Not sure what/where the heat riser is or what function it performs……
If you have a stock exhaust manifold, there is a half moon shaped counterweight just below where the four runners become one. Directly opposite, there is a bimetallic spring.
In the middle of all that is a flap that opens to allow exhaust heat to warm the carb, and then the spring closes it once engine is warm.
Find the part and see you can rotate the counterweight. Report back with findings. Also, search old threads here for tons of info on the topic.
