Actually Grumpy, this is less about modifying Victa's speed, and more about restoring it.
The engine in question is either a Series 70 MKIV or a Series 80, so at it's latest, this is still an early 80's machine, most likely running a G4, but possibly an early LM.
This form of issue is very common with these carburetors, after this length of time, particularly after disturbance to the carburetor (issue appeared greater after carby was rebuilt)
Over time the lower part of the cam stem wears, lowering the cam, and in turn the lifter, this lowers the idle position for the diaphragm, resulting in a higher idle.
The washer in question is a part made by victa to correct this issue.
Further, the large metal plate used to lock the diaphragm in place can cause a set of issues of its own, and at service time, many repairers simply remove it, however, it is always a good idea to replace it with the smaller type in that case, to prevent the poppet tip wearing through the rubber diaphragm, often here, due to the smaller clip being thinner, the same issue can occur, and the same washer will remedy the situation.
