Sounds like you've bought a Seagull PC-9?

Even lowering the supplied tank to the minimum position possible with the rod removed but without former surgery, the tank position remains higher than optimum relative to the carb for a 46 installed on the supplied mount.

There are a couple of specific circumstance caveats, but I'll deal first with the general and remove later to the specifc.

Have a realistic idle expectation. In an ideal inverted situ with the right plug, ideal tank height etc, you might be able to achieve around 2k idle, but with that tank height still high, you'll have to be idling higher. Use a tacho.

Use a good condition OS #8 or quality equivalent heat range plug, and use nitro. 10% is best. No more than 20% oil, preferably synthetic blend and 18% by volume. An A3 won't cut it period and an A5 will be too cold for low idle with an inverted engine without lots of nitro.

After setting the high speed needle, re-tune your engine's low end (idle mixture) accurately.

A couple of Seagull PC-9 ARF tank fitment induced idiosyncrasies.

If you fuel and leave the model sit for any period of time, the engine will start to flood. That is a function of that tank height. Solution: Don't refuel until just before you are ready to start. Long term solution is to replace and refit the fuel system which includes modification of the PC-9 so that the tank height is perfect in relation to carb height, or refit the engine as a sidewinder or upright.

Upon initial starts after a flooded situation or if the plug is oiled up, the engine will not idle well until it has cleared itself and/or the plug. Very noticable if you start upright and turn the model over to place it upon the ground, when it will quit.

Solution: For the first start of the day, turn the model over and start the engine upright. Remove the glow igniter and rev the engine up to burn of any residual oil and clear out any residual excess fuel. Starts thereafter should be able to be made from the inverted position without any problem unless you inadvertantly flood the engine. After you've started always burn off any residual oil shortly afterwards and you should find that your engine will idle reliably and at a reasonably low setting.

FTR, I've a PC-9 with an OS46 aboard. Tank is lowered insofar as the design will allow with removal of the rod, but without surgery. I'll get around to optimising it, but mine idles reliably so for now, I'm happy to simply follow the start upright for the first start of the day procedure and not refuel until I am ready to fly.

Try it and let us know how you get on.