Why is it there are a fair few aftermarket exhausts for the NC21 and NC30, but the NC24 has not that many availible?

You talking cans or whole units? Cans are cans - just get a 52mm/2 inch slip on

NC30 was a bike that had a larger production run and was imported over here (and outside of Japan generally) and was also involved in the UK Supersport 400 championship at the time..so lots of bits out there.

The NC24 is 23 years old..is rare and if you are talking full systems, then they are not easy to make and so only our man RCV4 is making them (although it seems no-one is taking him up on the offer!! Madness IMO)

Few reasons:

The 24 was only sold from 87 - 88, while it sold reasonablly well it was more of a prototype / stage setter for the NC30 which was sold from 89 - 94

The NC30 also had a UK release which opened up the market more for UK makers.

The NC30 sold really well and became a phenomenal cult success, if you've ever ridden one you'll know why.

The build quality is second to none, the frame suspension brakes and wheels were all much better than the NC24, you can still get fairly decent rubber for the NC30, and fit an RVF wheel and you can still get decent rubber.

There just generally a better bike when it comes to 400 TROUSER FIREWORK, you can ride the nuts off them :)

Not to say the NC24 is a bad bike, just not as prized, which is why they dont' fetch much money. They have more of a typical 80's Honda feel to them, soft suspension, twitchy.

The motor is essentially the same albeit crank / firing order and they make the same power from stock.

I like this 'character' they have.

Final point would be any major upgrade (ie full system) would require you to remove the engine to change it!

Never see big brand names like Devil,Yoshimura,Akrapovic,Micron on nc's