RE: g51 quits in flight
A 15% nitro content is a bit high, but won't hurt your engine at all.
You mentioned that fuel consumption is drastically higher. Is the nose of the model wet with fuel? That could mean that the fit of the crankshaft to the crankcase is getting a bit loose between the carb and the front bearing. That's usually sealed by a thin film of fuel. If the clearance gets too wide, then the engine will start to spray fuel out through the bearing. Front bearings in model engines do not actually make the seal. What many engines have is a dirt shield and not an actual seal. If there was a seal, then there would be no way for oil to lubricate the front bearing.
To check, put the engine onto a test stand with a brand-new fuel system. Run the engine and see if you are getting fuel sprayed from the nose. You can hold your hand behind the propeller even with the cylinder. You'll quickly see if raw fuel is being thrown out. A piece of card stock will become wet with fuel if you hold one at the same location.
That's about all that will cause such an increase in fuel consumption. Otherwise, you're going to have a fuel leak somewhere and the fuel is going into the model or being tossed overboard. A very careful check is needed.
Finally, a worn-out engine may require such a rich setting of the high-speed needle that consumption has gone up because your throwing a lot of unburned fuel out of the cylinder. You need a lot of fuel so that the piston can make a decent compression seal against the cylinder liner.
Again, check, check, check.