quite often I have the engines cut off during take off while still on the runway or just in the early phase of my ascent.
Especially when I take off at Grand Canyon with the Robin
Also I am hardly able to ascent. I cannot get faster than 60 to 70 kts and at that speed I am not gaining altitude. Flaps are at 0
I had a similar problem and troubleshooting for a couple of hours led to the sim thinking that there was no fuel in the tanks. Requested a fuel truck and the problem went away. Checking fuel levels is now part of my preflight.
There are more “technical” answers, but the easiest is to run the throttle up to about 1700 RPM, then adjust the mixture until you get the highest RPM available.