The OS Carbs have a "Thermal Insulator" that the pinch bolt bites into to secure the Carb to the Block. If this fractures or unbonds itself, it can be replaced, cost about $12,(rebonded with high temperture Epoxy). Over tieghtening the pinch bolt is very easy to do and not good.
I'd bet this is your problen.
And BTW, don't continue to run the engine with the Carb unsecured.
