@sender46 Thats very interesting, I never thought of the weather side of things. I did a fair bit of experimenting yesterday to try and pin it down and came to a different conclusion. Which may mean more than live weather might be at play here. I found this only happened when I had enhanced airport third party scenery installed, in particular EGNH. Also one location on this airfield had a different effect on the engine. It wouldn't start no matter what I tried. Even choosing ready for takeoff. Remove the third party scenery enhancements and the problem disappeared. So it may not only be the weather. But as you think, I am more inclined to think it's an MSFS issue.