Lots of reasons. Blow by from a bad piston ring (worn ring) can do it, as can high operating temps and/or poor oil. A cylinder hone will ruin the cylinder in a heart beat. Just use #600 sandpaper kept real wet with acetone, and don't worry about getting every little bit out. Then install a new Frank Bowman high performance ring, (about $11.00), and you'll love how the engine runs.

Frank Bowman also has a very good mod for the Homelite 25cc engine. If you send him your piston, he will install a pin in the ring land so that the piston ring cannot rotate, and then he will make a special ring for it to accomodate the pin. This ring will prevent blow by, add power, and it is a very good modification for the Homelite 25cc engine. Once set up this way, you can grind that bridge out of the exhaust port for much improved flow. I've done a bunch of them that way. I hate to quote numbers, but I have seen gains of 600 to 800 rpms with these mods if used with an 11mm carb and a free breathing exhaust.

AV8TOR