8 x 6 2 bladers sound too small for a 25, 9 x 6 at least. Are all the engines mounted sideways? I had severe vibration at certain RPM's and I solved this by rubber mounting the engines via backplate mounts. Filtering the fuel before it goes in the plane - not good enough IMHO, fuel filters directly before the needle and not those 2 piece aluminium things which leak air - Sullivan Crap trap 2 stage jobbies. If you have the height throttle back before you turn & then slowly apply more power when you have it straight again, also put in RUDDER trim, easy to say when your not sweating and your knees shakin! Don't run your engines too lean, lean them out till they just break into clean 2 stroke running & no more. Don't worry about synchronization, if one stops they won't be very synchronized! - John