Boyesen usually recommends leaning the jetting but IMO you should not change the jetting if the bike is already properly jetted. Remove your reed cage and closely inspect the reeds with a flashlight (torch for you English). If the reeds are are cracked or not fitting absolutely flat against the reed cage, they should be replaced. It is possible that even if they are sitting flat and not cracked, they have aged and are no longer stiff enough, allowing too much air and fuel in during starting.
For vent hoses, there are the ones on the carb, but don't forget the fuel cap vent. You may want to just remove the fuel cap and see if the bike starts. If the vent in the fuel cap or its hose is clogged, too little fuel will trickle down to the carb.
BTW, even though the spark looks good, it is possible to have an ignition problem. If the stator is going bad, you can get a decent looking spark when the plug is outside the motor, but the spark may be too weak under the higher pressure & more humid conditions inside the cylinder.
Also, if the woodruff key that holds the flywheel in the proper alignment is sheared, the flywheel may not be properly aligned. It could still be spinning and giving you a nice fat spark, but the spark will be delivered at the wrong time. The key is very inexpensive, but you would have to pull the flywheel to inspect.
Oh yeah, the little rubber boot on the new throttle cable is there to keep dirt and water out of the carb. If it is not seated properly, it should not effect how the bike runs. Some of the aftermarket cables have a poorly fitting boot.