Answer: Inverted engines need a few additional considerations as follows:  

 

1) Be very careful when fueling an engine when inverted. There is a tendency for fuel to siphon from the tank into the muffler via the pressure feed line and subsequently drain from the muffler back into the cylinder via the exhaust port. If this occurs and you then hit the engine with an electric starter you end up with a hydraulic lock which will bend the rod, snap the piston pin and/or dish the top of the piston. All of these are fatal and pretty much total the reciprocating components. A very effective but really dumb way to muck up an engine.

 

Always turn an engine over by hand before connecting the glow plug ignitor and electric starter. If you can't rotate the engine by hand, do NOT try to rotate it with the electric starter.

 

2) Try to avoid a 180 degree inverted orientation. If possible go with 150 or 210 degrees. The idea here is to avoid putting the glow plug at the absoluted "bottom" of the cylinder. At low RPM, oil residue and excess fuel tends to collect as in a sump and snuff the plug.

 

3) Set the low end mixture a bit leaner than you would with an upright engine. If your carburetor has a mid range mixture control (most do not), lean that a bit also.

 

4) Avoid prolonged periods of idleing or operation at low RPM.

 

5) Switch to a hotter plug. 

 

6) Use the maximum nitro content specified for your engine. This is usually stated in the operations manual that comes with your engine. Many modern engines specifiy 10% or less so don't go to 15-25% if the max is stated to be 10%.

 

7) Use an onboard glow driver that automatically increases the energy flowing to the plug when the plug cools (changes resistance) as it gets swamped with more liquid fuel and oil. In some cases this is the only way to keep an inverted engine running reliably below mid throttle.

 

8) Carefully balance your prop and spinner. This is not directly related to inverted engine operations but if you find that your inverted engine runs fine on the ground or bench but fails in flight, the mixture settings you are so carefully setting up on the ground are changing in the air due to vibration inducing more air into the mix. Balanced props and spinners vibrate less and your mixture setting will be more stable.

 

9) Carefully check the airflow around your spinner, across the cowl and over the inlet into the carburetor. In some cases when an engine is inverted it is nicely "hidden" and at some air speeds you end up with a laminar airflow that in effect chokes off input of air to the carb. This can usually be fixed by putting a small deflector around the carb neck either ahead of or behind the carb intake. The idea here is to "roughen" up the air flow around the carb to make sure there is no tendency to have a vacuum develop.