It can be hard to see the spark in daylight sometimes - and even then what do you judge it against. It is harder for a spark to jump a cold plug but you'd be unlucky to have an ignition system just good enough to run hot but failing when cold. A new plug, iridium or not, should help if things are marginal but it'll soon carbon up again.

Having said that I thought the main advantage of iridium plugs was that they're used as fit and (almost) forget items in cars as the tips don't erode like normal copper ones. I wouldn't have thought you'd see much cold starting advantage over a new ordinary plug - not until you'd done 50k on it anyway.

Back in the stone age there were a number of things we'd do if the ignition system was marginal - firstly close the plug gap down (to somewhere around 20 thou / 0.5mm (or even less if things were desperate)). That saved a few thousand volts and would often fire up a fouled plug two stroke. I've never needed to do it on a CDI system though.

Second, take the plug out and (after cleaning it) wipe some pencil lead on the electrodes. If you have a weak spark the graphite improves it (probably by burning but I'm not certain - this is folk wisdom, not science ) for a spark or two and may be enough to get things going. This has worked for me many times.

To check whether it's carb or spark take the plug out and tip a spoonful of neat fuel down the plug hole. Put the plug back in and try to start as normal - if it fires for a few seconds and dies it's probably a carb problem. If nothing happens it's probably ignition (or compression).

You could try using Easy Start (or the far better named Aussie version if you can get hold of it ) but a spoonful of petrol is cheaper.