912UL Miss Below 4,000rpm

When was the last time the carbs were cleaned and rebuilt?  If more than 500 hours ago, might want to have that done…

Two things and a couple of things to rule out.

First I agree with Mike. When were the carbs last overhauled and if they haven't then they need to be with that age. Did you clean out the idle jets? Mixture screw checked at 1.5 turns out? The one next to the main jet needs to be removed and cleaned out. What rpm on the higher side did you use to sync the carbs and are they synced at the higher rpm and idle?

Then you may want to unscrew the spark plug caps, trim the wire end back about 3/8" to get some clean wire, apply a dab of dielectric grease and screw them back on all the way down.

Carbs were cleaned and new rubber installed within the last 100 hours.

Thanks!

First I agree with Mike. When were the carbs last overhauled and if they haven't then they need to be with that age. Did you clean out the idle jets? Mixture screw checked at 1.5 turns out? The one next to the main jet needs to be removed and cleaned out. What rpm on the higher side did you use to sync the carbs and are they synced at the higher rpm and idle?

Then you may want to unscrew the spark plug caps, trim the wire end back about 3/8" to get some clean wire, apply a dab of dielectric grease and screw them back on all the way down.

I just checked the idle jets and the main jets for debris. They were clean. I went ahead and had him order new o-rings for the idle jets and idle mixture screws. Mixture screws are set at 1.5 turns out.

We are going to check the resistance on the caps, trim the wires back, add the grease and reassemble them.

Carbs were synced at idle, 3,000rpm and they are in balance up through 4,500 rpm where we stopped cause it was getting really windy.....LOL.

Thanks for the ideas!

The mind can plays tricks on the pilot(s);

Before I owned it, I used to fly right seat with the previous owner. We had many wonderful flying adventures. There was one period of several months,  where we used to get a miss in our faithful 912 ULS, during cruise climb (5200rpm), usually about 6000ft (probably 3500 above terrain) as we passed over an extended area of "tiger country". 

We tried new plugs, AvGas shandies, cleaned the carburettors, all to no avail. This phenomena only happened in the one general area, at the one stage of flight - in the end we just ignored it and what do you know? the miss went away.

We figured our anxiety over taking the risk of a direct line of flight, over very inhospitable terrain, somehow heightened our perception to the point where we separately imagined an engine hesitation/miss. We reinforced this perception by verbalising what we thought we had just felt and then went on to "trouble shoot" without success.