Mk3 Airbox Restrictors?

I currently have my Mk3 in pieces awaiting a new camshaft. Getting that airbox out was something else, it looks like that was the first part fitted to the frame on the assembly line.

I thought about changing it for the earlier breadbox type, but I try not to change things too much without a very good reason, particularly in this case given the nightmare that would be involve in putting it back.

So it's going to stay, but my question is this, there are two soft rubber venturi or restrictors? on the inlet horns. Does anyone know with any certainty what the purpose of these things is and the consequences if any of removing them?

My own best guess given the desperation to quieten the Mk3 down (very successfully) is that they are intake silencers....but does anyone know different?

Mick.............kettle738

I can't answer the question with an absolute but I suspect your reasoning is correct - for sound. My 92 ducati 900SS had the same sort of rubber venturi. Removing them raised the intake sound level considerably. Related to that, Fast by Ferracci, a noted Duck tuner of the time, recommended their removal and a jet change for increased power. The removal of the venturi provided increased airflow and larger jets could take advantage of that. A quick "test" of that on the Norton would be to check the stock main jet size for the 850 with that air box vs the 850 with the older style air box. If there is no difference, then there is probably nothing to be gained by changing to the old style anyway.

From experience I know that the MK3 air box can be removed without removing the engine :shock: , but not easily. As I remember the bike ran ok with it in place along with the super quiet and probably rather restrictive black cap silencers. (had this bike since new) The stock jetting was 230. I am now running a little higher c/r do to skimming the head a bit and it's bored +.020, otherwise stock. The silencers are now strait through pattern pea shooters on the original balanced connected header pipes and that air box was replaced with a simple K+N air filter. These small changes allowed me to go up to 250 jets with a real clean soot free exaust and I think much snappier running overall. Oh sorry back to your question :roll: . I think those rubbers around the snorkles may have been an attempt to speed up and smooth the air into the airbox but the snorkles also have some 3/8" holes drilled in the and that would seem counterintuitive, sooo..... Maybe Norton was just grasping at straws. :mrgreen:

I run a mk2a without the rubber snorkels and with peashooter exhausts, main jets are 260 and the bike runs well. No idea what the snorkels do but have a pair and if I can be bothered one day will test. I doubt the plastic air box restricts performance compared to the perforated air filter, looks to me that it was the black cap silencers.

kettle738 said:

So it's going to stay, but my question is this, there are two soft rubber venturi or restrictors? on the inlet horns. Does anyone know with any certainty what the purpose of these things is and the consequences if any of removing them?

My own best guess given the desperation to quieten the Mk3 down (very successfully) is that they are intake silencers....but does anyone know different?

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There doesn't appear to be any other logical explanation for them. They were also fitted to the 850 Mk.1A and 2A (black airbox) models according to the parts lists.

A properly sized air box increases performance (and reduces noise) over no air box or a set of air filters (like KN) on the end of a carburetor. The issue becomes whether the air box and snorkels were actually designed/sized for the breathing characteristics of the engine or whether it was just "stuck" on there to reduce noise. I have no idea but I suspect, given the situation at Norton at the time, it was strictly a noise reduction mod. The smaller jets that were used would tend to support that.

The black cap silencers may or may not be a performance liability, again, depending on the design. A quiet muffler can make as much or more power than a loud one but assuming both the loud and quiet muffler are equally efficient, the quiet one will have to have greater internal volume.

Well all I can say is that mine is long gone and I'm not gonna try replacing it. Too many other choices out there these day's. I'd heard that original breather box was good in foul weather though.

mike996 said:

A properly sized air box increases performance (and reduces noise) over no air box or a set of air filters (like KN) on the end of a carburetor. The issue becomes whether the air box and snorkels were actually designed/sized for the breathing characteristics of the engine or whether it was just "stuck" on there to reduce noise. I have no idea but I suspect, given the situation at Norton at the time, it was strictly a noise reduction mod. The smaller jets that were used would tend to support that.

The black cap silencers may or may not be a performance liability, again, depending on the design. A quiet muffler can make as much or more power than a loud one but assuming both the loud and quiet muffler are equally efficient, the quiet one will have to have greater internal volume.

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Agree 100% about air boxes and mufflers. The black plastic MK3 air box does not have a lot of volume though and that non pleated foam filter not much surface. A k+N is probably the best alternative to a really good size airbox ( 'cause there's really no room). And witness the silencers we're pretty much all using, mostly straight through peashooters and the odd Dunstall Decible here and there. The black caps were about the same size and lots quieter, I dare say not much volume in those things either. I think today a bikes intake and exahaust system are very well designed with optimal flow along with quite operation hard to beat in a street application. I get a kick out of the hundreds and hundreds of $'s spent on exhaust systems that are super loud and only gain like 4 or 6 hp at the very end of the rpm range often with a dip in the mid range. Boy do I know how to ramble!

What are these restrictors i have never seen any on my bike maybe they fell apart I went to some micro fische for 75 and didnt see any either ?

MkIIIDan said:

What are these restrictors i have never seen any on my bike maybe they fell apart I went to some micro fische for 75 and didnt see any either ?

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No No, I 'm not sure if any of us are positive what they are, ie. purpose, but, they are merely rubber grommet type things that snapped onto the two snorkles that come off the undereath of the box. Sort of like a trim ring really.

MkIIIDan said:

What are these restrictors i have never seen any on my bike maybe they fell apart I went to some micro fische for 75 and didnt see any either ?

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http://www.nortonmotors.de/ANIL/Norton% ... 14&Part=12

The description is wrong on the Old Britts website

12...06-4839.....MK3 Airbox....2

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The description should actually read: Intake Venturi...2
The item line [12] is also drawn incorrectly on the factory diagram.
http://www.oldbritts.com/nor_mk3_75_14.html
http://www.oldbritts.com/1975_g14.html

Abut that airbox...no question it's a nightmare to try and remove (I have a 75 Mark III). My understanding is that it was part of the US restrictions on exhaust emissions added in 1975. The main problem is the "oil mist" (as they so delicately refer to it in the manual) that is fed into the carbs and the engine. That is not a good thing for power, in my humble opinion. I'm going to leave mine on and find something else to do with that mist...that should help things out. Re the bean can mufflers on the 75s, they do restrict power over the straight through peashooters.

Waiting for my 68 Commando to show up from the painter...

alex100 said:

Abut that airbox...no question it's a nightmare to try and remove (I have a 75 Mark III). My understanding is that it was part of the US restrictions on exhaust emissions added in 1975. The main problem is the "oil mist" (as they so delicately refer to it in the manual) that is fed into the carbs and the engine. That is not a good thing for power, in my humble opinion.

Click to expand...

But then, the plastic airbox was initially introduced on the 850 Mk1A and Mk2A models for 'Europe' and oil mist was fed to the airbox on all standard Commandos.

Removing the box is just finding the right moves then it's suddenly out. I seriously doubt there's any real performance gain by removing them on a stock or nearly stock engine.

I can report that going to pea shooters required upping the jet size. The give away was that at WOT on the highway a slight closing of the throttle gave more power. The pure white spark plugs told the same story. I would not remove the box. Futz with it a bit with some patience and discover the magic position it goes in and out. No force needed.

I have an mk3, with non conected colectors and peashuters, New premiers and the original Black box.

The bike didn't run well, i took away the rubbers, ingrese the jetting to 260 and position the needles to bottom notch (rich), and runes fantastic.

Very happy with this set up