I am still using the stock carb so it's the enrichment valve but looking at it,it seems to be a choke with a butterfly that reduces air to the combustion chamber. It does take some futzing with the idle knob when it's cold (ambient) increasing initially and then backing it off as it gets warm. As far as propane conversions on cars, propane and gasoline have different properties and running propane in an engine set for gasoline, propane generally suffers in comparison. But if you modify the engine(high comp, loppy cam, advance timing) propane is great. Now with modern engines they install a new ECU that changes the settings in the ignition/fuel delivery for each fuel. Back in the 1960s my Dad ordered high performance engines in his work cars and they screamed on LP. I know, as teenager I surprised guys in their camaros and mustangs. Also I've done dozens of conversions on fork lifts, commercial lawn equipment etc. and they run better on propane. Generac builds a number of engine for their generators(that they sell to other manufacturers of lawn equipment) that were designed from scratch to run on propane/naturel gas and they perform at least as well as gasoline counterparts.
I not trying to talk anyone into propane, I'm just saying it is a viable alternative and has some real advantages. By the way to the guy talking about propane freezing up. That is in liquid propane delivery systems, where liquid propane goes to a converter where (in a separate chamber) engine coolant circulates to vaporize the LP into vapor prior to entering the engine. So once again in colder temperatures, before the engine warms up, the converter (years ago they called them vaporizers!) would frost up and cause difficulties. An engine heater, heated garage or start on gasoline til warms solves that. What I'm running in the 350 is a vapor system. The concern then is the tank frosting up because the surface of the tank(cylinder) cannot vaporize the liquid in the cylinder as fast as the engine is using it. I had that one time when plowing in 10 below zero. The engine starts losing power(I guess obviously) and I just opened up the other tank and finished plowing, by the time I was done (20 minutes) the frost had melted.