I live in the rural east of England and we can get a lot of dust in the air during harvest time. I have wanted to fit an air filter to the side draft Zenith carb on my 1935 car for some time and having traced a modern upgrade which copied the design used on export cars in the 1930's this was fitted in February. The carb specification made no reference to changing jet sizes or any other adjustments to the Zenith carb itself. However, since fitting the air filter the car runs very rich and it is impossible to adjust the carb to run lean. If I remove the gauze filter and leave the car air intake open without the silencer baffle in place the car runs better but the engine "hunts" and the tick over is uneven. What am I doing wrong - any ideas? Should I just junk the air filter assembly which was a quite expensive modification and leave my car at home during August?

Location: North East Essex

What problems have you had WITHOUT the air filter, David?

Location: Herefordshire, with an E not a T

David,
I have an air filter fitted to my 26va n my 38 Opal supplied by Malcolm Watts , I also have one that I made myself fitted to my Rosengart (copied from malcolms and my pal has one fitted to his Austin 10. we have had no such problems with fitting it and I believe that Austin fitted them for a reason, keeping tish out of the engine and that works. As for the question why fit one , why not. How many modern cars are not fitted with one, there must be a reason

Location: Oakley, Hants

Hi David. The Austin Seven engine is pretty puny, and I can't imagine that it draws more than about 30 cubic feet per minute of air at full power and much less at tickover. I would have thought it would take a significant blockage by any air filter to cause enough pressure drop to have a noticable effect on running.

The only mixture adjustment on the Zenith is for tickover, to change anything at higher power levels you would need to fit alternative jets. As far as I'm aware Austin didn't spec different jet sizes for export cars with a pancake filter.

If your car hunts at idle without the filter it is usually a sign of running too rich, but there are several potential causes for this.

I wonder if in fitting the filter you have inadvertently upset something else ?

Presumably even in the 1930's it was found that filtering the air cut down the abrasive effect on rings and bores from fine airborne grit, even on tarmac roads.

Location: New Forest

Austins fitted filters to the export cars. they were the oil dipped type--easily modified to take paper filters.

David,
what filter do you have ?. What type of filter, I suggest that you start the car and warm it up without the filter, adjust the tick over etc so that it runs properlyen when happy fit the filter. If it then makes any significant difference to the running I would suggest that the filter is either dirty or too dense I use a FILT617 from feked.com.
good luck

Location: Oakley, Hants

The export filter was quite coarse and more for keeping out small stones than dust.
A filter would have negligible effect at idle.
!950s manuals for other makes with externally vented carbs do show different jets for cars fitted with elaborate export oil bath cleaners.
It may not look pretty but some sort of shield between the filter and fan keeps the filter much more clean.

Bob Culver

Location: Auckland, NZ

If, and that's quite a big IF, it is the filter causing the engine to run rich, then it will also be sapping the available power quite considerably by restricting the air flow into the engine.

My experience with more modern cars when I was "in the trade" is that an air filter has to be very seriously dirty before it has a significant effect on mixture. Its measurable much sooner with an exhaust gas test, but not noticeable in the running of the car until absolutely filthy.

Have you tried blowing through the filter element to check that it does pass air reasonably freely? If it does, as others have suggested, I would look for another cause, possibly entirely unrelated to the filter fitment.

Good luck

Stuart

Location: Devon

Hi,

I have been using an air filter on my Austin for 15 years without any problems, the filters used on my Filter Units are originally intended for 1960's BSA & Triumph motorcycles.

My thoughts on the rich running would include have you removed and replaced the float Jet as failing to replace any washers under this jet will certainly make the mixture richer, indeed my experience is modern fuels cause the float to be slightly lower in the liquid which itself cause the engine to run rich, I have increase the thickness of the washer under the float jet to lower the level of the fuel in the float chamber.

Back to the question of filters.

The original Austin Zenith carb set up had a crude cover over the air inlet forcing the air to be drawn through two narrow slots, approximately 1.5 square inches in total. The air filter modification allows air to be drawn through an area of over 15 square inches, in a radial pattern around the air intake producing a far better air flow into the carb. Feedback from customers has indicated improvements in engine running but this is not something I would ever claim as it is subject to opinion rather than proved fact.

I would certainly look at other factors for the rich running.

If David feels that the air filter is the cause of his problems then he should remove it and contact me to discuss a refund. However as no irreversible modifications were made to his carburettor when fitting the filter unit I would suggest that he returns the carb back to its original design and see whether it resolves his problems.

Do feel free to contact me David.

Location: Falmouth