Fountain pens rely on gravity to draw ink from the top of the nib down to the tip via the feed prong that runs the length of the nib. The space between the two prongs of the nib and the space between the feed prong and the nib both act to determine the amount of ink that travels the feed and ends up on the paper. Adjust the spacing between the feed prong and the nib and/or the spacing between the two prongs of the nib to increase or decrease the amount of ink flow.
Test the present ink flow by holding the pen between the thumb and forefinger and resting the nib on a sheet of paper. Draw the pen across the paper exerting no downward pressure on the pen in addition to its own weight.
Look at the ink left on the paper. A visible, dry ink line means the pen needs adjusting. A thick, wet or gobbing line means the ink flow requires adjusting.
Try inserting a piece of paper between the feed prong and the nib. If the paper slides in, bend the feed prong to lie closer against the nib and reduce the space between the prong and the nib, thereby reducing the amount of ink that accumulates in that space. Immerse the nib in near-boiling water for about 15 seconds, remove the nib from the water and bend the feed prong to lie closer to the nib. Hold the reshaped prong for about 30 seconds while the nib cools and hardens in the new shape.
Close the distance between the two prongs of the nib by moving them closer to each other, thereby reducing the amount of ink that accumulates in that space. Bend one nib prong toward the other, then repeat with the other nib prong.