Roy ,I have a hemmens max2 engine that was a improved version of your caton i have not had that problem but it is nearly the same engine ,what i do know is they like running at a high psi looking at your topaz thread if that is the engine you are referring to you indicate you are running at 40 psi or under i thinkyou should run at least 60 or above its normal to run that engine at 70/80 that is something you could try,you also want the exhaust to be a large diameter as this will create a back pressure if it is to small,you could try running at a higher pressure with the exhaust disconnected,obviously there are more experienced people on here that will help you and you could also ring or e/mail john hemmens he should be able to give you first class advice regards Allan
