Is the vapourizer/regulator/heat exchanger functioning properly?
Every LPG system on a vehicle will have a preheater for the propane fuel. the preheater is part of the regulator/vapourizer assembly. It preheats the propane before it is injected into the engine. This helps warm a cold engine sooner and keep a warm engine from being blasted with a super cold fuel mixture. My guess is, without a properly functioning propane fuel preheater your engine is constantly being injected with a very cold fuel mixture. (just hold your hand in front of a propane bottle and open the valve slightly to see how cold).
If the engine is not warming up then the choke will be on. That rich air/fuel mixture should warm up the engine, but in the case of propane this will not be the case. Most people I know running a duel/fuel system. Use gasoline to start the engine and switch to the propane when the motor is up to operating temperature.
The vapourizer/regulator generally needs to be rebuilt every 10 years or so.
Here is a link to a catalogue page for the regulator that you probably have.
http://www.impco.ws/pdf/MASTER%20CATALOG%2010-10.pdf
Page 203.