Skip to main content
000000 000003 000006 000009 000012 000015 000018 000021 000024 000027 000030 000033 000036 000039 000042 000045 000048 000051 000054 000057 000060 000063 000066 000069 000072 000075 000078 000081 000084 000087 000090 000093 000096 000099 000102 000105 000108 000111 000114 000117 000120 000123 000126 000129 000132 000135 000138 000141 000144 000147 000150 000153 000156 000159 000162 000165 000168 000171 000174 000177 000180 000183 000186 000189 000192 000195 000198 000201 000204 000207 000210 000213 000216 000219 000222 000225 000228 000231 000234 000237 000240 000243 000246 000249 000252 000255 000258 000261 000264 000267 000270 000273 000276 000279 000282 000285 000288 000291 000294 000297 000300 000303 000306 000309 000312 000318 000321 000324 000327 000330 000333 000336 000339 000342 000345 000348 000351 000354 000357 000360 000363 000366 000369 000372 000375 000378 000381 000384 000387 000390 000393 000396 000399 000402 000405 000408 000411 000414 000417 000420 000423 000426 000429 000432 000435 000438 000441 000444 000447 000450 000453 000456 000459 000462 000465 000468 000471 000474 000477 000480 000483 000486 000489 000492 000495 000498 000501 000504 000507 000510 000513 000516 000519 000522 000525 000528 000531 000534 000537 000540 000543 000546 000549 000552 000555 000558 000561 000564 000567 000570 000573 000576 000579 000582 000585 000588 000591 000594 000597 000600 000603 000606 000609 000612 000615 000618 000621 000624 000627 000630 000633 000636 000639 000642 000645 000648 000651 000654 000657 000660 000663 000666 000669 000672 000675 000678 000681 000684 000687 000690 000693 000696 000699 000702 000705 000708 000711 000714 000717 000720 000723 000726 000729 000732 000735 000738 000741 000744 000747 000750 000753 000756 000759 000762 000765 000768 000771 000774 000777 000780 000783 000786 000789 000792 000795 000798 000801 000804 000807 000810 000813 000816 000819 000822 000825 000828 000831 000834 000837 000840 000843 000846 000849 000852 000855 000858 000861 000864 000867 000870 000873 000876 000879 000882
English French German Italian Spanish

Amal 375 bell mouth or air cleaner?

Forums

Chaps, having got the Navigator running happily I am reminded of my carefree teenage years by the evocative 'inlet roar' of an open mouthed Amal carb. My bike is missing the standard short plastic bell mouth so sounds even more asthmatic and I can't help thinking that it doesn't exactly help the less than abundant performance! The bike came with a 2 1/2" long aluminium bell mouth help in place by a jubilee clip which I took off when I rebuilt the carb. I have refitted it and it dramatically quietens the 'inlet roar', and I can even kid myself it seems to pull / rev slightly more freely at the top end. Has anyone experimented with bell mouths to establish the optimum length and can tell me where to buy one? I have checked the Amal website and the plastic bell mouth isn't listed as available but they do list various conical air filters which can only help engine wear. There is limited room behind the carb on a lightweight, has anyone tried these and do they stifle / enhance performance? I note they aren't listed in the NOC shop.

Nick

Permalink

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.

Permalink

Thanks for that Patrick, my memory may well be playing tricks, I thought the original short bell mouth was plastic so stand corrected. I know inlet tract length and therefore bell mouth / velocity stack length makes a difference to power / torque but very interested to hear your MGB experience with short giving top end and long giving mid range improvement. In searching the internet Hitchcock's (of Royal of Enfield fame) offer a conical air filter, a short and a long bell mouth for the 375 (see pics). I'm going to phone and check dimensions of the filter to see if it fits and report back.

Nick

Attachments Conical%20air%20filter.jpg Short%20bell%20mouth.jpg www.nortono
Permalink

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

Permalink

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

Permalink

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.

Permalink

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

Permalink

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

Permalink

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

Permalink

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

Permalink

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.

Permalink

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?

Permalink

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.

Permalink

 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?

Permalink

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.

 

Norton Owners Club Website by White-Hot Design

Privacy Policy