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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.

debris in fuel tank clogging pump
Pump diaphragm worn out

Thanks. There was indeed debris in the tank. I can't see a pump as such, but I changed the carb which came with a new diaphragm.

The diaphragm you changed is the pump
The long tube has a gauze on the end that gets clogged
The diaphragm has to go against the tank then the gasket & finally the carm on top.
Very common for people to get them in the wrong order

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.

the tank has 2 little holes in it which combined with the flaps on the diaphragm makes the pump