Not sure on the carb setup if it is a 2-jet CV, or 3-jet CV, but if it happens only when you punch it, I would be looking at the main jet, jet needle, and emulsion tube on the 2-jet, and the needle jet, jet needle, and secondary main jet on the 3-jet CV. Not all that tough, just take your time and clean off all of that varnish, just be sure to keep the rubber pieces and plastic pieces away from the cleaner.
Kelsey