The counterweight on the crankshaft is there to "counter balance" the weight of the piston, ring(s), piston pin, and about half of the rod. If the crankshaft counterweight has insufficient mass to do the job, a flywheel can be balanced in such a manner as to provide the extra counterbalance mass needed.

Some engines are entirely balanced internally, and the flywheel is "zero" balanced. (no imbalance). Other engines are "externally balanced." This is somewhat of a misnomer, because "externally balanced" engines are balanced both internally and externally with the flywheel in combination. I have no idea which weedeater or chainsaw engines are balanced in which manner.

However, if one finds that an engine has either too little or too much crankshaft counterbalance, you can compensate with an "unbalanced" prop, depending on where the weight is needed and how you position the prop.

(I bet this is as "clear as mud" huh?)

AV8TOR