Re:'heat' fuel additive
Do you have oxygenated fuel (Methanol/ethanol blends)? If so, it's already ten percent or more, if that didn't fix it then another bottle isn't going to help.
You don't specify what machine you're working on, I'm not so familiar with one of yours... Drain the fuel. Drain it from the RES setting if it's an option, and from the carburetor if you still have one.
Do your draining into a white or preferably clear container, that is CLEAN AND DRY. You will see if there is water, and you will know when water no longer presents it's self.
If it's just water (looks like gray blobs usually) then once you've let that out of the bottom, you'll be good to go, what's left will be just fine.
If it's pasty white, drain the entire contents of the fuel tank and replace it completely with fresh gasoline.
Usually water contamination isn't "pure" water. You may find that you end up with plugged passages. The only way to know is to remove the water (after you have confirmed that this is the problem) and see what it does. If it straightens out, great. If not, there's probably a chunkie stuck where it shouldn't be.