problems when under load...

[Coyote]

OK, I have ran into a problem I figured I'd put out and see if anyone has an idea on what steps to take next to fix it.
My Cub will start and run fine at idle. There is some surging, but nothing awful. It will rev up when you hit the throttle, again, with some surging, but nothing too bad. the problem comes when you put the tractor in gear, or try and spin up the belly mower. Any kind of load, and the tractor will bog down and quit. It seems to be running fine... just has zero torque.
So far, have;
cleaned the fuel tank, new fuel lines, fuel filter and new on/off flow switch.
cleaned the carb, adjusted the choke and throttle lines.
cleaned all the throttle linkage, greased the fittings.
checked for good spark from switch to plugs.
Not sure exactly where to go next. Any ideas what the problem could be? thank you all in advance for any help you are able to give, or any direction I haven't thought of yet.

[Coyote]

forgot to add...
distributor not generator, and we opened the govenour, cleaned it up. also, the mower was serviced, but also have a problem when not using the mower

[BigBill]

Did you inspect the hole in the high speed jet? Did you disassemble the carb and clean it in a carb tank.? Also that brass tube that’s on an angle requires a gasket under the nut that secured it. How’s your float level?

[Eugene]

Surging is typically a fuel related problem.

However, before starting on the fuel system, check the compression, ignition timing, valve tappet adjustment.

All that done and everything satisfactory - - check the linkage between the carburetor and the governor arm. Then everything BigBill listed.