First let me say I have no experience with Zenth carbs so my input will be based on general carburetor performance.
What do your plugs look like after running? Are they clean with just a light coating of gray or tan ash?
Just because you can hit rated speed with no load doen't mean your fuel delivery is adequate. Can you get rated speed under load? If you can't, either you lack fuel or your air cleaner is clogged, limiting power. To eliminate the air cleaner, disconnect it from the carb inlet. Unless you are in a very dusty environment, it won't hurt anything. Generally these carbs are of a pressure balanced design so that fuel air ratios are not affected by air filter restriction but power development, being directly proportion to air flow, is.
Do you have a fuel bowl drain on the carb? if you do, open the tank valve(if you have one) open the bowl drain and holding a suitable container uner the carb, verify a CONTINUOUS GUSH OF FUEL, like a cow peein' on a flat rock. If it dribbles or is intermittent, you have a fuel delivery problem.
When your engine stops, does it quit immediately or does it miss a few times and then stop. If it quits immediately, then imediately check for a spark. I mean right now!
You need to sort out whether your problem is fuel related or ignition related.Then you can find the root cause.
Remember that the most important thing about the carburetor is that all the passages must be squeaky clean and to get them that way, you need to soak the carb in real carb cleaner for 24 to 48 hours and then blow out the passages with compressed and run small soft wire or mono fishing line through the drilled passages to make sure you get all the crud out. That includes the passage with the float valve also. A small grain of sand, etc in there can inhibit flow enough to cause your problems. You changed the float setting. Which direction did you move ? To a higher float leve or a lower float level?
If you have a bowl drain, you can make a simple tool to measure the fuel level in the bowl externally without having to open up the carb except to change the fuel float setting. Using a brass fitting that fits the bowl drain outlet, attach a piece of clear tubing. Attach a 90° el to the tubing and another piece of clear tubing to that in such a way that the verticle tube of this the L shaped device is near the carburetor and extends a few inches above the fuel bowl. (Depending on your bowl drain orientation, you may need two els.) When you openthe tank valve, the tube will fill with fuel to the level in the fuel bowl. Since you measure the float level relative a split flange on the carb, you can see if the fuel level is at the correct height.
You say there is noadjstment for the carb. How does it adjust for power enrichment and ambient temperature changes? Does it have additional fuel circuits besides idle, part throttle and full power?
These types of carbs are relatively simple and if they have problems, a thorough cleaning and the proper adjustment are all that is needed to get them to operate properly