I have put a well used OS 46FX in a Sig Sun Dancer. It requires the engine to be mounted inverted. Problem is, I can't seem to get the engine to run consistently. This was a very good running engine on two previous planes. Is there a trick in tuning inverted engines?

Did the engine run ok when flying previous planes inverted?

Engines have no idea whether they're right side up or not when they're running. On the other hand, tank locations are often differently oriented from rightside up to upside down. And starting can also be affected.

This is an ongoing conversation, da Rock is correct the engine does not know up from down, when it is running...

The trick is to GET it running! [:'(]

As the fuel comes into the cylinder it tends to pool in the head which 'cools' the glow plug and may starting hard, SO... a hotter glow plug may be needed and most definately your tuning has to be very good (not too rich) to prevent the pooling. Also, when inverted the position of the fuel tank is important, as the carb is now lower the tank may have to be lowered to prevent siphoning.

Once you correct these little problems the engine should work like a charm! If you don't like this much work but still don't want a verticle engine, try mounting the engine at 90 degrees...

The biggest problem with an inverted engine is tank height. For some unknown reason, with many ARF's, kits and plans the tanks are designed to fit in the fuselage as if the engine were upright or at best, 90 degrees.

MOST inverted installations require the following:

1. Hogging out (or otherwise modifying) the fuselage former(s) to allow the tank to sit lower in the fuselage. What you want is to have the fuel outlet tube at the plug on the tank at the same height as the spray bar in the carb. If the outlet tube is higher than the spray bar fuel is going to siphon out of the tank and flood the engine.

2. A remote glow ignitor (saves the fingers and hands )

3. More juice to the glow plug. The plug in an inverted set up is a lot more prone to being cooled down by pooling fuel, especially when starting the engine. By using the glow plug connector hooked to a power panel you can minimize the cooling of the fuel by increasing the power to the plug (within reason so you don't burn the plug out ).

These 3 things will solve 99% of the problems with starting an inverted engine.

Thanks guys. I just assumed that the tank placement in the design of the plane would be right!! There was a significant amount of siphoning that I tried to solve by routing the lines at different elevations. That improved and I put an on-board glow on to help keep the plug lit. It seems to run fine on the taxi out to the runway (about 30 feet) and I turn up the runway about 10 feet and the engine quits abruptly. The same as it did before I put the on-board glow on.

try an os 'f' plug.

The answers you have received are all good advice, EXCEPT that the fuel level is what causes siphoning, not tank level. Even with a properly mounted tank, when it is full the fuel level is still much higher than the needle valve.