I’m pulling out what little hair I have over synchronizing the carbs, or more specifically the idle, on my Rotax 503 DCDI with a clutch.  No matter what I do, here is how I end up.  Near perfect pneumatic sync throughout the range up to 4,000 RPM.   But when I return to idle, it sits there close to 2250 with the prop spinning, but after about 10-60 seconds, it suddenly shifts down to 1500-1800 (keeps varying in that range) with the prop barely turning, but otherwise idling well.  When I start it, when it is warm, it starts instantly and runs at 1500-1800, purring nicely, but kinda slow, no?  But as soon as I give it any throttle, it goes back to 2250 at idle for the timeout period before returning to 1500-1800.  It is not the throttle cables hanging up.

One more thing, probably unrelated.  My previous 503 DCDI from years ago had fuel injection but no clutch, and I do not recall this behavior.  This one has mix fuel, and I am running 40:1 at this time, using what I have of Rotax 2-stroke oil.

Also, I am not sure what idle speed to set given I have a clutch.  I was trying for 2000, but that appears to be no man’s land, with it wanting to be higher with prop turning or lower with prop almost stopped, disengaged clutch..

Any ideas?  Thanks.



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