Fzs 600 Pod filter without rejet

It's not that hard to do

You can find a lot of resources and forums discussing about ditching air box and fitting a cool looking yet irresistibly cheap chinese pod filter but I got no definitive answer as to how to make it work on the cheap. 

Do It On The Cheap

Usually most online resource will tell you that you have to empty your wallet for dyno jet and hours of dyno time to get it right... some put some bike tyre tube around the cone.... none of them seem reasonable... and why o why do you put a tyre tube over the cone? That is defeating the purpose twice by ruining performance and appearance. 

Only way to find out? Do it myself and share with the rest of the world. 

It all depends on the way manufacturers design the carb but, most cv carbs seem to need fair amount of pressure drop between the ambient and the carb inlet to operate properly. That is why experienced bikers tell you not to mess with air box. It is very important to have right amount of resistance to have diaphragms operate steadily without jerkyness while maintaining good throttle response in these carbs. 

You can always install larger main jet but what it does is that it forces your carb to work within shorter cv slide stroke and engine becomes somewhat jerky or end up with delayed response from main jet since you are still using same spring.

So here is my theory

 Engineers in Yamaha design an air box that gives the exact amount of air flow resistance so that the slides lift just the right amount at given engine speed and throttle angle. So, in a way they did all the homework for you already. Too much resistance means engine runs too rich. Too little resistance means engine ends up running lean. I just need to copy what they did.

The inlet size on the airbox is approx 80mm x 28mm =2240mm sq. 

Divide that by 4. = 560mm sq.

That gives theoretical orifice diameter of 26.7mm.

Then multiply that by Air filter pressure drop at high flow... say 90~85%?

So the orifice size should be around 23~24mm.

You get theoretical orifice size to mimick the air box pressure drop to be around 23mm diameter round hole. Don't be so shocked, you can pull quite a lot of air through 23mm hole before reaching speed of sound.

0.5mm Aluminium sheet with milliput. It is not taking any load so it should do. 

This goes into the cone. 

Rubber is shaven to clear the breather hole for the pilot jets.

These are alternative restrictor plates made with Fimo and aluminium sheets. The hole is offset to line up with the opening of slides to provide sufficient pressure drop but reduce turbulence for the air travelling through the carburettor.

Version 2

Once these funnel pieces are oven cured at 130 deg C, these need to be glued onto the plate with epoxy glue.

These are the ultimate induction funnels. Throat diameter is 23mm. OD is 42mm to give bit of compression fit against the filter rubber connector.

Removing airbox

Despite what some people say, you can get it out without destroying it or taking the engine out. 

1) Remove fuel tank. 

2) Remove battery. 

3) Remove carburettor. 

4) Remove 2 intake rubber on the cylinder 1 & 2.

5) Take off air box bracket (small one close to the fuel filter)

6) Take off air filter lid and all screws securing air box to the frame.

7) Take off front part of airbox-lots of screws and you need stubby phillips driver and disconnect all breather hoses.

8) Gently wiggle it out towards the No.1 cylinder side. Have patience... it will come out. It works.

The end result is promising. No popping, no hesitation even above 9000 rpm. Engine sounds happy with 23mm dia hole in the end. 

I will try version 2 orifice plate later on.

https://docs.google.com/leaf?id=1M5SRISsei56PipVCmm3O8qCXxOQ-OPIr

Also, crank case breather filter must be installed.

Version 2 restrictor plate also worked. No issues. But engine felt slightly more ready to spin up and responded better to the throttle movement. I think I'll keep the version 2. Also no back fire or hesitation. 

It ends up tiny bit lean:

 Later on, after installing the 23mm ultimate version induction funnels I found there is a bit of flat spot around 6000 to 8000 rpm. I moved the main jet needle to No. 4 groove and it seems much happier now. That means possibly the stock air filter causes higher pressure drop than anticipated.  Probably 20 ~ 21mm may have been just right for the stock carb setting. 

The pilot jet screws are set at stock setting of 2 turns.

On the road

It performed just like it used to with the airbox. No issue with low speed maneuver, overtake was just as responsive. Bellow 4000 rpm under low load, it sounds bit burbly but anything above 5000 rpm  especially under load starts singing with distinctive dry high pitched induction sound and my mate behind me heard it from 30m away. It is not intrusive like a stubby can with no db killer...it is a lot milder but every time you open up the throttle it reminds you that it is a 140hp/litre highy tuned beast although it is only 600cc. This project deserves a stamp of approval. 

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