You can make the holes marginally bigger, but bear in mind the ‘non-reserve’ port has to take a tube to create the reserve. That said, if you can find a slightly larger bore tube (doesn’t have to be nylon. Alloy tubes are available from good Model Shops) to fit the counterbore of the ‘non-reserve’ port & enlarge it to suit, then all the better.
I think the real trick in increasing the flow rate is to ‘port’ the valve face. That is the item that is turned through 90° to ‘ON’ then through a further 90° to ‘RESERVE’
What I do is enlarge the kidney shaped slot with a small round burr of the type that fits a Dremel or equivalent. In actual fact, I extend the slot so that it covers 180°
(In fact some taps used to be supplied just as I am trying to explain, but I haven't seen one like it in a while. Of course, I'm presuming that your slot only covers 90° GULP!)Before you conclude that I’m mad (you might be right) I enlarge the slot so that @ the ‘OFF’ position, the ports (as you look @ your second photo) are connected from 9 O’clock, clockwise to 3 O’clock. Think about that. ‘OFF’ will still remain ‘OFF’
Now, imagine the valve turned through 90° to ‘ON’ you should conclude that there will still only be the ports @ 3 O’clock & 6 O’clock connected although the slot has now been extended to sit over ‘blank’ 12 O’clock
Finally, the valve turned the extra 90° to ‘RESERVE’ then the ports @ 3 O’clock, 6 O’clock & 9 O’clock are all connected
I can assure you that this trick works & the beauty of it is that if you want to get an even better flow rate for some reason (racing is a good example) all you have to do is remove the tube from the tank side ‘ON’ port though the result will mean no reserve
This is what some of us used to have to do before fast flow taps became available for sale. Race port our fuel taps!
