Villiers Mk25 running speed

Completely runout of ideas. We have a Mk25 with a type 3/4 carb.  Can only get it to run at tickover or just a bit faster with the throttle fully open. Tried adjusting the needle and both a 3 1/2 and 4 1/2 needle. Any ideas about getting it to run properly in a normal throttle slide range will be gratefully received. Plug is black so it must be running rich.

Governed speed set too low? Does it have a separate butterfly in the inlet linked to the governor?

Good suggestion, but it’s not running fast enough for the governor to kick in.
I suspect either carburation  or ignition timing. The engine is in a museum and I only see it once a week, tried all sorts with the mixture, so next week I check the timing again. The tappets are a bit sloppy but surely that’s not it.

AdrianRJackson wrote:The tappets are a bit sloppy but surely that’s not it.

Not sure exactly what you mean by sloppy.   
With both valves fully closed there should be a gap of 6 to 10 thou between tappet cap and valve stem.
Definitely check ignition timing - flywheel isn't keyed to shaft so easy for it to slip having set the contact breaker/gap.
I've had a backfire cause the timing to go off.

Thanks Paul.
Will check timing and tappets again on Tuesday when I see the engine again

Another couple of thoughts:

Is the engine physically 'tight', ie a partial seizure somewhere?

Exhaust port and/or exhaust partially blocked with carbon?