Hi
Ive been running a new os 70 surpass in a cub I ran this engine ritch for a whole galon and it has been very reliable however sence i started a second galon out of the same case of fuel ive noticed if its just a touch lean i get a preignition type knocking sound at high rpm i back the needle out about 1/4 turn from peak as a rule with no troubles its been running a 14X6 prop and rarly gets ussed at full throttle i try and give it one or 2 blasts when dooing a llop just to clear it out and het it broken in right cant baby it forever. yesterday i decided to switch to the wooden prop as my take offs and landings have greatly improved and i no longer feer nosing it over. However when i did this shange over i also notices that the knocking noise was more previlent and happened a lot easyer as i tried to lean this thing out im guessing its just the way a 4 stroke runs or is it that the compretion is improving? when it was fresh out of the box i didnt notice it nearly as much prope size i feer may be to big it performs great on a 13X8 but it land at idle it wants to keep on going i broke the 13X8 and the 14X6 master air screw black plastick slowed it down for landings and i was thrilled also the 14X6 wood has caused it to come in a little hotter tempted to get a drag shute or crab it in for 1/2 mile before getting over the field. so prop size and engine noise durring lean opperation are my troubles tempted to try a 15X4 thank you all again for your time and safe flying