It depends on how slow you are running the engine. I have seen quite a few people try to idle a engine too slow in RPMs. They don't quite idle as slow as many people might think they do.
The other thing, is a long idle time isn't really needed for most purposes. Then if the engine is new it won't idle all that well anyway. Most all of the engines require a break in or running in for a long time before they start to idle good. Plus some engines, especially the low end inexpensive engines may never idle well. It depends on how well the engine was made, how good the carburetor is, and the muffler exhaust system as well as the fuel tank location and how long the fuel lines are plus the muffler pressure. As a engine idles the fuel draw is much weaker, and as the engine idles for a longer time period, the muffler pressure gets weaker compounding the weak fuel draw issue. Some engines will idle pretty slow, but only after a good long break in period. The main design issue is that internally the fuel tends to start pooling and oil is pooling plus the engine is cooling off and one good droplet of raw fuel is enough to quench out the glow plug. It doesn't matter if the engine is right side up, sideways or upside down.

As mentioned try to always center the fuel tank on the center of the carb and try to keep the fuel lines short.

Now granted, if you are trying to idle a engine on a plane and you are taking off of pavement, then it can be a problem. Trying to get the engine to idle slow enough so the plane doesn't roll forward at idle may not be acheivable with many engines. In this case you need to setup the nose wheel or tail wheel to have some drag or friction to it so the plane does not roll forward at idle.One can even fabricate a simple brake mechanism to operate off of the throttle servo to stop the plane at idle.