaning carbs, what needs cleaning and best methods, air passageways etc etc
also a jetting sheet, which jets / needles / settings work well with certain airbox's, pipes and filters etc etc.
it always gets asked and would make a good rule of thumb for people to start with ?