Rebuilding a old 5 hp Briggs. The gas tank (side mounted on engine) has a lot of black gunk in it almost like tar. Ive scraped and flushed but cant get it all out. Anyone know what will desolove this stuff?? Thank, Don Wertz

Most "gunk" left from old fuel can be removed with carburator cleaner or lacquer thinner.  Take the time to let it soak.  most of these solvents evaporate rapidly so keep the lid on the tank while it soaks.

To bad they outlawed this in most states.

Methyl Tertiary Butyl Ether

We would use it as fuel for the spring tune-ups.

Run the motor before the work was done.

Cleaned the tank to the carb.

I would try pouring some "Sea Foam" into the tank and allow it to set for a few days. Pour it out of the tank and strain it and put it back into the tank again. Sea Foam does wonders with bad gas. You may want to try some Marine Boat sales or yards and see what they use in restoring tanks that are corroded as yours is. A good friend bought a large boat with permanent tanks, (non removeable) anyway his carbs kept fouling and the filters would plug up. He went to a large Boat Dealership and ask the parts man what they used. He bought a few cans and it fixed his problem. Not sure what the product name is.