what does it do?

hey guys

i bought a scorpion can for my gsxr 750 yesterday............got the old one off .....

1........i can belive how heavy that silencer and cat is :o

just about to go out and fit new can but ive noticed something

just inside the exhaust there is some kinda butterfly valve? looks similar to the yamaha exup ............the pully on the outside is slighlty corroded. what does this valve do?

its in wide open position and it doesnt seem to move with revs or anything.

i assue it moves in correspondance with throttle position?

if it doesnt move is this a problem?



cheers

Dan

Hi Dan,
You're correct, it's the same as a Yamaha EXUP valve, I think all the Japanese manufacturers started using them once Yamaha's patent expired in the late '90s.
The valve is "closed" at lower revs to give more back-pressure in the exhaust to improve low-down torque, and then opens progressively with rising engine revs and throttle openings. With the engine off it should be closed. They are prone to sticking if the cables or spindle suffer corrosion, but they normally stick in the closed position. If yours is stuck open I'd say that a previous owner has locked it open in a misguided attempt to get more power. The valve is actuated by a little servo motor that's probably under the seat, and you should hear it cycle when you turn on the ignition. If the motor's been uplugged there should be an error signal on your clocks somewhere, might be worth looking into... :-?

no error on clocks............there is a like servo noise when u flick igniion on.

im having a guess the valve shud be closed at idle?

Yes, it should be "closed" at idle. When they first came out on the FZR1000 EXUP in 1989 I think that the valve was only fully open above 6000rpm, on a higher-revving new 750 it might not be fully open until later, maybe 7-8000rpm or more.

Take the seat and maybe a side panel off to have a look at the motor and cables, then you can see what's up.

The valve will be open when running at any revs IF its failed (it goes into a Fail Safe mode), there should be a fault code showing on the dash, but have a look at the ECU, if its had a black and orange wire snipped the fault light wont show.
its a common mod on the suzuki to "get rid" and doesnt cause any problems.
EXUPs did use use a valve to gain midrange, it allowed them to use pretty radical cams, and the valve increased the resistance to flow, it was the only way that the "Carbs" (remember them?) could be made to accurately meter fuel.........fuel injection gets round this problem, and the valve isnt really needed, engine designers are now cleverer in their exhaust port and header design and can acheive this without the valve, and accurate metering of fuel is a thing of the past...

lots of people now speculate that the valve is used to keep the noise down and removing it isnt a problem (ive done it on 2 gsxr's)
il bet that yours had a race pipe previously, and the owner put the standard back on to sell it......

put your new can on and enjoy, altho getting rid of the cat may require a remap of the feulling if you dont like how it rides.....

see you on a rideout soon mate :-)

JayDee.