Chainsaw Repair

Hi Guys.

I have an Efco mt4100s chainsaw. Having said that the symptoms apply to any saw really. I have recently serviced this saw and according to the customer it was running fine prior to me servicing it. Now he has got it back and used it, he says it starts and cuts fine for about 20 minutes and then it starts to bog down in the cut and lacks power. I've had it in and factory set the carb- again (slightly richened the L setting) and done some test cuts and handed it back. Now it's come back again with same issue. I have swapped over the module with a known good one and re-checked the carb settings and changed the plug. What I did find though, was the new fuel filter I fitted at the service had become detached from the fuel line and is rolling around in the bottom of the tank.....is it entirely possible this could be the fault. After all, this is something I have done when I had it in for service, and the guy did say it wasn't doing it before?

How old is the saw? Could be crank seals.