RE: Saito 100
JWander - what the other guys have said - especially Aussie Steve.
I have got the Saito 100 in an H9 Mustang so its tank position is probably very similar to yours relative to the engine. My engine has a very reliable 1800 - 2000 rpm idle. Check the tappets, use a fresh OS Type F plug, fresh fuel and let's start working on those needles...
With many inverted engines, you MUST get the idle mixture absolutely as lean as you can get away with before your idle and transition to high throttle settings start to deteriorate. On a Saito, that means many turns in from where it was when you pulled the engine out of the box. You will never damage an engine with a lean mixture when it's idling.
There is no such thing as a "factory setting" on a model engine - this is the single most misunderstood issue in the history of aeromodelling! There are guidelines on where to start but every engine's low speed and high speed needle must be adjusted to where that particular engine runs best in that particular airframe with that particular fuel.
Good luck and do report back.