They should be a 5mm hole atm. I'd go up .5 of a mm at a time until you find what works for you and your pump.
Take a cordless drill and a few drill bits with you next time you go out. Only takes a minute to take them off and drill the holes.
I've never encountered this problem before.
That...
Pull the end off that's not glued and slide the whole lot out.
Either the end caps arnt glued in or the other end of the spray bar at the top of the hopper.
Either way you will get them off.
G'day Ben,
Sounds like you could run that a bit steeper with a bit more water, but in saying that 3 shovels full in 30 seconds is a lot of material especially with the amount of black sand in the Eldorado area. It needs time to clear.