Air filter or bellmouth on a 1958 99?

Dear Mark, There will be lots of Norton motorcycles at 'Norton 100' that have never seen an air filter and have also been running well for very many years.  Your machine will probably be fine depending on use. With best wishes, Colin Cheney.

The bellmouth doesn't do anything.  It'll run as well without one.

IF you have the very traditional long bell mouth (2") as favoured by the Ace Cafe Racers, it will actually slightly reduce the pickup/bottom end punch. You need the Amal type bell mouth that puts the air hole for the float chamber on the outside. This is true for ALL engines. If you continue to run without an air cleaner you will eventually shorten the life of your bores and carb parts, but it is unlikely you will see the problem in your life of the engine.

An air filter is not essential but may avoid the odd stone or other debris from maintenance  finding its way in  to the detriment of the pistons.

...and state that I believe the use of an air filter will stop ingestion of fine dust and dirt, the enemy of your bore and piston rings. See that nasty fine scoring in your cylinder bore? Dust and dirt. 

An effective air filter is essential! There is a reason manufacturers fit air filters, air is dirty and that dirt wears engines out very quickly. You do like your engine don't you? Or maybe you like rebuilding engines again and again... A few years ago some of our club members were on a ride out. A dry breezy day and rode through a cloud blown off a field. The two bikes without effective air filters ground to a halt. The damage cause by the ingested dirt meant they had to be rebuilt ( they seized solid) A gauze mesh often seen is NOT an effective air filter, but it will stop gravel, stones, nuts and bolts and the like getting sucked in, better than nothing. The problem with some of our bikes is that there is not enough room between the carb and frame,oil tank,toolbox to get a good filter in there. An engine run without an air filter will wear the bores out within a few thousand miles. Probably less than 5K

Lots of us have done lots of riding with no air filter.  It's not usually a big problem. But I do use an air filter, because it filters the air(!) and there's no reason not to.

I have  had bikes with no air filter that have seen a fair bit of use  with no issues at all. I also have a box filter on my 99 with a modern paper pleated filter . The bike has been in use for 30 years and has not been stripped and has good compression with no smoke.  The Atlas has suffered very  heavy wear  of piston rings but that was caused by not having its oil changed . Kids dont listen !.

British did not normally fit them in the home market for two main reasons. First: customers didn't demand them, because racing bikes did not have them. Second: because if an engine only lasts 5,000 to 10,000 miles before a rebore, they sold more spare parts and the dealers made more money. But air quality in cities was appalling by today's standards.  Full of abrasive soot from coal fires.  And engine oils had no dispersants.  Soot in the oil from combustion starts off quite safely as minute particles.  Without dispersants, these join up and become abrasive lumps, which caused early engine wear.  So our old engines should last much better than they would have done in times past because air is cleaner and oils are far better.

I stand to be corrected but didn't the bikes supplied to the military have to have an effective air filter? Granted modern oils are far better than the stuff available back in the day, but to say 'our old engines should last much better' should have had a caveat that also said 'so long as you can guarantee a supply of  clean air to said engine'.   To run an engine without an effective air filter WILL wear your engines piston and barrels out. Leave your car parked on the roadside for a few days after it has been washed, then go look at all the dust and shite on it, That stuff wasn't applied by gnomes, it is in the air and that is what your engine tries to breath in. A filter stops that.