My muffler on myOS 90 4 stroke doesn't have a back pressure connection. Where do I put the back pressure tube from the gas tank?

The 91 's with a fuel pump did not have a pressure nipple on the muffler

The old original 90 four stroke did not use or need muffler pressure

its a pumped engine, no back pressure valve needed

If it's the pre "surpass" 90 then just vent the tube out side the plane somewhere indiscreet, like the bottom of the cowl, just like if it was a gas plane

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

While I plumb my tanks exactly as John has suggested and there is no problem loosing fuel out the tank unless you stand the plane on its nose, Modern practice seems to be to wrap the tank vent line around the tank front to rear before going to the outlet position higher than the tank.

This way, if you stand the plane on its nose, no fuel comes out the vent. However it will still possibly come out the carb.

Thanks for all your input, that answers my question.