Hi Silvio
I still have to use 2 hands to start when the engine is on my back, but this does not bother me anymore. Don't spend hours messing around with settings like I did
The only change I made that made pulling easier was moving the pulley to the right hand side, this makes the rope more inline with the hole in the assembly.
Another mistake I made when I first started was with the engine on my back keep trying when it does not want to start until I was so tired I had to pack up and go home. If it does not want to start after 6 pulls, put the engine on the ground, start it and let it run for a few minutes. Then try again with it on your back.
I now always start and run the engine on the ground before putting it on my back.
Don't worry after about 15 hours it gets easier to start, I can now always start my engine with less than 4 pulls, but I do have to use 2 hands.
I believe this is because the zenith pulley is too low on the frame, and/or does not swivel,.
This was pointed out by morgy an Paul b, but I foolishly did not believe them back then
As a last resort you can start it on the ground, lift it up on a Cooler box or something that is about 50cm of the ground and then get in the harness while the engine is running, this is a bit dangerous, but at least you will not have to go home without flying.
Jaco