Will Marvel Mystery Oil or Seafoam Fix My Carb Issues?
I am not going to say adding some fluid to your bike's gas or oil will fix a problem. These products are marginal means of preventative medicine/maintenance/preventative steps, at best. They should never be used to fix a problem on your bike.
If my bike is going to sit for a long time, I drain the gas from the carb which prevents a buildup of varnish. When gas is left to sit, it evaporates and leaves behind varnish. Much like cholesterol in veins, it reduces the ID of the passages in your carb and in your jets, even a few thousandths of an inch can change a 125 main jet to something like a 115... which would mean your bike will no longer run properly.
Reactive fixes after leaving a bike to sit with fuel in it are rarely if ever a good idea and the issues most likely could have been avoided with proper preparation. If a bike has badly corroded jets, just replace them.
Now using something like marvel mystery oil or Seafoam won't do any harm to your motor and when used during your riding season or when your bike will be running through fuel regularly, it can help keep things clean, but again, they are really only maintenance and preventative fuel additives.