There's a power jet in the bottom of the float bowl which might be blocked - as RW says if it's been standing a while you can get 'varnish' in the float bowl which can block things. You'll have to take the carb off to clean it - nothing else will work really.
Also, the vacuum advance on the ignition unit can stop working. Test it with it hot and idling by folding the pipe (from the manifold to the ignition unit) to block it and then pulling it off the ignition unit. If it slows down or stops it's working; if not it's broken. They're not really repairable although I've got one working before now.
The ducts just feed warm air in - not really necessary in warm weather but I just got some silicone ducting from
. If the exhaust manifold is leaking it won't have much effect, but the inlet shares the gasket and if that's leaking it will. So probably worth changing it. They do crack as well so hopefully it's not that.
How does it run generally? Smooth, spluttery, smokey? Does it start OK? Does it go better if you pull the choke out a bit? Will it pull fifth gear OK at 50mph on the flat?
One more thing - there is very little reserve throttle pedal travel especially if it has a cab carpet so make sure there's hardly any slack in the throttle cable. It adjusts at the carb end with a clippy thing. That's made a few of them faster!
However, they're slow when they're running right. Just be glad it's not the 1.4