I periodically pull the spark plugs on my car to check their colour and hence the mixture. I always do it after a run when the engine has well and truly warmed through and when it has not had a chance to idle before I turn it off.   Why is it that the plugs for numbers 1 and 4 are always showing slightly leaner than numbers 2 & 3? Not a lot but noticeable and if I swap the plugs (1 for 2 and 3 for 4), then next time I check the outer two will still be showing evidence of a leaner mixture