You've got half a forest in that tank! Wow!Well, I found the problem and I'm guessing this is probably a first for most of you.
Knowing the tank was plugged, I cut an inspection/clean-out port just over the fuel sump. This involved carefully cutting a 1-1/2" hole with an exacto, which took a while. The following pic (after debris removal) is the hole plugged with rubber compression plug found at Menards for ~$3.00. After filling, it doesn't leak at all, so this is a good alternative to the 8 hours+ or so it would take to pull the tank.
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Early last summer, I made a couple passes around the yard after refueling and forgetting to put on the fuel cap. Apparently, I must have brushed by the pear tree as the following pic shows what I found inside. Of all the damned places for a pear/branch to fall, where does it go? A swish shot right into the tank that would have impressed Michael Jordan. No surprise, however, as I've known for years where I fall on the bell curve of fate.
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Anyway, after pumping the tank nearly dry, I was able to suction all this crap out of the tank with pump and various grabby tools. Refilled with about 1/2 gallon of diesel and repeated the suction. Small pen light showed nothing of any concern left in the sump, though I fully expect to have to replace the first fuel filter again. Not a big deal as I now know how to do that quickly.
I'm sure when I mowed without the cap, I was filling the tank got waylaid with someone else's problem for an hour or so and forgot the cap, as I have never done that before. While this remedy was certainly faster than pulling the tank, I have learned that from now on, I don't care if the house is on fire ... I will not stop the refuel until the cap is back on.
Also probably wouldn't hurt to make some sort of micro-filter insert for the filler hole in the tanks as well.
Glad you were able to figure it out.