The heat riser only job is to warn up the carb faster then the engine can warm it up. Once the exhaust manifold gets hot and the spring relaxes, the flapper valve diverts heat away from the carb and out the pipe. The manifold will be just as hot with a non working heat riser as a manifold with a working heat riser. The only difference will be the amount of time it takes to heat the carb.
A cold start icing problem will benefit from a working heat riser, but again, once the engine is hot, it will make no difference. A warm air pipe to the air filter will also help with icing issues.
Be sure the manifold has the flapper/diverter valve or you can get a serious vapor lock problem from a over heated carb. GM new they had a heating issue, thats why the isolator was added.
Ideally you want a cold carb with a hot intake. The cold carb keeps the fuel dense and the denser it is, the more power it makes. The hot intake manifold keeps the fuel vaporized and as you know, fuel only burns in a vaporized state.
Joe
