ORIGINAL: britbrat
Questions:
Is the knocking sound a common feature of this engine? My previous GMS 76 did not make this sound, although it was a performace pig like this one.
Is it a sign of impending doom?
I have looked at the bearings & they appear to be good -- no play in axial or radial directions. I haven't fully dissassembled the engine, (I prefer not to disturb its innards) but I wonder if the con-rod bushings are wearing.
Does the addition of extra castor oil make the sound go away? I am thinking of adding castor to the Omega to bring the total lube percentage up to as much as 25%.
I have worked with 4 GMS .76s and a couple of their .61s. They have generally been fairly quiet at idle; I haven't heard one "knocking" at idle like yours. I have heard one (actually a .61 GMS) with a strange "grumble" at high throttle while flying. When you checked your bearings did you also check the rod big end play?
Does your engine have the finned muffler or the Tower-type muffler? They come in both forms here depending on the shipment. The Tower-type OEM muffler provides quite a performance boost; the GMS .76 I saw with this muffler did 10,400 rpm with an APC 13x8 - not too shabby. Your 11k with a 12x8 seems consistent with what I've seen the GMS .76s with the finned muffler do. If your muffler is finned, have you removed the baffle cone from within it?
If you haven't already done so, read post # 303 in this thread and try your HS needling experiments with different LS needle settings to see if fuel draw and atomisation are affected by the LS setting. John got a big improvment in draw, fuel consumption and power once he'd sorted out the optimum position for his LS needle, due we believe to improved atomisation.
Good luck. Let us know how you go.