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Old 10th June 2013, 12:52   #1
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Default Secondary Air Injection in Scooters & Motorcycles

I came across a Secondary Air filter in my Dio, and the manual had mentioned the same too. Though it had stated how to maintain the filter element, there was no mention of the actual purpose or troubleshooting WRT the system. I did search the internet for information and I could only find international references, that too mainly for cars.

However, I was curious to learn more about this with respect to two wheelers. I welcome any sort of information, reference etc from members and hope to learn more. As per Wikipedia and some other links, SAI was implemented to control emissions, and this was done by injecting air into the exhaust manifold of the vehicle.

This is how the parts are laid out in my Dio:
1. A Secondary air filter with a sponge element soaked in oil.
2. From the filter box, a hose goes down to a cylindrical object, not sure what it is called.
3. To the cylindrical object, another hose comes from the exhaust manifold and a third hose comes from the inlet manifold.

My findings about the system:

1. Removed the hose from the air filter and choked it. The engine did continue running but idle speed reduced. Discovered that there was suction being created, ie the air from filter was indeed being used somehow.
2. Removed the hose coming from inlet manifold and left it there. The engine refused to start. If it was choked manually, the engine ran fine. Once I let go of the thumb and let in air, the engine stalled.

This put me into thinking on how exactly this setup works. Is this similar to the EGR? Or as per the article I read is it providing air to the exhaust?

I had an issue with my dio making a growling noise once the engine got warmed up. When I showed it to the Honda guys, they did not even aknowledge that there was something abnormal. However, I know this is not normal as I have another dio in my native which doesnt get this growl anytime. I took it to a nearby mechanic who told me that it is related to the secondary filter and the whole setup has to be checked. So I did not go ahead. Apart from an annoying growl while accelarating, there are no other issues WRT performance or economy. Just that during initial accelaration, the scoot sounds like the air filter box has a hole. I checked all Hoses for cracks and ruptures and everything is fine.

However, if I choke air supply into the cylindrical device, the growl disappears. That meant the mechanic was right.

Would like fellow members to throw light on what this setup really does, and how it can cause this growl when engine is warm.
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Old 10th June 2013, 16:56   #2
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Default Re: Secondary Air Injection in Scooters & Motorcycles

Originally Posted by audioholic View Post
I came across a Secondary Air filter in my Dio...
Audioholic, this is a pulse air injection system. It works by injecting fresh filtered air into the exhaust manifold just downstream of the engine, to help the still-hot and partially burned or unburned charge combust before it leaves the exhaust system. Improves the exhaust emission levels apparently.

It uses the negative pressure generated during engine operation to open a valve (the cylindrical thingy you refer to) and draw in fresh air through the filter.

So the hose coming form the inlet manifold is a vacuum line to control the valve. The hose from the filter is the fresh air intake to the valve, the hose going to the exhaust manifold from the valve is the injected air.

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The pulse air system operates optimally at idle or just above idle. When the secondary air inlet at the filter is closed, air injection stops. Did the revs drop because the exhaust circuit was, in essence, gagged?

When you opened the control vacuum line from the inlet manifold, the engine was drawing in extra air from this line, leaning out the mixture enough to stall.

Did you try blocking off the air outlet from the valve while at mid-throttle? Ideally it should not affect anything.

This is just my understanding of the system, and is open to correction.

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Old 10th June 2013, 17:42   #3
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Default Re: Secondary Air Injection in Scooters & Motorcycles

That was a detailed explanation. Thanks a ton. Speaking in the same line, the exhaust gives a pop note during decelaration when the engine is cold. Is this supposed to be normal? When the engine is warmed up, this wont happen. How about the working when the engine is worked up? In my case when I ride for a distance, the secondary intake develops a growl. Is this also related to normal functioning?
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