On 18 January 2017, Gary Lock in Queensland, Australia wrote:
"Any suggestions to clear a blocked pickup line? The tank has is in excellent order after sitting for 20 years with stale fuel inside. I've cleaned out the insides with POR Marine Clean, but the intake line appears to be solidly blocked. I've tried compressed air but no joy. As you know, the brass right angle attachment to the tank does not encourage any wire or otherwise to be pushed down the line, because of the right angle. My next attempt would be to drill a 1/8" hole in the brass fitting to try pushing something straight down the line, and braze the hole up again later".

Sounds like he is well on his way to both diagnosis and the fix. You can try all kinds of solvents, but when it is clogged so tight that compressed air won't clear it, then solvents are not likely to get through.

If you can rig up a pressure tight fitting with a 1/8-NPT pipe thread, you could use a grease gun to apply much higher pressure, and that would most likely work. Fill the pipe with mineral spirits first to eliminate as much air as possible, Then go after it with the grease gun. Once unplugged flush through with your favorite solvents to completely clean the pipe.

No useful pusher wire will get around the 90-degree turn in the external fitting. Alternatively you can drill a small hole (maybe 1/8-inch) in the elbow for in-line access for routing it our with a stiff wire (suggest speedometer cable and electric drill). When finished you can solder the hole shut or drill and tap for a screw plug (maybe #10-32 screw with a sealing washer). I would not recommend open flame for brazing or soldering, but you can solder using a large copper soldering iron that is heated with a torch (keeping the torch far away from the fuel tank).

Addendum February, 2022:
Here's another idea from Greg Reynolds
I'm cleaning out my fuel tank and read that sometimes the pick up pipe can get clogged. Of course, mine was too. I had success unclogging it with carb cleaner and an 1/8 inch steel cable.

Instructions:
Dribble carb cleaner into the outlet of the pickup pipe. Let it stand of a day, add some more and let it stand for another day.

Get 2 feet of 1/8 inch steel cable from the hardware store. Use a pair of pliers to bend the last 1/4 inch of the cable. Use a cold chisel and a good sharp whack with a 2 pound hammer to cut the cable for a fresh tip.

Slide the cable into the outlet elbow with the bent tip facing the fuel tank. With a little fiddling the tip will catch in the pipe and the cable can be slide down to the clog. Give the clog a few pokes just in case it will give up without a fight.

Slide a piece of small diameter rigid tubing over the end of the cable. This provides a handle and keeps the cable from whipping. Chuck a portable drill on the end of the cable. Turning the cable one way will tend to unravel it, the other way will keep it tight. Turn the cable with the drill in the direction that keeps it tight and work the cable back and forth against the plug until it breaks through.

What looked like a gob of tar came out of my fuel pipe. Turning the cable in the direction that unravels it will expand it and clean up the pipe. If you get the cable stuck in your fuel pipe, I don't want to know about it. -- Greg