I actually use carb cleaner from an autoparts store for blowing dirt and stuff out of motors. Old fuel does work in a pinch, but I worry about leaving "cold" fuel in an engine, it seems to promote rust. The carb cleaner will wash away oil as well, so when I do that, I follow it up with a heavy dose (not drops, big squirts) of oil. I prefer automatic transmission fluid for this, as it penetrates and is good at getting rid of dirt and water.

I also pop the head, backplate, and muffler off so I can see if there is grit in there and get a clearer shot. For example, if there is dirt inside the crank, and you just shoot in through the carb, but don't pull the backplate, you just forced more dirt down deeper in to the engine. So pulling the backplate allows everything to come out before it reaching the piston/liner area.

If you don't pop the head, at least get the backplate and muffler and glowplug and have a good look.