Here’s a useful tip for finding a dud plug. Using an analogue multimeter (NOT a digital, these can give false readings) connect to the top of the plug on one side and the earth the other; there should be no reading on the resistance setting for ordinary plugs (i.e. the non-resistor type) but if there is then it is a dud! But before you panic the carbon deposits if the plug is sooted up could be creating a short so clean it first! I just replaced a set of plugs in a car that were all dud making it very hard to start if it went at all. The worst part was that it had been to a garage TWICE and they failed to spot it!
There is also the other side to it and that is the amount of times the carb gets the blame for bad running or starting yet despite much tinkering with there is no difference. The fault is ignition not the carb!