Question about sludge from old gasoline

I cleaned an old boat gas tank using and acid for cleaning wheels. I tried everything else to get the stain off of some glass parts and nothing but that acid would touch it. Unfortunately I don't really know what it is, but they call it acid and it does sting your hands after a little while.

I've used a product called" Berrymans Chemtool " for that purpose...worked well. Its a liquid carburetor cleaner thats availible in aerosol spray cans , or by the gallon. Most parts stores will have it.

I use petrol for removing old fuel in fuel lines,blown clear with a air line,stained glass filter bowls I use wire wool and soap.
Paraffin is also good.I also remove fuel tanks and hot pressure wash them.Its the best way to clean them.

I always new old gas turned crappy after a while but a had a real experiece with it a while back. Had a 302 ford engine overhauled including new valves and guides in the heads. Dumped a little gas in the tank just to be sure, but didn't empty out the old stuff. Engine started fine, ran nice. Shut down after about 10-15 minuites. Next day went to start up and it would barely run!! Upon closer inspection, found almost ALL of the pushrods bent, some even punched clean through the rocker arms. The valves were almost seized solid from the varnish after only 10 mins of running. A very expensive lesson. I now get rid of old gas.

Go to your local welding supplier and get a steel liner out of a mig welder, use it to go through the lines with some berrymans carb cleaner. The liner will go right through most fuel lines and scrub the gunk out of them. It is flexable enough to turn most curves in the fuel lines.

The fuel valve to turn from Starting gas to Main fuel tank on an old John Deere was locked up solid by glassy looking varnish. We never did get it cleaned and replaced it. I may look for some Chemtool to soak it in.

GWH - The gums and varnish that collect when gasoline evaporates over a long period of time can usually be removed with carburetor cleaner that comes in a spray can, such as CRC brand .. or paint thinners.

MEK (Methyl Ethyl Ketone) and acetone are well known and regularly used solvents. MEK is especially useful as a final tank cleaner.
There are numerous types of paint thinners, each one especially formulated specifically for acrylic, enamel or urethane paints. Toluene is generally a major component, but xylene is also used. All these volatile compounds are good gasoline gum and varnish removers .. but use great care when using them, they are highly volatile, and you need good ventilation when in use.

If there are "pot metal" parts in the carburetor (I can't recall the material the Sixty carburetor is made from), this zinc alloy will often react badly with the decomposing fuel, and leave some nasty deposits that are hard to remove.
I've known of carburetor repairers who have used a combination of acids, such as phosphoric acid, citric acid and acetic acid (vinegar), in weak solutions, to clean up the pot metal. This takes a deal of experimentation, and generally none of the experts are prepared to reveal just what combination and strength of acids works best on carburetor pot metal clean up. Soda blasting is a good fall-back treatment when deposits are difficult to clean. Soda blasting is good for items such as carburetors, because it does not damage the underlying metal.

Re the tank .. a common trick is to remove the tank, fill it with a handful of small nuts, plus a liquid solvent .. attach to a rotating mechanism, and rotate it for a period of time to clean up the inside. I've seen posts where restorers state that they strapped the tank to a tractor wheel and drove around, or strapped the tank to a concrete mixer agitator drum.
A solvent such as water with a cupful of laundry powder will work just fine on this style of clean up .. but the tank needs to be pressure cleaned and dried after this method is used, and the tank interior then coated with a good tank sealant such as POR-15, Kreem, or Red-Cote.

Others regularly advise a 5%-10 solution of Hydrochloric (Muriatic) acid as a tank cleaner. This solution is very aggressive, can't be left in the tank too long (an hour or two), and requires great care in use, with gloves and face shielding necessary.
I have know people take their tanks to a radiator shop and ask them to dip their fuel tank in their radiator bath (which is usually a 5% solution of Hydrochloric acid, sometimes with other cleaning additives as well. The radiator shop may or may not be interested in doing this, it all depends on the manager or owner.