Why would you think the stock carb wouldn't work at full throttle? I have even used "2 stroke" carbs which don't have the "helper" spring on the diaphragm, and they even pumped enough fuel, even though they typically have much stronger pulses from crankcase pressure, rather than using the vacuum in the intake port like on a 4 stroke. The biggest problem with the stock carb is that it has no adjustments. It's jetted for a string trimmer, when you increase the load (with a propeller) it needs more fuel. When I first got my engine, I ran it all by itself, the way it comes out of the box, and it ran great, with unbelievable throttle response for a 4 stroke, of course it wasn't turning anything. Then once it had a prop on it, I realized it could idle much much slower with that large flywheel, (if you deactivate the compression release) and it needed to be richer at all throttle settings. I thought about changing jets, but that is trial and error, so I ended up buying a carb with a high speed needle, (they all have a low speed needle, if you can dig it out)