I have been trying to get this zenoah engine tuned so I can have a reliable idle, before I maiden. Not sure how new this engine is, but it looks pretty new. I got it on an CMP Beaver about 10 years ago. I replaced the ignition module with a new Rexcel. Turning a Graupner 16-8 on 91 octane 32 to 1 gas (zenoah oil). It will turn 8000 easy, has reasonably good transition...but it will idle down around 2200to 2400 for about 10 sec then falls off to 1200 or so and dies. Rebuilt carb (twice), set float needle exactly to walbro specs. ( yes, I have the walbro setting gauge).I have tried several methods mentioned here, but to no avail. Started with 2 turns on each needle, with radio, set rpm to 3500, leaned low needle till rpm stopped climbing, brought rpm back to 3500 via radio, and kept doing that until I got no rise in rpm. Then set high needle to max rpm (stopped leaning needle when rpm quit rising). Runs great, except the idle hangs around 2200 for about 10 seconds then it dropped to about 1000 to 1250 for a second or two then dies. If I'm quick with the throttle stick, I can save it. Not sure why. But it should be around 1500 to 1650. If it wasn't broken in would that hinder idle?
also tried the tune high needle first, then the low, but it would not transition, it would just bog down.
Any suggestions?

Try changing your throttle servo. It happened to me once when the feed back pot developed a worn spot. Dan.

If you keep the engine on the rich side (a little richer than what it sounds like you have it now), I think you'll see you can get a little more consistent idle out of it. This true even more if the engine isn't broken in yet. The engine is running hotter now than what it will once broken in. This means you need to keep extra fuel running through it to help cool it.

You don't want to waste too much time getting these carb settings just perfect when on the ground, as they are almost certainly going to change once airborne. Close is good enough. It really is. Once airborne, then you can really see what needs to happen. What it takes to get it running right at that point is what really matters.

And last, don't expect the engine to idle at it's best when new. It's not going to happen. Actually, I prefer to keep them as high as I can. If you can set the plane down on the grass without rolling, the idle is low enough. If you have to go around because the plane doesn't want to slow enough to land is a much better thing to deal with than one that quits on final........

The Zen G26 is an absolute gem of an engine but when new can exhibit a few tuning issues. First off it really prefers a bigger prop than that 16x8 as an 18x6 is considered the norm here which would also aid in a more reliable idle. As Al mentioned if the engine is new with little or no run time on it then don't expect a great idle, it just won't happen until it its been run a few times, so as already suggested get a decent tune on it and go fly being careful on the first few flights to keep wide open throttle or long up lines to a minimum.