Sounds to me like your butterfly valve is rusted in place, possibly shut or darn near. A friend of mine had a Pontiac motor that was running like crap. They pulled it to replace it, and discovered the valve was rusted shut. All their exhuast from the passenger side had to travel through the motor and out the drivers side. When they freed up the valve the motor ran like it had been rebuilt. If your spring is broken you should be able to move the valve by hand. Hope this helps?
(Their motor was a v-8 with single exhuast)
