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Removing the Air Injection Valve

Post Received 17 May 2020

By Mark Noel

The March of Progress formulated by Eurocrats in Brussels has forced manufacturers of motorcycles, large and small, to reduce noise, improve safety and minimise emissions. This is good news for the planet but inevitably is increasing mechanical and electronic complexity to the point where 'system malfunctions' are more likely to occur. Imagine you are on a lifetime's trip riding through the wilds of Matabeleland when suddenly your bike coughs, splutters to a halt and the multifunction colour TFT dash flashes the alert:

'Subsystem interface mismatch error. Failure in Module B'.

Trying to ignore the emergency flashers and beeping horn, you hook up your laptop to the multipin connector, start the diagnostic app and scroll through menus to reach 'Module B Errors', only to witness the PC's battery extinguish. Heck! You decide that your next trip to Matabeleland might best be done on a Velosolex or on a camel when rectumfrying a 'system malfunction' will be much simpler. In the case of the camel - with a fresh bale of hay.

Anyway, I digress. The reader will appreciate that I like motorcycles that are simple, primitive even, with bits that can be seen and understood, and with a minimum of black boxes. Even better if these boxes are brown and made of Bakelite!

In the garage I have two quite new bikes - a Honley 125 which is an exact replica of a 1970s Honda CG125 made in Turkey, and a Chinese Herald 250 Classic which is a replica of the gorgeous Suzuki TU250X which sadly never made it to these shores from Japan. Both machines have something called a carburettor which your granddad will remember as the bit that vaporised petrol before it entered the cylinder. 

Such designs are on their last legs, soon to be abolished by the aforementioned March of Progress because the exhaust gas may contain a proportion of unburnt hydrocarbons that enter the atmosphere to make your hair turn grey. For proof just look at mine. 

Manufacturers of these Neolithic motorcycles have invested considerable sums in tooling and production and to extend their life have managed to comply with Euro 3 rules by injecting air into the exhaust stream. The idea is that the additional oxygen will burn off any traces of fuel vapour so that the exhaust will be as pure as the alpine air at Chamonix, only hotter. In the case of the Herald the silencer also contains a catalytic converter to trap any further harmful residues.

Call me old fashioned but the plumbing and ugly lump that comprises the Secondary Air Injection System detracts from the vintage lines of my two neo-classic bikes. First there is a port on the inlet manifold that connects by a twisty pipe to the valve unit bolted beside the cylinder, which connects by another pipe to an inlet on the head above the exhaust. Another pipe links the valve to a spout on the airbox, the whole gubbins snaking above and around the top of the engine.

The workings are pretty clever: at tickover and on the overrun with the throttle closed there is a deep vacuum in the inlet port which forces a diaphragm to open in the valve unit, enabling air to pass from the airbox into the exhaust stream. The system is thus designed to only operate when there is likely to be excess unburnt fuel in the exhaust and results in the cleanest fumes possible with carburettor technology. Some forums on the Interweb state misleadingly that the system feeds exhaust gas back into the engine but this is obviously untrue when you dismantle the valve and suck on the ports to demonstrate its function.

Atop of the clean cylinder head is fitted a small smartly made blanking plate over the hole
Blanking plate on Herald cylinder head
Another small yet well engineered and finished plate bolted to blank off where the air intake used to be
Blanking plate on Honley cylinder head

I have removed the valves from my Honley and Herald for aesthetic reasons and took care to make snug fitting blanking plates to the cylinder heads, plus sealing caps to the manifold ports. Removal makes no difference to performance, although the loss of 550 grams has boosted top speeds by several inches per hour. 

A possible downside is that the catalytic converter in the Herald's silencer may die sooner due to an increase in contamination. On the other hand failure of a Secondary Air Injection System on a standard bike can also be an issue if the manifold pipe comes loose or if the valve stays shut or open. The former will lead to a weak mixture and overheating, while a stuck valve will also interfere with fuel:air ratios. Either way I am happier to see the classic lines of my bikes restored, even if it means that my beard as well as my hair is now turning grey!

For more information on the working of these injection valves please see the excellent article by David Angel at:

If you'd like to edumificate the great unwashed with your motorcycling wisdom then click here.

Reader's Comments

Ren - The Ed¹ said :-
Just look how well made and finished those blanking plates are. Makes me SICK! What's wrong with a blob of silicone? Huh? Huh???!!
28/05/2020 08:59:26 UTC
Bob said :-
I've done similar on several bikes, Honda FX650 and SLR650 benefit from reduced weight and plumbing as you say.
My Euro 3 compliant KLX250 has an air injection valve, but it also has a catalytic convertor and an EFI system so I have left it (the air injection) in place because I don't want to upset the catalytic ocnvertor and mainly because when active on the overrun the incoming air creates enjoyable popping noises from the silencer!
28/05/2020 09:33:33 UTC
Ren - The Ed¹ said :-
Now you'll have to help me out here Bob. I cannot recall where the air injection is on Sharon's similarly engined Z250SL is. Does it have one I wonder? Where does it go into the head/cover/exhaust on yours?
28/05/2020 10:58:09 UTC
Ren - The Ed¹ said :-
Aaah no I've found it on the parts diagrams.
28/05/2020 11:02:24 UTC
Ian Soady said :-
Very nice job (and nicely written). I agree this mod may lead to premature failure of the cat, however I wonder whether in fact this item is required especially on UK bikes where emissions are not tested. It may be possible to replace that by a bit of straight through tube which may even liberate another couple of mousepower.

The MoT manual only makes reference to exhaust noise, leaks and markings indicating the silencer is not for road use. Interestingly, on the last point, it doesn't (as some people seem to think) specify that it has any EU / EN number stamped on it although this may be covered by Construction & Use regulations.

And at least you still have some hair to turn grey......
28/05/2020 11:21:26 UTC
Upt'North said :-
Con and Use Regs..
(3) The silencer which forms part of the exhaust system of a motorcycle first used on or after 1st January 1985 shall be either—

(a)that with which the vehicle was fitted when it was manufactured; or
(b)clearly and indelibly marked with either—
(i)the British Standard marking indicating that it has been tested in accordance with test 2; or
(ii)a reference to its make and type specified by the manufacturer of the vehicle.
(4) A motor cycle shall not be used on a road if it is fitted with an exhaust system any part of which is marked with the words “NOT FOR ROAD USE” or words to the like effect.

(5) Instead of complying with the provisions of paragraph (2), a vehicle referred to in paragraph (1)(b) may comply at the time of its first use with Community Directive 78/1015.

Boring innit, even worse if you get a ticket.
28/05/2020 14:05:05 UTC
Ren - The Ed¹ said :-
Yes it was well written, much to my chagrin.

I do find this blog to be enlightening. Like most things it's actually quite simple once you understand it. The link to F2 motorcycle's blog regarding the confusion between EGR and Secondary Air Injection was something I was labouring under. I now understand what the PAIR valve is doing on my CB500X and it's not the same as the EGR on my diesel Astra. Not *quite* certain what the EGR does on the Astra but I'll get around to that as and when.
28/05/2020 18:49:24 UTC
Borsuk said :-
Nice to see I am not the only person who appreciates the little Honleys.

I think Euro 4 killed the idea though. I still see a few advertised but I don`t think Earnshaws are doing them any more. I wonder if they made a profit out of it as it was quite a short run I think. Only 460ish were registered.
Posted Image
28/05/2020 23:31:55 UTC
Ian Soady said :-
EGR (Exhaust Gas Recirculation) is a device which feeds a small amount of exhaust gas into the induction system. As far as I know it's only fitted to diesel vehicles as they don't have a throttle as such (engine speed is controlled only by the amount of fuel injected) so at low speeds / light loads you can get incomplete combustion and the recirculated exhaust gas helps to clean this up and reduce nitrogen oxide emissions. However, most people think it actually does very little.
29/05/2020 11:33:49 UTC
Ian Soady said :-
EGR (Exhaust Gas Recirculation) is a device which feeds a small amount of exhaust gas into the induction system. As far as I know it's only fitted to diesel vehicles as they don't have a throttle as such (engine speed is controlled only by the amount of fuel injected) so at low speeds / light loads you can get incomplete combustion and the recirculated exhaust gas helps to clean this up and reduce nitrogen oxide emissions. However, most people think it actually does very little.
29/05/2020 11:34:45 UTC
Ian Soady said :-
Oops added it twice somehow.....
29/05/2020 11:35:10 UTC
Upt'North said :-
The real purpose of the EGR Valve is to become blocked and for it to then cost the owners lots of money to fix all so a blummin Shady car company can cheat on emissions testing.
Strange world innit.
29/05/2020 18:15:39 UTC
Bob said :-
Not just oil burners - my 1600cc petrol Vitara has an EGR valve. It's actually about the only part of the fuel and exhaust control system that hasn't needed attention in the 5 years we've owned it!
I do know they block up a lot on diesels, which I guess makes sense when you think about the difference between the emissions from a diesel (sooty and orrible) compared to a petrol engine.
Luckily my van is so old it doesn't have an EGR, or a turbo, or airbags, or ABS.
29/05/2020 23:01:59 UTC
Pocketpete said :-
Ren, you mentioned blob of silicone. Surely you meant bit of chewing gum off a park bench. Silicone that's just to expensive.

But I enjoyed the article a lot. I love that these bikes are direct copies of older type bikes. Somehow they seem a bit easier on the eye than their modern equivalents.

My neighbour has a new camper van and wanted to put a manual cut off switch on the fuel pump to prevent someone stealing his 45k pride and joy. Despite it having tracker fitted.

He installed the switch and it works perfectly and is disguised as a light switch above the bed.

He tested it and next time he drove engine management ight stayed on. Eventually he found out his switch causes a change in resistance across the pump which the onboard computer gives as an error code to dashboard system. This requires a dealer to pop on computer and reprogram the pump with a new resistance figure. Total cost £ 150 for 2 minutes work on the computer.

Suddenly these old bikes look a bit better. You may have noticed I turned on my spell check..... after subtle complaints were made

30/05/2020 11:25:54 UTC
Ren - The Ed¹ said :-
Cheers, another thing to worry about. I'll have to learn how to clean the EGR on the Astra now. Mind you it only does around 2,000 miles a year, should be a while yet.

Pocketpete - well obviously yes some used spuggie from the underside of a park bench is the way to go normally BUT chewing gum, to become flexible enough to mould into shape, needs chewing again. I wouldn't mind normally and if you add a drop of menthol flavouring it's quite nice. During this Covid 19 pandemic though I was wondering if it would be safe to use previously masticated gum? I can't find any science on the infection rates of used park bench offerings.

Glad to read the speel chucker is woking.
31/05/2020 21:08:35 UTC

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