only speed does it

I have a VW kit car it is a 1600 engine with a single carb (I took the twin Kadrons off to try and cure this problem) the engine is just getting up to 10K miles properly serviced electronic ignition and I am told 'lumpy cams' (it can go!)
The problem is (and I have always had it) if I start from cold it will not tick over and hesitates and you need to keep the revs up this condition will continue for miles and miles even when the engine is hot. I have changed the auto choke a few times, plugs, it has a pancake air filter.
However even from a just warm engine if I give it some welly or fast road work just for say 5 minutes its a perfect car with tickover and smooth running amazingly just now after an 80 mile run with motorway and running like a dream I pulled in and filled up (same pump same petrol as before trip) when I started it up again it was back to no tickover and hesitation until I got a chance for a quick burst of speed and all was OK again.