As for the CoroCub, the problem was in toe-in. I got great explanation what's going on. We fixed it and there is no more a problem. More info in CoroCub thread.

As for the Poulan 25 - the engine is running great. We measured now 7000 rpm on the ground. Looks like the ring is settling in. More compression too. I started it by hand just starting opposite way. This time it worked . I tried to see how low it will get and at 1700 rpm Zinger prop started going backward. I guess this wooden prop is to light. I will repeat this test again with my MA Classic.

Now there is a weird thing. On the ground the engine runs 7000 rpm - it is screaming. When the plane is flying the full throttle it sounds more like 6000 rpm (I need to measure it recording the sound). I see there are two possible reasons.

1. The mixture is getting lean why flying, but weird thing is, that engine sounds OK, just has lower rpm.
2. When airplane is moving the prop starts to work more efficient and loads the engine more (takes better "bites" of the moving air), while on the ground it "slips" through the air not loading the engine as much (it is like clogged vacuum cleaner - it screams but does not push the air)