RE: Inverted Engine Performance Question
What Gray Beard said and here's some literature from Saito's website.
Upright, Inverted, or Side Mount ?? Any engine orientation is OK. A frequent question is can I run the engine inverted ? A scale or pattern type model looks better with the engine inverted. Inverted engines run fine but can be a real inconvenience when starting. Attaching a glow-plug ingniter, and keeping it on, can be a real pain. Also fuel may accumulate inside the piston when inverted, causing hard starting, or possibly engine damage when forced over with an electric starter. This latter problem with inverted engines is minimized if you turn the engine over by hand 5 or 6 rotations before attaching the glow plug battery; any excess fuel inside the piston will run out the exhaust (if the exhaust points downward). If you are new to 4-stroke engines it is preferable to mount the engine vertical or sideways. It is important to note that the centerline of the fuel tank should be level with the carb or a little lower; if you invert the engine the tank may be a long way above the carb and you may have fuel running down into the carb making it difficult to get the fuel mixtures correct. Therefore, before deciding on an engine mounting orientation remember to consider the tank location. If the idle mixture is set correctly there should normally be no need for an on board glow system.
The dreaded <u>“hydro-locking”</u>. Often mention is the term “hydro-locking” with inverted engines. It is easy with an inverted engine to get the piston full of fuel and it will be difficult (or maybe impossible) to turn over the engine by hand and application of a strong electric starter may damage the engine. The instructions with engines purchased from the Horizon Hobbies (distributor in North America) suggest that in such a situation you should remove the glow-plug and drain the excess fuel. While this will work, I suggest that procedure is inconvenient, unnecessary, and undesirable. It does nothing except wear out the glow-plug thread, maybe strip the thread in moments of frustration, and certainly waste a lot of time. There is a simple fix; (with glow-plug igniter removed) just pick up the model and tilt it sideways so that the exhaust port points down, turn over the prop by hand and you will see the excess fuel that was in the cylinder run out the exhaust.
For an inverted engine you should first make sure you can easily turn over the engine by hand before applying an electric starter.</p>