I just went thru the same process with a frozen heat riser valve. After removing the manifold I sprayed it with pb blaster inside and out ,let it sit then repeat for a couple days. Eventually with a pair of vise grips i got it to move and then reapplied more PB blaster. Bit by bit it freed up. Basically the manifold was all coked up with black exhaust crud and required scraping and careful chiseling to remove the carbon from the passage ways. Good luck. After cleanout I ordered the spring that operates the valve. Does anyone know if the spring requires presetting the tension by winding the spring up or if it is installed straihgt without preloading? thanks all....
