This is a problem I have been looking at for a while. There are two separate issues here:
1) Increasing the load on the engine.
2) Opening the throttle.
If you increase the load on an engine it will naturally slow down. This will reduce the airflow speed through the carb and hence the suction at the jet, reducing the fuel flow. Most model engines drive propellers in boats or aircraft where the load drops with the speed so this problem is not so evident.
If you open the throttle the vacuum in the inlet drops, this also reduces the suction on the jet and reduces the ability of the air to hold the fuel in suspension so some of the fuel condenses out onto the inlet pipe wall causing leaning of the mixture. This is the main reason for accelerator pumps (or other enrichment devices) on full size engines.
I have been playing around with a constant depression type carb in this thread:
http://www.modelenginemaker.com/index.php/topic,9601.0.htmlHowever as George says you then need a constant fuel level which needs pumps and float chambers or similar.
If you want to build an accelerator pump you will be halfway to making a fuel injection system anyway. My 1”bore 2” stroke fuel injected engine has a 2 mm diameter pump plunger and a working stroke of around 0.5 mm to deliver the full quantity of fuel per stroke.