You sure you got the spring mounted correctly? The heat riser deflects exhaust to the underside of the intake. Spring is supposed to dampen the baffle's movement during operation. Im unsure how much the spring actually 'moves' the riser, like the spring choke in a carb. I'd think that as RPM increased, the exhaust gas flow would press the riser baffle more open, with the spring trying to force it shut. But you don't want it to close more when it gets hot, cuz that could boil the gas in the carb & cause vapor lock or other strange performance probs.
You sure you didn't take the temper out of the spring with that torch? Make sure the riser is not stuck, and the counterweight is intact and in the right position when you install the spring. Does the riser move when you revv the engine?
good luck
chip
