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Amal 375 bell mouth or air cleaner?

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Amal 375 bell mouth or air cleaner?

Posted by Nick Clarke at May 10. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by patrick_mullen at May 11. 2018

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.

 

Re: Amal 375 bell mouth or air cleaner?

Posted by Nick Clarke at May 11. 2018

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

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Re: Amal 375 bell mouth or air cleaner?

Posted by Steve Adkins at May 11. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by Dan Field at May 11. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by Nick Clarke at May 11. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by patrick_mullen at May 11. 2018

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.

Re: Amal 375 bell mouth or air cleaner?

Posted by Mikael Ridderstad at May 11. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by Nick Clarke at May 12. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by Steve Adkins at May 12. 2018

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

Re: Amal 375 bell mouth or air cleaner?

Posted by Terry Grimwood at Monday 18:06

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

 

Re: Amal 375 bell mouth or air cleaner?

Posted by andy_sochanik at Wednesday 08:41

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.

Re: Amal 375 bell mouth or air cleaner?

Posted by Nick Clarke at Wednesday 11:09

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?

Re: Amal 375 bell mouth or air cleaner?

Posted by robert_tuck at Saturday 19:01

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.

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