injection pump timing

Having just gone through timing my B5100 (two cylinder, 500cc, same gear setup) I can give you a few hints.
First one shim changes the timing a lot, so start there.
Second the timing marks on the gears as shown above only come around every 138 revolutions of the crank
This threw me for a loop but if you consider the idler gear has 69 teeth the math works out and the timing could be correct but look wrong.
After you set the gears correctly as shown above and turn the crank two turns that normal thinking (mine) is the lines will line up again, not so
This is because the idler gear has 69 teeth that is in no way related to 25 teeth on the crank or 50 teeth on the cam and pump and two of the reference points are on the idler.
This had me going for several hours

The pump it's self does not have a gear, there's a camshaft that drives the pistons on the pump that stays in the block.
Before attempting to remove the pump read the manual carefully. On mine there's a cover with 4 bolts on the side below the pump that needs to be removed and a spring disconnected. This is because there's a pin on the "rack" that's on the pump that is operated by a slot in a fork and the engine block casting has a area that allows this pin on the pump to be removed. The fork is thin cast iron, expensive and easy to break, ask me how I know this.