cold weather

When my bike sits for a long time the fuel will evaporate out of the carb. The clear fuel line and filter will be empty.
If I take the gas cap off and pressure up the tank with the fuel valve on I can then see some fuel
in the line. Put the choke all the way on pedal and pop the clutch it will fire right up.
If I don't pressure up the tank it will take a while before it starts and by then I'm cussing it.

To pressure up the tank use a rag to keep the gas for being blown out and just a gentle amount of pressure will do it.
This can be dangerous so be careful if you try it. no smoking, fires or open flames.
Do not put more than just enough pressure which will be less than 1/8 lb( just guessing) on the tank more pressure is not better you could blow the tank wide open. I wipe off the tank opening and pressure it with my lungs this is another not good to do recommendation.
if people see you doing this it will make them wonder what your doing.!!?? Do not inhale either gas will make you sick or poison you. Your girl friend or wife will not like it either if you smell like gas. So I warned you. This is kind of like lifting a horses tail rubbing its rear with your finger then wiping said finger on your lips. I won't cure chapped lips but it will keep you from licking them.( I have not tested this by the way) I heard it works............ I'll suffer with chapped lips.

You might try turning on the fuel valve and let gravity do it for you. Takes longer and I don't know if the float needle sticks shut or sediment plugs of the fuel strainer in the tank. You can shake the bike to move the fuel around in the tank and tap the float bowl with a screwdriver handle gently that sometimes helps. If the bike has set and the fuel line looks empty they can be a booger to get started.

you mean you removed the spark plug and it was black and oily looking.
If so then it was running on the rich side could be the spark plug is fowled
you want to replace the spark plug and gap it 0.017 to 0.020 is what I use. Some of the guys like to run more gap I don't.

I went and reread your post you did change the plug out and the plug wire.
it still won't start? did you try it with the choke lever all the way up held the throttle wide open and it didn't try to start or even pop once or twice?

might not hurt to flush out the tank. new fuel filter,clean the carb. place a clean new spark plug in it. Check the plug gap as a spark plug is gaped from the manufacturer with no specific engine in mind. Out of the box the plug may be right, then again it may not.
I run 0.017 to 0.020 some recommend more gap I prefer less.

How did you determine you have spark? Did you remove the plug and lay it on the engine while the plug cable is attached and kick it over, and look for a spark?
If you are sure you have spark, you need to figure if you have fuel.
It's so easy to remove and check the carb. I suggest you do that just to verify you don't have evil goo living in there, or a stuck float.
My best guess is the UK is humid and moisture is probably reacting with the oil in the gas and making sludge.

Rod S

A few ideas. It could be too rich especially if you see fuel coming out the exhaust, or the timing is off which is probably imposible unless something slipped. Weak spark? You have a new plug and HT lead.
Best guess is it's too rich. maybe the float or needle is off.

If your engine was running before with the needle as it is, you should leave that for now and just clean the carb and make sure your float is floating, and your needle seat is seating.