I am having the same problem with two G26's.

Engine #1 was mounted in a H9 Taylorcraft. I was going to fly this airplane last Sunday. I checked the RPM and it was turning 7200 with a 18/6 zinger. Using my fish scale I was getting 10.1 pounds of thrust. Throttle response was good. I started my takeoff roll and the airplane lifted and at the same time the engine started to slow down. I was too slow in taking action and lowering the nose and it stalled on me. That part was my fault. No damage to the engine or wings. Fuse will need major repair.

Engine #2 mounted in a Ultra Stk 120. Checked the RPM and it was turning 8000 on a 16/8 prop. My fish scale was showing 10.4 pounds of thrust. It looked and sounded good so I put it the air. Same thing, the engine started to sag and slow down. This time I got the nose down came back around to land. The 120 was very slow but no problem with the landing.

I am at a loss on these two engines. I have checked everything I can think off. The choke butterfly sticks out in air and I wonder if this could change the airflow going in the carb. No sign of overheating on either engine. I have two G23,s and I have never had any problem with them.

Thanks in advance for any help.
Tom