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MG TD TF 1500 - Running rich caused by air filter

Since the restoration my TD always runned rich whatever I tried and re-adjusted. Always soothy plugs inspite of carefully going through Dave Brauns great SU instructions and endless attempts with colortune plugs. Also lean needles did not do it for me.
Recently I joined our MG Carclub technical day at the Speed Centre in Geldermalsen and my TD was on the Dynotest facility. The outcome: only 43 HP and above 2000 RPM far too rich.
The owner, Jaap van Wuijckhuijse, challenged us to think about the engines processes and it was one of his questions that put me on the right track: do you realize the tremendous amount of air that an engine needs. And that all has to pass the airfilter.
When I started my restoration, the original air filter was missing. So, I bought a remake that to me looked like a perfect copy.
I took measuments of all air passage gaps in there and found the devil: the gap just under the wingnut was only 3/4 of the surface from the pipe that goes down towards the carbs. So, this acted as an extra throttle! All other passages were several times that surface. The pictures show that particular gap.
I did some panel beating ( in fact widened the hole the stud goes through and replaced the surface the wingnut is resting on). Now that passage is about 1.2 times the surface of the pipe.

That makes it a newborn car: I feel much more power and for the first time I can see beautiful light brown plugs. And on an outing with other TD's, we have the same fuelconsumption whereas befor I always needed 10- 15 % more.

If you measure from the top of the airfilter, the surface the wingnuts rests upon, is 21 mm. After the job, that surface is 15 mm from the top. So, the wingnut is higher up now.

Measured from the underside as shown in a picture, the gaps height improved from 8 till 14 mm.

Sometimes learning takes a long way!

Huib Bruijstens

Pointing at the too narrow gap

Huib Bruijstens

After widening the gap. The larger stud hole was necessary to allow the panel beating. That larger hole is closed by an extra large washer under the wingnut.

Huib Bruijstens

Huib - A very interesting situation you discovered - I am going to check that gap in the air filter on our TD. Thank you for posting the information.

It has been a long time since we have seen you on this BBS. I have often wondered where you had gone - glad to see you back. Cheers - Dave
DW DuBois

I've had the same issue before on the TD. Over tightening the wing nut over several years caused the air filter cover to crush down partially cutting off the air supply. This is yet another cause of a rich mixture, but not the one fixed by messing with the carbs.
(BTW, carburetor is an old French word, translated it means "don't mess with it".

My cure involved turning the cover upside down over a 2 pound empty coffee can and gently tapping the dome back into place. Problem solved.

Lew - Thank you for the today's language lesson - a good one : ) Cheers - Dave
DW DuBois

Ran into that years ago with a local TD. The car ran fine when really cold, but as soon as it warmed up it was rich with no power. I recall the support legs can bend/collapse also, causing same symptoms. George
George Butz

This information is very, very interesting as I have always had a chronic rich condition. The underside of my filter shows 12 trapezoid windows through which the metal gauze is visible. Yours shows round holes. Were there two different styles of filter? Mine says Patented AC Made in England on the top. I measured my gap (using an L shaped piece of card stock) and mine is 10MM. Before I go beating on the wing nut depression from the underside, what is the recommended gap? On mine, the distance from the surface next to the wing nut hole and the lip that contains the cork O ring is 21MM. I'm thinking a few blows with a hammer on a 1.25 inch socket with an extension might do the job.
John Quilter

I have read your comments, but the original filters are an oil bath design. You didn't mention anything about oil level. Without oil, they are just an air "strainer" and would run lean.

If you want to truly know what your air/fuel ratio is, you might insert an O2 sensor in your exhaust pipe and observe it with a gauge.

If you think the filter is choking your engine, remove it an then run it around. If the plugs turn white, then you may have a case to blame your air filter. I doubt that's your problem. Read your plugs (or an O2 sensor). There are many other factors that are more likely the problem.

Some pics of a an original AC oil bath air filter. The support legs/tabs mentioned above by George are shown. Huib's filter appears to differ somewhat from the original. Cheers
Peter TD 5801

P Hehir

Can't load the second one as the file is apparently too large. This shows the underside & the legs. Cheers
Peter TD 5801
P Hehir

Huib: Is your filter like Peter's picture on top or is your top plain without the raised lettering?

I have never seen the underside on an early filter (smooth surface that used a decal) before so I don't know what it looks like.
Christopher Couper

Just for the record, here is the underside of what I believe to be the original AC oil bath air filter. This one has the raised lettering on the top.

John Quilter

Thank you for the welcome back Dave and indeed it has been a longtime.
To answer questions:
John: I measured all air passages in there and as a reference I took the amount of square mm that the downward pipe into the elbow has. Every passage should be at least same or larger.

Jim: it is an oil bath filter and there is a layer of oil in it (about 8 mm). The three "legs" of the filter keep it at an appropriate distance above. So, i my opinion: enough space for the air to flow through and enough near by oil available for dust particals to be catched.
The Dynotest involved an air/fual measurement at all RPM's as well as power measurement. That's how I discovered the issue. Using my colortune I hardly went up above the 2000 RPM.

The results after widening up that particular gap are soo clear: clean plugs, much more power ( I was missing 20%), fuel consumpsion now same as collegues) that I believe it is ok now. But one day I will goback to that Dynotester and have it all measured and finetuned.

Peter Chris: my filter is a remake. No lettering on top.
Greatings to all, Huib
Huib Bruijstens

The underside of the original oil bath filter. This is not the one you refer to Chris as it has the AC & the instructions embossed into the top. See pic above. Cheers
Peter TD 5801

P Hehir

Huib: Yours did look like a remake but did not want to call that out.

The very early ones did not have the embossed lettering and used this decal instead.

Christopher Couper

Still on the filter topic, but this time for my TF 1250. When I still had twin carbs on it I replaced the original elements with K&N. The ones I got were a little small, but pushed up from the bottom they looked OK. I also took out the perforated rings that surround the elements and fitted steel tube spacers on the bolts that hold the cleaners together. I did this just after a dynotune, which I tried again with the new filters - 4 extra HP at the wheels, which isn't at all bad.

Can't recall the type of filter I bought, but it was the closest I could get to a matching size.


Chris, there is or was no decal around my airfilter. I bought it from one of the four well known MG part suppliers in UK and it was announced as a remake and ready on the shelf. So, most of us TD owners will have their original air filter with big enough air gaps inside.
But since this remake was not a specially made for me one, I suppose there must be quite a lot of TD owners suffering from naughty soothy plugs , insufficient power and too high fuel consumption.
Huib Bruijstens

This thread was discussed between 20/09/2016 and 26/09/2016

MG TD TF 1500 index

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