Looks like it was the choke seal having gone belly up, Carb kit arrived yesterday, so I pulled things apart today. Shaft seals were tired too. Running much better now.

May be miles off base, with the engine running place your hand on the coil. If to hot to touch it could be shorting out internal. Our mini went rich and ran poorly. when I opened the hood/bonnet and leaned over the engine to check the choke position I laid my hand on coil and it burned me. Only bad coil I have ever had. Steve (CTR)

And also first check that the choke/choke lever itself has fully returned to the off position. As you likely know, on the HIF it's a separate mechanism/fuel feed from the fuel feed stream @ the needle/jet tube.  Mine has stuck just a bit open in the past and I can tell by how it's running and that there's a bit of slack in the choke cable when I pull on it.  Once the mechanism is closed, the slack is gone. Could be a frayed choke cable as well.

I't check that one of the breather hoses hasn't popped off or split before mucking about with the internals, just in case.

i concur with mike....

Your choke seal just died and it's bleeding past it. In other words, you're driving with the choke filly on. It happens a lot. When you change it out make sure the bore is clean and smooth. I run a soft wire brush on a Dremel tool to clean/buff it out.

 

The piston could be stuck in the dashpot or the damper in the dashpot could be stuck or have some other issue. Like the float this is easily inspected.

Our Mini has started running extremely rich all of a sudden. It is a single HIF4 carb. Popped it off and check the float, it floats and level is OK. Needle and seat seem to shut off. Any common causes for a sudden change to go seriously rich?