I've never tried modifying the carby to the extent of changing springs or jet, but I could only imagine the power you'd get from installing the higher output jet and heavier spring, coupled with the correct poppet valve position. That said, another component which may relate is the flywheel. If the different flywheel generates a different amount of vacuum, that would affect the suction to the top of the carby and subsequently how the rest of the machine runs.
That's why I keep suggesting knowing exactly what was supposed to run on what mower in the first place. With that information, we may be able to tell the cause of the initial issue. Is the PT from an 18" designed to run on a 600 correctly? Are there required changes to the flywheel, carby, etc.? I don't know, but as I keep coming back to here if the mower didn't work... if everyone had that issue... wouldn't Victa have known about it? Addressed it? Why would they keep selling it if they knew it didn't work? Wouldn't every person who has ever purchased one reached out to Victa or reached out here with the issue?
I believe the stock power of the VEX60 in the last generation of PT's was 8.5Nm? I always laugh when I see massive 4-stroke engines, twice the physical size of the VEX60, boasting 10Nm. I often just think... if we kept going with the PT, if we got those figures where they could be with the mods listed above, they'd blow it out of the water.
That said, that may actually be a reason for the 8.5 rating. That may be with those specifications, so as the engine has max power while keeping under the old emission restrictions. If you scrap the fuel efficiency and fuel economy, who knows where the PT would be at. Put in a Green spring, plain face jet, correct poppet valve position... mmmmm haha.
