RE: back pressure question
This is the very old FS 90, it is not the later FS 91. The fs 90 did not at first come with any pressure taps or had the need for it as mentioned by W8 and Scale 4 me.
If what you have is in fact the FS 90. Run a short tube from your fuel tank vent just above the level of the tank and facing forward into the propellor stream and secure it. This facing forward is highly recommended so do not let it just hang.
The crankcase tap on the bottom run a short length of tube to just outside the airplane and let it hang.
If this is you engine in the pictures as you can see there is a dangerous safety problem with the forward glow plug so it is agine recomended to not use a glow ignitor directly but instead use a remote glow 'connector'.
John