Hi Pe

Carb pressure test ok.

Your question about the exhaust made me think

I had to make a maifold for the engine to mvvs silencer for the model I am using it in , the standard MVVS would not come close for alignment

I attach the photos of mine

I did smooth the gas flow in my version so itis not the angled face theoutside would lead you toconcluded,but even so I can now see that there could be a back pressure into the cylinder. do you think this is the cause given we have tried everythink else!

Ian