SP250 Air filters

[Vortex O'Plinth]

The original AC air filters fitted to the SP were a pretty rudimentary design - the oiled metal mesh element would probably keep some of the rocks out, but not much else. However as part of my rebuild I decided to replace my originals with pattern new ones, since after years of damp storage the mesh had started to decompose due to corrosion and having small pieces of steel wire finding their way into the carb chokes would not be good!

The reproduction filters available were a disappointment. The mesh is crudely fitted in the canister and the original UNF captive nuts securing the filter to the carb have annoyingly been replaced with metric rivnuts - which prevent the filter sealing properly on the carb flange.

I wondered therefore if I could somehow refurb my originals with improved filtration while retaining near-standard appearance.

So - I first removed the innards. There was a surprising length of mesh - nearly 2 metres secured with a metal backing strap....



....that I decided to replace with modern open-cell plastic foam. I bought a 600mm x 400mm twin layer foam pad from Ramair Filters....



which I cut into strips 400mm long x 32mm wide...



...then formed into a ring and glued the butted ends together...



The foam ring inserted easily through the choke hole in the filter body and was expanded to fit the circumference (with the butt hidden behind one of the pillars).

To prevent this new foam filter being inadvertently sucked into the carb, some rigid but highly porous backing was needed. After some experimentation Plastic gutter guard mesh seemed to provide the necessary combination of rigidity to support the foam and flexibility to allow it to be inserted through the choke hole in the can....



This was cut into 32mm wide strips of a length that provided a firm backing to the foam with a small overlap then formed into a ring and the ends tied together (with nylon fishing line) ...



This was inserted through the can choke hole and expanded to back the foam throughout the circumference, and Voila - the finished product!



The 'new' filters were fitted to the car.....



...and a test drive over a range of engine speed showed no decrease in performance - in fact if anything the car seemed more responsive, so the original carb needles are probably still fine.

My initial purchases leave me enough material for several sets of replacements when needed (the maintenance stickers are purely to lend the filters verisimilitude - washing these in petrol and oiling is probably not a good idea!)

[Josef Eckert]

Thanks, looks like well thought through. Will do the same.

[Automospe]

Excellent solution! I've been considering something like this for mine. I'll try this. I also used your relay setup for my overdrive and love the way it works. So simple to use.
Thanks,
Tom

[Vortex O'Plinth]

I also used your relay setup for my overdrive and love the way it works. So simple to use.

I'm so glad someone else has tried the two-relay overdrive switching Tom - and pleased that you too find this mode of operation simple. The general consensus (among those who haven't tried it) is that it's serious overkill for what is a straightforward job. To me it seems eminently more logical and straightforward than the basic on/off switch.

I played around for a while to find the most efficient type of control and the best position for it. My present layout is a momentary action long paddle toggle switch mounted to the right of the steering wheel above the Fog and Long Range lamp switches...



Since the switch only requires a downward press to select or deselect overdrive, this is easy to operate with the fingertip without the hand leaving the wheel.

There's an indicator lamp sited directly in front of the driver, visible through the wheel between the two main gauges, wired across the overdrive solenoid to show when overdrive is selected. Certainly for me this layout now works very well.

[JSabah]

This was my solution after fiddling around with the original barbed straps (keepers). I chose to use a length of stainless cable as when shoved through the chike hole, it springs open and compresses the original mesh. As you can see in the pictures, I added some barbs of tie wire so it wouldn't slip.

http://archive.dloc.co.uk/forum/topic.a ... Air,filter

[Automospe]

I mounted a small push button momentary switch just above my shifter, very simple to access. I enjoy using the OD as you hardly even think about it as it downshifts automatically when changing shifter position. I appreciate your work on this it makes for a fun driving experience. I don't know why everyone wouldn't enjoy it's benefits.
Tom

[klindsell]

This post was just what I needed, as just getting to the filters having rebuilt my carbs this week. So thanks for posting. I guess there is no reason why I couldn't put a ring of the original mesh in first and the foam filter behind it to give a look of originality?

[andrewatcourtfarm]

Nick.

I have followed the instructions in your post of Sun Apr 30, 2017 regarding air filters.

A couple of comments:

The strips need to be wider than 32mm to accommodate the extra width at the inner circumference due to the domed shape of the filter housing.

I had difficulty making the mesh apply enough pressure to the filter material evenly all the way around.

I am going to have another go next week using wider strips and I am going to try and stick the filter material to the in side of the filter housing using Hylomar non-setting gasket sealant.

Andrew

[andrewatcourtfarm]

Hello Nick.
After following you excellent instructions to the letter earlier this year, I found that the engine started to run a bit rich about three or four weeks ago.
Yesterday I had time to have a look and I found that the filter material was partially clogged with oil and that there were signs of dust and engine oil in the carburettor plungers.
I replaced the filters with a wider strip of the same material (38mm instead of 32mm) and I have directed the breather pipes down to below chassis level.
The filters fit much more snugly and I did not need to stick the filters into position.
It will be interesting to see what the result is in 3 months time.
Kind regards - Andrew

[Vortex O'Plinth]

Thanks for the feedback Andrew. Obviously the engine breathing through the filters will eventually contaminate the filter foam to the extent that it needs replacing. Mine so far show no signs of oiling up, but they have been fitted to a newly overhauled Russ Carpenter engine. It does sound as if the foam width needs to be wider on your filters than on mine ( I will check the width again in case I have misquoted it)

I presume your re-routing of the engine breather pipes means they now discharge to atmosphere rather than back into the engine through the filters?

Nick