"I bought a moped that had been sitting for many years and the fuel had dried up in the one-gallon tank in the frame. I was able to fill it and get it running but over the years, the gas would quickly turn brown and particles would cause problems in the carb jet." How old is this machine - 30 years? It sounds like you bought a Moped that had sat for many years and that you have had an ongoing problem for many years since then. The sludge is the result of oil and gasoline being left to evaporate many times. I would clean all of the fuel system out completely. Then be sure to keep the tank filled with fuel and use it regularly. "I decided to fill the tank with NAPA carb soaking cleaner and with the use of air bubbles, sand, soaking time, removed all the deposits, which amounted to about 2 cups worth." Two cups of sludgy deposits?? Wow!!

I have never tried to repair a plastic gas tank, but I bought an old 1981 Troy Bilt that someone has tried repairing the tank with silicone and it is coming loose. I have no idea how long ago the repair was made but I will be trying to repair it in the future with something else. I have had pretty good luck using E600, Goo or maybe goop, I have also used a product called "through the roof". But none of these repairs have been made on a gas can. I often use some type of webbing like fiberglass or dental floss embedded in the glue. I have a plastic mirror mount on my pickup that was broken several years ago and it is still holding. The E6000 and goo or goop both can be found at walmart, the through the roof product I buy at ace hardware.

probably kids sticking thing in holes or you live in the drought area of the country? I just use pressure washer or garden hose (course pressure washer would be better), then bottle brush, pressure washer, rinse with gas or alcohol. Course you just can't stop there. replace the fuel lines, filter if it got one, clean the carb. change the oil, and check the air filter. Remove the spark plug spay 5 second blast carb. cleaner down the hole. motor the engine (pull the rope several times) to blow out any carbon while priming the combustion chamber. Clean the plug, check the gap, hit it with carb. cleaner install it and fire it up after you put gas in the gas tank.

I am not so sure that the 2 cycle engine is always Echo. I have parts diagram of a lot of the Mantis, they do not look the same. They have at least 3 different 2 cycle engine that I can tell. Did you tell the Mantis people what model serial number and ask who make the engine for them? You might want to download your IPL and manual first before you do further search. they have it on their site, I think you just need to match the serial number.