I recently trashpicked a PT Victa and got it going pretty easily except it would behave like it ran out of fuel at a certain level in the tank when there was still plenty left.
I took out the brass filter that's attached to the fuel tap and tried acetone in a small jar in my ultrasonic cleaner at first, but after a few minutes, no sign of varnish residue in the liquid. Then carby cleaner. Same story. I thought something acidic, something acidic. Aha! Lemon juice. So a freshly squeezed lemon later, I immersed it in a tiny jar of the lemon juice and ran the ultrasonic cleaner for a few minutes then let it sit a few more before rinsing and blowing it dry. It looked cleaner and the mower ran fine, although I had quite a bit of petrol so will need to be ran some more before I know for sure what difference it's made.
Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!