Microlighters

My 503 dcdi,has a very inconsistant idle,idles at 2200 rpm( approx) when cold, after a circuit or two it has dropped to 1800 rpm. Any ideas why. I'm flying from sea level,burning 96 octane and castrol 2t oil .About 600 hours on the engine

The idle speeds you are giving are a little high - if i recall they should be in the region of 1500rpm warm. There is a small variation between warm and cold engine, but i think it should be about 100 rpm. A cold engine gives a very inconsistent idle speed and may fluctuate a bit. My recommendation is that you allow the engine to warm up a little. Then increase the rpm a bit by blipping the throttle to make sure no residial starting fuel/fouling and then see what the idle value is. Compare this to the post flight idle speed. Im sure the boffins out there can give more info.

It is quite difficult to get these engines idling very smoothly.

Check the following

1. make sure the Idle jets are clean. Remove float bowl, remove idle jet and clean (see pic)
2. Turn the Idle air screw all the way in, then out 1.5 turns
3. Check that the two pistons are exactly the same height from the bottom when looking into the carbs from the front with the throttle fully closed. Insert two straws of welding rods and make sure they are at the same level. Use the Idle screw to adjust (The large screw). Warm up and set for about 2000rpm
4. There is air leaking somewhere, check the rubber boot for cracks between the cab and the intake manifold , the rubber stopper on the side of the carb (see pic circled in red) or intake manifold gasket

Cheers Guys,many thanks again Morph,will check your list this weekend ( have you ever considered shifting here )