Hi Ron,this could be a possibility...The first wooden prop (Ritz propeller) was far away from being balanced.I tried to balance it statically but could not get it perfect.Since the main usage of that prop was for the break-in and some trial runs,we used it for a couple of hours only.Next one is a brand new Ivoprop which supposed to be balanced at the factory.A big difference with the first one but you can feel some vibrations past 5000....Hum.....Not a big lot but some and it does increase until top RPM...One friend is an AME and has the outfit to dynamically balance certified aircraft and I was waiting to receive a new spinner before doing it.The last prop I tried (GSC 68 like yours) has vibration somehow between the Ritz and the Ivoprop.Now ,if you read my first post...One of the last thing I mentioned, it worked fine with the GSC after I changed the pitch to get somehow 6300 on the ground.The RPM was stable at all ranges for a good 15 minutes and next day,it was decided to redo the test just to double check.Being perfectionnist (too much), I decided to change the pitch on 1 blade slightly to have less than 1 degree difference between both blades for less vibration and the problem is back!!!So dynamic balancing is a must for sure.
Update...today,we put back the Ivoprop, changed the needles position to top notch to get it leaner, we also disconnected all wiring from the aircraft except the starter ,knowing we can shut the motor with the choke, no success.Tomorrow,we'll be very really aggressive.We are changing the motor for a rebuild one that needs a break-in.Balancing will be done asap!!!I plan to also install one of those prop balancer on the hub.Anybody ever install one of those ???
Guy
