When a running problem /needle seeting problem occurs between an upright and an inverted iengine nstallation, the culprit invariably turns out to be the tank; and more to point, the position of the tank in reference to the carb.

If you can visualize the tank installed in your airplane, and draw an imaginary line through the middle of the tank ... that imaginary line should extend right through the center of the carb, generally through the position of the high speed needle. If this is not the case, either the engine has to suck more fuel up an incline (if the tank is below the carb), or the carb has to stop the over-abundance of fuel (if the tank is located above the carb). Fuel, like water, always runs downhill.

The case of both of the two Harrier models from Seagull is that the tank is located at the very top of the fuselage and the carb, in an inverted installation, is at the very bottom of the fuselage, creating a tremendous downhill run for the fuel. The best solution to this problem is to drop the tank position in the fuselage to better align the carb with the centerline of the tank. Without changing the position of the tank, you could lean out the low end of the carb to the point where the engine with run very well. But as soon as you invert the airplane, now the engine has to suck fuel (because the tank is below the carb) and the chances are that it will quit lean in this installation.

Talking much more generically, if you are having a problem with an inverted engine installation, look first to the alignment of the tank, and then try to trouble shoot the engine. If the engine runs great while it is upright, there is nothing inherently wrong with the engine. The problem is a fuel feed problem, either too much, or too little.

I hope this helps.

Pete