ORIGINAL: jester_s1
Engines do seem to be more sensitive to idle adjustments when inverted, I suppose due to the fuel tank height issue we've been talking about.
The engine doesn't care that it is inverted once started.
There is NO measurable "fuel pressure difference" over the 1/2" differentials we are talking about.
Your engine should not change when inverted in flight too even with the throttle pulled back to idle.
PRIOR to start fuel can siphon which can lead to initial idling problems until the excess fuel is flushed out ( which can take longer than some may think ).
In turn that can cause you to mistune the engine which makes it seem that the engine is more sensitive.
Typically though you can tune the engine upright, then mount it inverted and it will run the same way.
If it doesn't look to starting proceedures and plumbing problems. Fuel may be flooding the head because of tank height/siphoning.
On my inverted engines I will sometimes loop the fuel line over the mount, to raise it and prevent siphoning altogether. (I drain the line when I'm done flying).
However this causes fuel to siphon BACK into the tank, so I typically have to hit the engine with the starter for 10-15 seconds before it grabs enough fuel to start, but once started there is no problem...
... No need to do anything as drastic as what has been suggested, e.g. cutting up your ARF.
This is a much simpler solution BUT once fuel fills the line, it can continue to siphon... so I merely keep the throttle fully shut down after each flight.