1978 Mercury Thunderbolt 500 50 HP Throttle Issues

Hi everyone, I recently bought a boat with a Mercury Thunderbolt 500 50 HP engine on it. The guy I bought it from was an as-is where-is how-is kinda guy who didnt want to put it in the water but had it I'm a bucket on the trailer and it fired right up and purred like a kitten. However after buying it and taking it to the water the first time I discovered that yes it starts and idles fine but once I'm out and open it up it does very little. I discovered I had a broken throttle plate so I replaced it. Headed back out and again once I go to open it up theres very little throttle. Now if i pop off the covers and manually open the butterflies on the carb with my finger i can achieve wide open throttle and I'm ripping around the waterways as long as i hold it. If i adjust the position of the throttle plate so that I can achieve wide open throttle I end up with a very high idle and an engine that wont start. Not sure what to do

Sounds like the linkage to the throttle plate needs fixed.