I bought a balsa usa kit where the engine will be installed up side down. I haven,t had any luck keeping a 2 stroke at an idle. what would be the best .46 2 stroke for this plane, an is there a trick to get them to idle for a min or 2 .

IMO, the best 46 size engine is the OS55AX.

Tuned properly, there is no difference when they are mounted inverted except maybe the starting procedure as you have to be careful not to flood it.

To me the key to success with inverted engines is tank height, tank height and tank height....

The spray bar in the carb should be around the upper level in the tank.

You may need an on-board glow igniter for low throttle operation.

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.

Or just reconfigure the kit to accept the engine mounting to be on it's side, or even upright. I do this with even ARFs, and reposition the engine upright for my personal preference. I really hate reaching under a plane for the glow igniter with the prop spinning. I haven't had idle problems though, just safety ones.

Only if your LS needle is set too rich.

I guess thats way my new engines would idle inverted,were they were set to rich because of brackin?

No offense but why would you want to idle for that long? Even perfectly tuned upright, sideways or upside down I cant see idling for more than 30 seconds or so.

Maybe I'm way off here but I get mine fired up and quickly tune them, set them down and away we go. After landing taxi back and kill it.

At a busy field, you might get your engine started and have to wait 3-5 minutes to take off safely. My engines will do that.
If they won't Idle that long there is a tuning issue, probably too rich on the low speed needle.

????????? what are you saying????? This is question to Fastbnack

I am saying that you need to get the LSN adjusted properly for a crisp clean transition from idle to full throttle without gurgling or dieing.

Even my classic Pattern planes can sit and idle all day if I wanted them too but I much prefer to fly them.

If you do have to wait 5 minuets to fly then shut it off and save the fuel or start your timer and fly a shorter flight.

The LSN is a critical adjustment, even more so in a inverted application where the fuel will pool in the head/plug instead of in the crankcase.