Mine's toast, it's crumbled to bits and fallen out the hosing in tiny particles. Despite my fascination for buy new parts at silly prices, I don't fancy the idea of paying a fiver plus VAT for a piece of foam that's about a third the size of a railway ticket! Can I use a piece of thin, fine foam to replace it, and does anyone have a link to a handy source?

Rick Oliver does sub air filters for 1.50 doesn't he?

Personally I don't use them.

Hadn't spotted that on his list. £2. Good call. Might be getting some bits from him in due course, so I'll get one then.

It does just vent engine pressure to atmosphere if i remember right(?).
So technically could suck in nastyness if it wasnt there.

I wasn't sure so I looked it up the other day, and now I can't find where I saw an explanation with pictures. If I recall correctly, it's about getting clean air at atmospheric pressure to the carb diaphragm that doesn't have fuel in it. In theory, you could end up with dust or worse causing the carb slides to get sticky or jammed. I'll try and find that page again because I've probably got that all wrong... lol

The sub filter stops shit from getting into the slide cylinders and stops them seizing,

So for what its worth its an important filter

There you go I'm naughty! I've removed countless sub filters from nc's and there is more chance of the sub filter breaking down and causing a slide to jam. Keep the carbs and the air box nice and clean and it will be fine.

Viper biker off here used the scarring pad of a kitchen sponge for mine. Said that's what he used on his. Haven't had an issue with it!

NC24 never used one.

I think the carb slides are just vented to atmosphere, I really cant remember.

Needless to say many other bikes have ran without them and been fine... I put the NC30 one down to over-engineering on Hondas part due to the 80s economy boom.

Since it is there for the sake of a couple of quid and how long they last id replace it myself but many people dont use one with what seems like little to no ill effect.

mine also dosnt have one, but iv just bought one to fit, for 2 quid i thought why not.