universal air filter conversion for hs2/hs4 carb

[ratrodmog]

Hopefully someone might find this useful, but possibly not one for the purists I'm afraid. This is a little mod I've just completed to adapt a standard 'universal' air filter to fit the stock Minor air filter base plate. This could be used on the standard Hs2 carb or the larger Hs4 as I've got here.

Completed set-up
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1st, open out the throat and re-profile with a nice smooth curve. A Dremel was used for this and just some fine wet & dry paper. I've also opened out the mounting holes slightly as the offset was wrong. (This part is only needed if using the larger Hs4 carb).[frame][/frame]

You then have this...
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The rubber neck on the filter needed trimming to fit around the bolt heads (you could use countersunk bolts here).
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I've used the original butterfly to clamp the filter in place. This needed shortening and re-tapping as the thread wasn't long enough, but easy enough to do and it clamps with enough pressure to allow the rubber filter neck to seal against the mounting plate.
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That's it, ready to mount. I'll probably replace this filter as it's just one I had in the garage but by doing this conversion it opens the door to using a wider range of filters.

[panky]

That's great, damn site cheaper than a K&N

[amgrave]

That looks good, it's something I will have a go at in the future as I am going to fit a hs4. What's the noise level like though.

[ratrodmog]

Yes Panky, cheaper and more choice i suppose. Not sure how well this one will flow but it's got to be better than the terrible stock air filter.
Amgrave, I'm not sure about noise as I've only just finished it, however I'll be putting it to the test within the week. I'm guessing it'll have a little induction noise but will probably be over-shadowed by my side exhaust!

[panky]

Don't know about this type but I fitted a K&N to the HS4 on my traveller and it was a bit too noisy for me. I ended up using a Mini filer box with 1" holes bored around the side with a K&N element inside, As quiet as the original but much better flowing
Be interesting to see how this one performs though, I like the fact it's cost nothing to make

[bmcecosse]

Looks fine - you must not obscure the little breather holes into the carb - I see you have exposed one -is that them all? I can't remember and don't have a carb handy.

[ratrodmog]

I can't actually remember BMC, I'll have to check. Obviously only one on an hs2 as there's only one hole in the filter plate but not sure on an hs4.
Had it running over the weekend but not taken it for a drive as the exhaust isn't fully secured yet. Seems to run OK, however I have replaced the choke cable with a crappy universal long one that needed cutting and it just doesn't want to stay out! I may replace it with a locking one as I'm currently using a clothes peg to hold it open while it warms up. Overall I'm happy though but the proof will be in the driving! It has a slightly irregular idle, rises and falls slightly on idle so will need to look at that.

[bmcecosse]

When Dremeling the inlet hole to enlarge for the bigger carb - it's also worth nicely rounding the lower leading edge of the piston - the side presented to the air flow with the slot down as in your picture. Just nicely round off the leading edge to the air entry - again helps slightly with the 'vena contracta' effect as the air rushes into the gap with the bridge. I don't advise touching the 'bridge' although I do believe some 'round that off' too. You can of course go on to thin down/knife edge the throttle plate and remove the splayed ends of the securing screws - but again - i think the gains will be minimal. I did do it in the past - but no obvious benefit.