emissions moreso i think. a smallport 3k-h with just a fast idle valve and pcv made 73bhp. a smallport 4k-c with all the emissions spaghetti and exhaust gas recirculation made 60bhp on the same scale. realistically, you'd probably be lucky to coax 40hp at the treads out of either of them
if you take off all the emissions crap and vac secondary, the 4k-c carb actually has larger secondaries than the 3k carb, and hence more potential for flow and power
either way, it's just comparing crappy engines with a 40 yearold design. i've driven a car with a stock 3k, stock 4k, mild 4k and now my fairly decent 4k. they're all nothing to write home about in stock form, but are quite alot of fun in the lighter shells, if you have the suspension and traction to go with it