Extract below taken fromt the Lycoming key reprints web site:
http://www.lycoming.textron.com/main...nts/index.html
Have you also considered as well as below how long you leave the engine after priming and before starting? Try leaving it 30-40 seconds to allow the fuel to fully vaporise in the cylinder and see if that makes a difference.
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Probably the most important factor in starting an engine is achieving a fuel/air mixture that is satisfactory for combustion. Since the engine usually starts very easily, many pilots are unaware of or ignore the change of starting procedure needed to successfully start under varying temperature conditions. In warm weather the air is less dense and therefore must be mixed with a lesser amount of fuel than in cold weather. In addition to this, in warm weather the fuel will vaporize readily and make starting easier. Simply stated, as temperatures go down it becomes more and more important that we have a plan for priming that will achieve the correct fuel/air mixture.
When priming a carbureted engine, the pilot’s plan must consider the temperature, the number of cylinders which have priming lines installed, and the number of strokes of the primer needed to produce the correct fuel/air mixture. The primer lines are ordered or installed by the airframe manufacturer and not all aircraft are configured the same. Some aircraft have actually been produced with only one cylinder being primed, and these engines are extremely hard to start in cold weather. The number of cylinders that are primed must be considered since the total fuel delivered by the primer will be divided and sent to these cylinders. As the air becomes colder and denser, the amount of prime used must be increased, but the number of strokes to be used should be planned as a result of some trial and error experimentation for each aircraft a pilot flies. When the correct number of primer strokes for each temperature range has been established, the engine will usually start very quickly. We may find that an engine starts easily when one stroke of the primer is used in the sixty-degree range, two strokes in the fifty-degree range, three strokes in the forty- degree range, etc. This is an example of the trial and error we might use to establish the number of primer strokes to use under any particular temperature condition.
While discussing the priming of an engine, there have been situations where primer lines become clogged. This makes engine starting difficult and negates any trial and error experimentation that may have been done. When maintenance is done on an aircraft before the start of winter, it may be wise to have those primer lines checked to insure that fuel will flow through them.
The amount of fuel needed to achieve the correct fuel/air mixture for starting a fuel injected engine is controlled by timing rather than number of primer strokes. With the electric fuel pump on, moving the mixture control to the rich position allows fuel to flow to the cylinders. For cold weather starting, it may be necessary to keep the mixture control in rich somewhat longer than in warm weather.
The fuel part of the fuel/air mixture may be the part we have the most control over during the engine start, but keep in mind that the amount of throttle opening does have an effect on the air that is pumped through the engine. Just as we compensate for cold/dense air by adding more fuel for start, it may also be appropriate to reduce the air part of the mixture when the temperature is very cold. For example, if the throttle is normally set open one half inch for warm weather starting, it may be helpful to reduce this to one quarter inch in cold weather. Again, it will require some experimentation to determine what is needed to achieve the correct fuel/air mixture for any particular aircraft at any temperature range.