Hi Byomarimba
I will say it has been a fair few months since I did any work on my pcn4040, so a bit hazy memory.
If your idle screw it that far out (ie throttle shut that much) and it still revs mad, I would say there must be an air leak down stream. Flip the saw over and there are 4 bolts that torque down the cylinder (yes the whole cylinder is torqued down by bolts through a plastic case - asking for trouble).
check none of them are loose.
There is a rubber intake boot that is behind the carby the retaining rings are easy to inadvertently pop partially off when moving and wriggling the carby off.
check this rubber boot is properly popped into the plastic plate behind the carby. check the retaining ring is properly around where the boot meets cylinder (this is fiddly)
What is the state of the metering diaphragm under the plate shown in first photo? It is some derivative of a c1u zama carby, so this sort of diaphragm would fit
https://www.ebay.com.au/itm/121652830910?Pull those limiters straight off with flat nose pliers
Regards
