I'm working on a Peugeot moped that was running great until something happened. The bike was filled with high octane fuel (instead of the regular grade that was usually put in) and ran for awhile, but then ran rough and stopped running at all. I've done a little troubleshooting. The problem seems to be related to spark. If I take the spark plug out of the cylinder and ground it against the head it sparks great. Seems like a consistent, fat spark. But once I put the spark plug back in, it doesn't spark. I have a timing light that you put over the spark plug wire and it flashes if it's sparking. So the timing light flashes when the spark plug is out (and I can see it spark), but nothing when the spark plug is installed. I've pulled off the muffler to make sure it wasn't clogged, and it made no difference. Any other ideas? I'd really like to get it running again. Thanks.