My girlfriend's Vespa 50 has a really bad misfire when the engine gets a little on the hot side, for example, after riding at a decent for pace for a good 20 minutes or so. The misfire will then appear out of nowhere and continue to do so every few seconds, making it very difficult to ride. The misfire quite often causes a bit of a backfire through the exhaust, leading me to think that it is an electrical problem.
The ignition coil looked pretty rusted up and the HT cables were knackered, so I replaced these items, along with a brand new plug and cap. It now has a good strong spark and I thought I had solved it, but unfortunately, it still misfires
I now need to diagnose the problem properly. Any ideas what could cause this and where I should starting looking?