If the carb was rebuilt it may have a non genuine main jet and its hose. I had one on my 1412, one day I got a bad load of fuel from Caltex and it behaved as yours is. Barely made it home with the choke out. Investigation showed the fuel had swelled the hose shut. Replaced it with a genuine jet and all was good.

_________________
DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Probably is a fuel related problem but don't discount ignition altogether. A dud condenser can cause similar, makes spark weak. That's my 2c worth.

if it's been sitting with the same fuel for more than a couple of months, dump it - easiest thing to rule out

You can put the old fuel in your daily driver - it'll just mix in with whatever's in there now

_________________
did I tell you that I won a trophy?