Even though airborne clean and flush the tanks out thoroughly before fitting them. I believe after a month or 2, small bits come off the tank side from manufacture. The cause of our grief. When brand new, and other I have looked at, had clean fuel checks. As time goes by stringy type clearish small particles begin to appear.
During my drain valve replacement, I completely drained the tank and then ran a bit of extra fuel through as well. After doing this I no longer get particles in my fuel drain check, and my valve doesn't leak at all.
Other than the small amount of fuel that was in the tank when I picked up my trike, every single drop of fuel that has gone into the tank has gone through a Mr Funnel.
This funnel has to be the ultimate in funnel filters, it even has a water trap though being careful where I buy my fuel from I have not found any water to date.
I do also use another one at home for diesel refueling if I use a drum for refueling. I often get water filtered out of that one.
Tools & Equipment :: Mr Funnel 8" Fuel/Water Funnel - Aviation Pilot Supplies Shop | Flight Gear | Aircraft Instruments
Yes my instructor used to fiddle with the valve as well. They are usually in a worse situation as their trikes are regularly replaced and usually always new.
My recommendation is after a couple of months - Dump the fuel out and replace with new. You don't need to waste it - put it through a Mr Funnel into you car / mower etc
The other thing I have done at others suggestion, rightly or wrongly ( there are varying opinions on this ) is to only use BP Ultimate.
There are some other posts on this but in a nut shell the theory is.
Ultimate is a better quality fuel and will not have any ethanol traces in it so water in the fuel is less of a problem.It is claimed that the octane rating of fuel quickly diminishes with time so starting high means by the time it is used is has a better chance of being in the correct octane spec. Start with a fuel with an octane rating close to spec and you can only go down and out of motor recommendations. Be careful if you use Shell 98 ( or their resellers) brand fuel as it has compounds in it may cause an interaction with plastic clear tanks where exposure to UV exists. With non clear tanks or no UV exposure Shell is fine - according to what I have read elswhere .