912 ULS Power Reduction

A 912 ULS installed on a EuroFox is giving problems during glider towing. The engine will only develop 5100 RPM static and about 5100 to 5150 RPM while operating at 65 knots, This is creating safety issues while launching and slow climb rates as it should give 5500 static and 5450 at 65 knots. The gearbox has recently had its scheduled overhaul.

The carbs were removed and have been fitted with the new floats ref R and the float arms checked, the air correction, idle mixture have been cleaned and blown through with air, the pistons and needles cleaned and new diaphragm's fitted. Carbs reassembled and mixture screw set to 1 turn out from the closed position 

Carbs then set up using the standard mechanical setup to get the engine started ( full throttle forward stop checked and idle stop set to 1 turn in from the clear position) the carbs were then balanced using a carbmate balancing kit, initial start up is slightly rough but the engine then runs smooth

This work has made no difference to the static RPM or the climb RPM which is still indicating 5100 RPM. Any help or suggestions on where there might be a problem or something that has been missed would be much appreciated.