Auxiliary carb on 3.5l 1946

Taken carbs off to discover the auxiliary starter carb has been blanked
Have looked though various workshop manuals but not worked out how the aux carb feeds into the engine- through the small pipes in the main carb spacers or through what appears to be another opening blanked off?image image

Yes, it’s that hole in the head with the brass hex head plug in it now.
There was a 90 degree threaded fitting screwed in there which had 1/2" copper tube soldered to it. The copper tube was bent in a 90 degree ELL to meet the port you see in the starting carb. There was a straight threaded 1/2" compression fitting with a brass olive going into there. That fitting is what appears to be sitting by your carb and plugged up with solder.

Thanks Rob- so the pipe was brass/ copper and solid- doesn’t sound like much leeway in measurement or very forgiving for vibration
Think understand now some of the comments about backfire knocking out plugs in block if this empties randomly inside the engine casing

And the smaller opening over the actual needle and jet? Should this remain blocked?

Richard Carter

The factory probably had a special fixture to make up these pipes in 1000’s. For us it’s trial and error.
There wouldn’t be any vibration or relative motion between the carbs and head.
Here is my '38 2-1/2 L pipe.

A backfire would blow fuel up and out of the top of the starting carb, where the air normally goes in.
About 1952 the factory added an anti-backfire valve to XK120 and Mark VII for that reason.
I suppose if the backfire was big enough in our pushrod engines it might blow out core plugs at the front and back of the head, if they weren’t in very good.
I don’t know about that opening on the side. Was that screw in there? My '38 has something there but it seems to be a blind hole. My Mark V doesn’t have anything there, nor does my 120.

And I’ve seen at least two Mark V cars with evidence of previous fires in the carb area. Given I’ve only seen a couple dozen Mark Vs in person, that suggests the carb fires were not rare.