McCulloch Chainsaw Sorry must of posted in wrong forum before

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. tried another Spark Plug and still the same.

Pulled apart carby but didnt remove adjusting screws. Checked diaphrams in carby and could see any holes in them. Cleaned out with 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.? Have since done a check and that would appear normal I think.

I also found that since now adjusting the main jet it has no effect on the motor. With jet fully wound in it still peforms the same and did note when jet out by about 1 1/4 turns and it conked out at this jet setting I could see a very thick spray from main jet as it stopped. Hence trying to adjust mixture with no luck.

Since doing this I can stop the motor and start immediately and idles fine. But leave the motor about 3 minutes and it wont start again.

Someone else suggested Muffler but unsure why that would affect it but can give it a clean

Any one help with any ideas please.