Have you removed the governor? That will give you more control over throttle position.
I don't know just how much more control that will give. If everything's set up and functioning properly, there should be a smooth transition from idle all the way to WOT, even with the governor. I have several governed small engines, 2 on karts, 2 on riding mowers, 2 on push mowers, 1 on a generator, 3 weedies, 1 snowblower, and several other assorted. The governor is intact on each, and each throttles smoothly, including the clutch equipped units.
I think the problem lies elsewhere. Namely, a bad throttle linkage arrangement.
With nothing else to go on, I'm guessing too much slack in the cable, combined with a short pedal throw, or something along those lines. Got a pic?
If the governor is intact, make sure the throttle cable is connected to the proper throttle arm, not the carb butterfly. Secondly, with the engine off, slowly depress the pedal, making sure the cable begins to move the throttle arm almost immediately. Third, ensure that as you continue to depress the pedal thru it's range of motion, that the cable smoothly and proportionately actuates the throttle arm. Fourth, that the pedal travel allows WOT on the throttle arm. Fifth, that the pedal STOPS at WOT. Don't rely on the throttle to stop the pedal's travel, fabricate an adjustable (or fixed)pedal stop.
Lastly, ensure that you have return springs in place to positively return both the pedal, and the throttle arm.
