1959 12hp problem

While on vacation in July, we took our Elgin 12 hp for my son to use on the boat that came with the place. This motor hasn't been run in years however I did replace the impeller and fuel pump diaphragm before we left. We put it on the boat and with the exception of it "sneezing" at low speeds, it ran pretty good. I then proceeded to take the carb off and clean it up. To my surprise, it was spotless inside so I gave everything a spray of carb cleaner and put it back on. The motor started great, usually on the 1st or 2nd pull, it also had a smooth idle. However, it had no high speed, it barely got the boat on plane - which it did before. I tried all different settings on the high and low screws with little change. While it neutral, it will rev up as normal, however once you put a load on it, it's as if the throttle is only half way. This model does have a key which is long gone, I did remove the whole assembly so I know that's not it. The carb screws are all tight so it isn't an air leak, however pushing on the carb one time did bring it to life and it ran much better but only for a very short time. We don't have a boat to put it on so testing might be difficult, I would need to see if running in a barrel in gear is good enough. Any thoughts on this?
Thanks, will give it a try, makes sense considering I didn't replace any gaskets and pushing on the carb made it run better that one time.