Typically, there is a small divot in a reinforced portion of the backplate. you drill that STRAIGHT in with a small bit. (1/32 is OK, 1/16 is too big) Enlarge the end of the hole to the outside of the engine case and tap for the brass nipple. (look for a small dia nipple... 8-23 is too big. Can't get one that's too small.) Be sure to clean up all metal filings. (the backplate should be OFF the engine while doing this!) deburr the inner end of the hole.

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its basicly a waste of effort on an O.S. .46 FX. The engine's already very reliable. Unless you plan on putting the tank more than 8 inches aft from the engine... don't do it. Properly tuning the engine will be all you need to do.

The O.S. FX series engines do very well when mounted inverted. The pump will NOT prevent you from mis-tuning the engine... It will make your adjustments easier to get WRONG! The pump pressure means the needle needs to be closed further, and a small adjustment will have more effect.

Mount the engine uproght on a test stand. Tune it properly. put it in the plane and LEAVE the needle alone! It'll work.