I'm certain a slasher with a PT won't need 3 kg as you have the spinning weight of the blade plate and shaft / pulley with a slasher Norm.
I would say the 3 kg is correct for the PT TAC even though I've never looked at one in person ,in the parts diagram it looks the same as the 3 kg pulley from eBay.
The reason the edger needs 3 kg is because there is very little spinning mass without the 3 kg ( just a small flat blade on a shaft with bearings and pulley.) Also more mass helps the motor keep it's rpm without stalling under load. Disadvantage of a heavy flywheel is the rotational speed of the engine does not rapidly increase even when the throttle is opened quickly ,not a concern for an edger.
We know the full crank will need less weight than the PT and the restrictor on the decomp valve is worth doing so you don't have to add too much weight.
Also we know all the Briggs ride on mower engines that have a sheet metal pulley on the end of the crank use a heavy cast iron flywheel.
I have a couple of non Victa edgers here but never looked at them ,I would guess they have heavy flywheels ,I can always check them.
