Very often, people who have converted their vehicles to run on propane complain of hard starting first thing in the morning. Once the engine is fully warm, the vehicle starts and runs fine afterwards. The boiling point of propane is -42.1 °C / -43.8 °F so it will vaporize at pretty much any winter temperature found in the continental USA. If you've got cold start problems, it is most likely due to either incorrect setup or poor maintenance.
It is very common for propane systems to have a long hose between the converter and the mixer (aka, vapor line). Since propane absorbs heat as it vaporizes, heat must continuously be added and this heat comes from the engine's cooling water system, which means propane vapor supplied to the engine is about 180°F. The mixer requires propane supplied at a constant pressure and this is usually -1.5 WC (Water Column) for Impco systems. When the engine cools down overnight, the vapor in the vapor line also cools and its pressure drops below way below -1.5" WC. When you try to start a cold engine with a long vapor line, the converter must first build the pressure back up to -1.5" WC before the engine will run. The longer the vapor line, the longer it takes. It is best to keep the vapor line as short as possible. Ideally, the converter should also be mounted so that the vapor outlet is pointed downwards to allow heavy end residuals to drain into the vapor line.
A Start Assist Valve (Impco SV) or Vacuum Power Valve (Impco VPV) is often used to automatically provide a shot of fuel directly to the mixer for better cold starts. The difference between the two is that the SV is an solenoid-actuated valve that is wired into the starter circuit so that it opens whenever the starter motor is engaged. The VPV is vacuum-actuated and it opens whenever manifold vacuum falls below 3" Hg. The VPV is also used to provide a richer fuel mixture under heavy acceleration. Alternatively, a primer may be used to manually add a shot of fuel to the mixer. There are both mechanical (cable-operated) primers (Impco AB1-28) and electrical primers (Gann PR-4604) available.
Because propane systems run so well and are so reliable, engines are often poorly maintained. Since the propane system relies on the engine's cooling water to operate properly, it is extremely important that the water has enough antifreeze in it and the corrosion additives are not depleted. It is possible for the cooling water to turn to slush in cold weather and this will prevent water flow through the converter. Crud or corrosion in the converter's heat exchanger could make this situation worse. Another issue could be the use of an undersized converter for the size of the engine. Worn propane propane system parts could also be a problem.
Normally, there should be no need to adjust the fuel mixture of a propane mixer. Before making any changes or repairs to a propane system, it is important to ensure that the ignition system is in good working order. Use only top-quality ignition parts (distributer cap, rotor, spark plugs, spark plug wires, etc) when maintaining a propane engine. Any fuel mixture changes should ideally be done with an exhaust gas analyzer and a chassis dynamometer.