I just notice the test tube for 5-8 is missing. This is the extended part that comes up for easier testing. The one for 1-4 is there. How important is this and is there a suitable replacement or can I just cap it off like the other one? Would effect does it have without the cap on?

 

Hi,
Without being capped, the air drawn in with upset the O2 loop. It will think mixture is too weak, and richen it up.

You need to cap it...

 

I capped both of mine by cutting off the pipes and stuffing bolts into the resulting holes. JB Weld seals them in place.