On most of the Homelite weed whacker 25 & 30 engines, there is a little hole above the exhaust port that is used for (EGR) exhaust gas recirculation. Everyone taps the hole slightly and put a short 10-32 set screw in the hole. Don't tap very deep or you will penetrate the cylinder wall and cause some of the Nickasil to flake off around the hole.

On some of these, there is another hole above the intake port that has a 1/8" NPT valve in it. This should be removed and a pipe plug put in the hole. This hole and original valve is the compression release for the electric start versions.

To my knowledge, the Zip Start deal was a different ratio on the pull cord pully that made the pull cord easier to pull?

Enjoy,

Jim