Gas rc engines
 

LOL Whats funny is a few years ago I was edging my lawn and I touched the edger to shut it off and it shocked me pretty darn good. Scared me to. Thats a good question though, my guess would be the ignition system

Gas rc engines
 

It's the spark plug. Gas engines either have a magneto or a battery to run it. What happens when you touch it, is you are grounding yourself and thus creating a passage way for electricity to flow. And yes, I figgered' that out the hard way too. It'll wake you up better than caffine...