I'm guessing that the motor is a Tecumseh based on it being a 1977 machine. Considering the engine starts and runs at all is a good sign. It sounds like the high speed jet, which is actually the bowl nut, is dirty and will need to be cleaned. A thorough carb cleaning and replacement of Tecumseh part number 631021B would help greatly. Donyboy73 has some excellent youtube tutorials on cleaning and adjusting Tecumseh carbs.
Just a thought, have you tried loosening the gas cap and see if the engine will continue to run in fast mode? The cap vent mat be clogged with dirt.
Toro 8/24 (38080) 1989
Toro 721-QZR (38743) 2015 (Hemi Predator Power)
Toro 421-E (38585) 2008 (New Owner)
Experience is something you don't get until just after you need it.
Last edited by Grunt; 12-06-2016 at 05:27 PM.