GQ Carburettor question

I have a 1988 GQ Patrol with the TB42 petrol engine and the Nikki carburettor. It's set up as a dual fueller, and on LPG it works fine and idles spot on at 650 RPM. Problem is it doesn't idle at all on petrol, and so you need to keep the throttle open a bit at the lights so it doesn't die on you. It also smells very rich and the exhaust pipe is too black for it to be running properly.

Pulling the filter housing off and staring down the primary, I see that the fuel is leaving the venturi like it's being poured out of a garden hose. Now it's been a long time since I've had a look down the throat of a carb, but I'm pretty sure I shouldn't be seeing THAT much fuel, even though it's a pretty big engine.

My questions are:
1) Should I be seeing a torrent of fuel (and hearing its lovely accompanying gurgling noise as it gets sucked in past the throttle butterfly) at idling speed? I'm used to seeing this sort of thing only when you operate the accelerator pump. As far as I can recall I shouldn't be seeing any liquid at idle, because after all the carburettor's job is to atomise the fuel into a spray, not act like a lawn sprinkler. Unless there's something different about the TB42 engine.

2) How do you check the float bowl level on these stupid things? Where the sight window should be, there's nothing but a steel blanking plate. Even in the workshop manual the photo shows a carb with a blanking plate. I'm thinking the reason the fuel is pouring out is because the float level is too high or the needle isn't shutting off the supply, but how I can check that without seeing the float level? I didn't bring my X-ray glasses.

If there are any peculiarities to the TB42/Nikki setup that can be causing my problems with petrol apart from the float level, perhaps somebody out there has had experience with them.