Q: I’m a long-time reader of your excellent mag and I also suffer from petrol addiction! I have a Moto Morini Corsarino 50cc four-stroke that has started to develop a running issue. After a couple of minutes on tickover or at speed, the bike will start to miss a little and eventually cut out. I have fitted a new plug and lead along with a brand new carburettor found in Germany on eBay but with no noticeable effect on the original problem. Fresh fuel is used and the flow is good, but the plug is quite black after running: both before and after the new parts have been fitted. I have no real experience of vehicle electrics so haven’t started on this area yet. Please can you advise on what else to try?
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A: So, just to clarify, the engine starts OK and runs normally for a short while including actually riding the bike, but then stops? You tell me that the fuel flow is OK and I assume you have checked that you can run off a good quantity of fuel proving that the air hole is not blocked. What I really want to know is after the engine has stopped, then what? Does it go again after a few seconds or does it need to cool down completely before it will work? I think you need to be ready with your spark plug spanner next time to see if you can see a spark. I suspect not as I think this is most likely to be an ignition fault. It sounds like something is failing after it has been in use for a while, and I wonder if that something is the capacitor? If the dielectric (insulating layer between the plates) is breaking down, perhaps as it warms up, it would short out the points and the sparks would cease.