Thanks. When I took the carb off, which has been undisturbed for over 20 years (and since new) the gasket was definitely next to the tank. If you're sur I'll give it a go. But I must admit it would make sense.
Ok so the engine is about 20 years old but it has never ever let me down before. From cold engine starts and runs as normal. But after about a minute the revs start to slow down and then it shuts down. Remove spark plug whilst still hot, it sparks fine but appears dry. Any ideas please.