McCulloch Chainsaw Eager Beaver 2116 has been working good then very hard to start and when started if kept running at about max revs it will keep going. As soon as you slow it down or stop it and want to start again it becomes very hard to start

Checked spark plug by removing and pulling cord and earthing body of spark plug and spark still appears.

Pulled apart carby but didnt remove adjusting screws. Checked diaphrams in carby and could see any holes in them. But a bit of rubbish in gauze filter in carby and cleaned all out with compressed air and assembled and fault still the same although appears primer is working better.

I just realized that it appears that when priming the fuel cap has to be on and the fuel pipe to the tank which doesn't have the filter on [ return pipe.]? When I remove this pipe fuel is under pressure and squirts out. Maybe thats the fault.? Does the tank have a breather.?

I might add when I pulled the fuel tank out to clean I noted a breather on one side.? And found a small black sponge floating around and assumed it should go against the tank breather. As couldn't see where else it would of come from.

I did note a white sponge material also in the same place.

Does the black sponge not long there causing pressurizing.?

Any one help with any ideas please.