losing power at wide open throttle

[motorhead255]

i have a 78 lf with a 77 lf 440 engine when i'm on hard pack or in deep snow and hold it wide open for longer than 3-4 seconds it starts to lose power and pretty much just drops off. i've done abunch of reading on reading plugs and have done the tests and from what i'm seeing is the engine is running pretty rich the carbs are still completely stock (jets and settings) now my question is, will it do what its doing if its running rich? or is it something else? it seams to be worse the longer i ride it. also i've seen the info in the fine tune manual, i wonder if i should give that a shot?

[AirborneX4Special]

Check the choke (enricher) settings. If it's anything like the carbs on a newer LF, the thing will just run worse the farther you go if the chokes are not set to proper length.
A fraction off and it fouls plugs like crazy.