The big union nut at the bottom is a normal R/H thread. Probably better to bolt the carb to the cylinder first if it's tight to undo. There is a bleed hole just above that union nut on the left side, which will often leak if your float bowl is not set up properly.
That method worked great. Now to find the leak. I read that people lap the float needle into its seat using Brasso - is that right ? Ive replaced all the gaskets and it still leaks but once fired up the problem goes away. Ant
firstly I would remove, clean and assemble the float bowl. Turn it upside down and blow through the fuel pipe banjo. You will soon tell if the needle is letting by. operate the tickler at the same time and you'll see if it's seating properly.
I've also heard about lapping the needle in, but it's difficult to keep the needle central while you do it. Maybe some sort of simple jig to keep the needle in the middle?
Also to note, there are three depths to those union nuts which will alter the float height, depending on the application, and an M20 should have the shortest one.
I found, and still find a little, that the carb tends to leak a little, or at least be moist. I've not been brave enough to open the float bowl up and attempt to improve things, as a previous owner has glued or otherwise bonded the float bowl top to the body so well that I'll only attempt to undo them when I have a replacement to hand.
Like you it is fine once running and is at worse a small drip on occasion.
Thanks guys. I gave the float bowl a good clean out and simulated a full bowl of petrol by holding the valve shut / blocking the carb feed hole until the float took over and held the valve in place. No leaks so I will rebuild and hope for the best. Ant
I had the same issue on a new type 29 on my Vincent rapide. I lapped the needle, replaced gaskets, checked the jet block seating but it still leaked. Finally, I took the big base gasket out from the big nut that secures the jet block out from my old carb and put it in the new one…no leaks using the 70 year old jet block gasket. Cheers.
My original 1940 carb leaked badly from the same place, so I got a new carb and the new one did exactly the same!
Like mentioned here, it’s all about setting the carb up which I’m learning about.
It seems to be better now but not perfect.