Re: 1500 chuggy
It sounds to me like dirt in the carbs. I'm guessing that having taken them off the Engine you may have disturbed some muck in the bottom of the Float Chambers, espcially as you mentioned a bit of a leak from one of them. If you can, strip down both carbs (Float Chamber lids off and Piston Covers off) and blow out any muck with compressed air, or failing that with some aerosol Carb Cleaner. Hopefully this should sort it assuming you've put all the Gaskets back as they came off and they aren't damaged..
Hope you get it fixed and out on the road again soon!
Re: Re: 1500 chuggy
Update: I took the carbs off again and found that the front gasket between carb and heat shield had dropped across the throat. It had split and was flapping about disturbing the mixture flow. The carbs on this are held on by bolts with screwdriver grooves. These are just gash bolts cut by the previous owner. They make installation really difficult so after the above I decided to use new studs so that all the layers can be hung in place prior to the carbs being installed. Looking through the manuals and parts catalogues the bolts on the inlet were or appeared to be the norm, how stupid. Now with the carbs off I blew through the fuel channels and replaced both float chamber valves before reinstalling on the new studs. Job done and it runs like a dream again.
A quick observation / moan:-
I have to say that the 1500 engine may have been fine in a Spitty where side access is not compromised by wings but in the Midget some bits like the fuel pump are a pain. I have to admit to having a soft spot for the old A series 1275, perhaps my dealings with Mini's for years makes me biased My experiences to date with the Midget 1500 have been all but easy, brakes have been a pain to bleed, clutch the same, dare I go on, unlike my old 1967 Midget 1275 which was easy and basic, perhaps too basic. It is perhaps typical BL in the seventies where changes were made to suit management changes rather than to improve the cars. The Mini is a prime example. Sadly I sold my 1275 when prices were deflated, typical.