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MG MGB Technical - K&N Air Filter Installation Question

I recently installed a pair of K&N air filters on the HIFs in my 73 B. I happened to note in Linday Porter's Restoration Guide that the extra air holes on the carbs must be kept clear.

Do I need to drill two more holes in the rear filter pan to match up to these extra holes? The car seems to run ok with them being covered.

What are these holes for? Seems to contradict the notion of having air drawn in through a filter if these are wide open to the engine compartment.

Mike
Mike McKinnon

Mike. The air holes on the filters should match the air holes on the carbs. Take a closer look and you should see how they should be installed. Screwed this up about 30+ years ago. Boy, was I embarrased. There is a learning curve for everything. Ask questions and most problems will be solved. Les
Les Bengtson

The parts I ordered (Moss # 222-935) has the 1.75 in thick filter in a can that is about 6 inches in diam. The can only has two holes in it for mounting bolts vs four on the carb mounting surface. Unless I get the wrong part it would appear some mods are required.

On the carb, the two holes furtherest apart are threaded and are the ones the cooper cans attach to. I found that the holes in the K&N cans did not exactly match the carb holes and needed to be reamed to allow them to line up with the carb holes. There are two other holes slightly above and inwards of the threaded holes. These are the ones that I'm wondering about. In the cooper can mounting plate, there are holes that line up with these but there are no bolts or anything that attaches through them, they seem to be there just to allow unrestricted air to enter inside the filter chamber.

Mike
Mike McKinnon

Mike:
Your assumption about the upper air holes at the mounting flange of the carbs is correct. They are an air pressure equalization device allowing atmospheric air pressure to enter the space below the air piston but above the venturi. Without this equalization, that space could become more of a damper on air piston travel and slow the response of the air piston to throttle demand. Therefore, you want to keep it unrestricted so that the air piston's response is as SU intended.

It does sound like they sent you the wrong ones, but I might suggest that it doesn't matter in a way, in that you could do as I have done on a set for an MGA I had and cut a centered hole in the back plate K&N provided, the same size as the hole in the back plate of the Cooper's filter cans. This hole will accomodate the cast aluminum mounting plate (reuse the rubber gasket!) which also has a built in "stub stack" or "shorty air horn" for better, smoother air flow into your carbs - as SU and the factory intended. This is a good modification even for the K&N "pancake" air filter housings and also works well with the deeper and less restrictive K&N standard replacements as used in the OEM Coopers cans. If you want to read a good description of THAT conversion, go to Roger Hotelling's site, http://www.hotelling.com/mgb7.htm . Good luck!
Bob Muenchausen

Bob:

I've installed the "Pyramid" type K & N filters on my HS4s as I have the power brake booster. I've heard these are not the best flowing K & Ns. If I accomodate the cast cooper can bottom plates, will I get a better flow of air or will the loss of space degenerate it even further?

Luis
'69 MGB
Luis

This thread was discussed on 30/09/2002

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