Amal 375 bell mouth or air cleaner?

Hi Nick,

It's interesting that you mention a plastic bell mouth. All my carbs have what I thought was the original short alloy bell mouth. The bell mouth is threaded on to the carb which is also alloy and after a lot of years are pretty much a permanent fixture - they would probably break rather than unscrew. As you say there is little space behind the carb to fit an airfilter so probably some design of an air box in modern style would give best results. But is it worth the bother ?? Your decision.

I don't think there is one optimum length for a bell mouth. We did experiments many years ago with an MGB on a rolling road. From memory short bell mouths gave best power at the very top end and longer bell mouths gave the better mid range; the one consistent finding was that power was always better with a bell mouth fitted than without. So even the standard B on 1.5" SUs had short rounded bell mouths fitted as standard INSIDE the air filter.

But you have a Navigator with a 375 Amal so my preference would be to fit the standard short bell mouth for best overall performance with minimal effort in normal usage.

Greetings,

I actually fitted one of these to a concentric installed on a 1961 model 50. At least it keeps the rocks out, as there is no room whatsoever for any sort of 'real' filter.

http://accessories.hitchcocksmotorcycles.com/Amal-Carburettors/Alloy-Polished-Air-Intake-With-Gauze-Short/12959

If there is a performance improvement, so be it, the poor old thing needs all the help it can get!

Rgds Steve

 Yes I have one on my 1962 navigator deluxe. Ex military. The short bell mouth does not reach the air box which I assume once contained an air filter. This is why I am looking for information on air filters. Any help to any one?

Amal make a screw on alloy ring to fit in place of the bell mouth. ,A rubber connector goes from there to the air box. Inside the box is a pleated length of filter that is not made but can be fabricated from an Austin mini one. To get the filter to seal in the box ,I greased the iinside of the box and coated the cut edge of filter with bath sealer and clamped it up till set. It works well and gives peace of mind and long engine life. My box was made by Norville  and fits poorly but I made it work.

I seem to recall putting a piece of old tights over mine!

The conical filter is a K&N or a copy of a K&N. K&Ns have been around for a long time and have a good reputation as efficient filters-even though they are from the USA !! I have a pair similar to these fitted to the 34mm carbs on the Yamton; they have lasted for the past 25 or so years and do the job OK. And I have another pair (slightly different fitment) on an MGB as well. You need to keep them lightly oiled to work best.

The short bell mouth looks very like the original Amal item.

Modern cars of course now have variable length intakes controlled via the ECU so you don't have to choose which bell mouths to fit.

On my classic racing Honda, I put foam air filters over the velocity stacks, bought from Vincentspares, as my son has a habit of entering the gravel traps. As they are soft they adapt to parts of the bike which prevent use of conical filters. On the Honda gap between frame and carb bell mouths is very small. No difference in power and jetting. If you don't want filters stamped HRD, I believe Ramair makes the sock type.

Mike

Greetings,

Try here, not the same as the 'mesh' type but probaply much more effective:

http://amalcarb.co.uk/air-filters/threaded-cone-filter-375-monobloc-289-pre-monobloc.html

There are a few here too even with hose clips. Nice to see one for a 276 too.

https://www.feked.com/carbs-fuel/bellmouths/

And there are all sorts here:

https://burtonbikebits.net/air_filter_assemblies/

Good luck and I will measure the space between my Model 50 concentric and see if a 'real filter' will fit, such as :

left bottom repeat-x rgb(255, 255, 255); ">Details for Threaded Cone Filter - 600 Series MK1

A spin-on conical air filter with billet adaptor, chrome end caps, stainless steel mesh with washable element.

Overall length of carb & filter is 13.5cm from flange. The filter extends beyond the rear edge of the carb by 66mm

But there is not much room between the carb intake and the oil tank:

Good luck.

Rgds Steve

Previously Nick Clarke wrote:

Dan, a lot cheaper and probably just as effective given where the carb is on a Navigator - although knowing my luck it would break loose and get ingested into the engine!

Steve, a bell mouth with a gauze screen would be ideal but sadly they don't list one for the 375!

Nick

I got a short bellmouth with gauze screen for my 375 from Surrey Cycles (surreycycles.com) at one of the local shows. They don't appear to show it on their website so you would have to phone them. It's a nicely made item but quite expensive as I recall.

Terry

Previously Nick Clarke wrote:

Patrick, my thoughts exactly on the conical filters and given K & N's reputation they are worth a try provided they will fit.

Mike, the joys of over exuberant youth! Thanks for reminding me of foam socks I will have a look at Ramair.

Interestingly the Jubilee & Navigator spares book lists an air filter as an optional extra at a whopping £2/9/1! I've never seen one or a picture of what it looked like or where it fitted but have often wondered about the various holes in the main frame behind the carb. Out of idle curiosity has anyone got one or a picture of one?

Time for a proving run on the Navigator today, and the on going battle to stop the miniscule and illusive fuel weep from the new fuel tap!

Nick

Hi Nick,

I have never seen an air filter either, in all the years of buying/selling & pulling bikes apart - though, as you say, its listed in the Parts List.

The two big holes in the framework behind the carb, were of course to hold the DeLux oil tank under the seat on rubber cushions.

Previously Nick Clarke wrote:

Thanks Andy, I should have guessed those holes were for the Deluxe oil tank!As for the optional extra air filter I can't help thinking if you haven't seen one then no one has! Do you use the standard short Amal bell mouth, a longer one or a conical K&N style air filter on your Navigator?

If the Deluxe oil tank has threaded holes in the back then these are for a filter box,which may be the same or similar to the one supplied by Norville for a DL Dommy. Expensive and fiddly to fit.