That's good you didn't have to replace the spin-on which I think is about $20. The primary must have protected it I guess.

so what is the procedure for removing the fuel tank screen? I've never had to do that. Do you need that huge 46mm socket?? I guess mine is OK but for future ref.

What I really need to check though is the one in the WVO tank, I think I know how to do that one - inspection plate on top, cordless screwdriver.
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PLYMOUTH DELUXE

 

If what was in the tank was a blackish gray and slimy you have something growning inside of the Fuel Tank.

There is a product made by Startron that is a boicide (will kill the infection) and has an enzyme that eats up the bodies of the microrganizm so it can pass through your filters.

If you use that you most likely will not need to pull out the Fuel Tank.
If you go to buy some besure to read the label because Startron makes several products that do different things.
If you buy another companies product be sure it tells you there is an enzyme in it; or the dead bodies will plug up your Filters.