This pump is a self priming water pump mounted to an ACND Wisconsin engine. I have never seen this and wondering if anybody knows. Is it for psi, flow, suction? or none of them.
I am just speculating and maybe someone has a parts list but I would guess that is a trash pump and that might limit the size of the aggregate or somehow keep stuff from becoming lodged in the impeller. I think it has something to do with regulating the flow due to the fact that if it runs too slow all the crap simply rattles around in it and this lets the flow be reduced while letting it run at higher speed but thats just a guess.