I ran my Mk1 for the first time in my lifetime on its Solex last weekend after rebuilding the Solex at Christmas.
Between then and last weekend, the fuel banjo on the side of the float bowl had become slack and leaked.
It turned out to be corroded & stripped threads in the top of the bowl.
Supporting this in situ, I drilled in situ and tapped in situe for a helicoil, It's M12 x 1.25, shortening the helicoil a few turns first. I ground the taper off the tap once it bottomed to be sure i got the thread to the very bottom of the blind hole. I vacuumed and blew out the swarf as I went and then removed it to rinse it out in petrol.
I still had a drip from the base onto the dynamo. That was because the the threaded plug screw that sits at a steep angle at the bottom of the carb was bottoming out "dead tight" before it fully sealed on the fibre washer. It looked and felt in order normal, so it took me a while to figure this one out..but two washers solved that one.
Your leak sounds like a gush, and the valve above the float could be responsible. My valve is working well and I was impressed when I checked it that it seemingly works so very much better than an Amals reverse-tapered valve through the middle of there floats. I checked it by holding the carb and blowing through the fuel inlet and then turning it upside down to confirm the weight of the float closed the valve. I also sucked on the fuel inlet while upside down and then turned the carb back up he right way. This way I could thoroughly feel for even the slightest air leak, much like sucking the air out the top of your soda-pop bottle and sticking it on your tounge. (Well, come on.. we are all just boys with toys eh?)
I came to learn/read that the bi-starter choke block on the top of the carb is actually a secondary carburettor in its own right, feeding both fuel and air. So if this is not fully closed with its lever full over towards the clutch side of the bike (perhaps its lever has slipped up the choke button piano-wire rod), then the engine could well rev as you describe, perhaps more so if there are still any air leaks.
I hope this helps. Good luck.