Hi, one week ago I changed spark plugs and air filter my NC30. I also changed the oil & filter.
Started the bike ok, revved nicely. Shut it off and left it.
Haven't started it in 1 week.

Now today, I pulled the choke, flicked kill switch to run position and turned fuel tap to on position. Started engine ok, released choke and then engine cut out. I proberbly let the choke off too early, I realize this. Tried to re-start again, this time engine rpm hovered around 2,000 rpm and would not rev up using the full choke to around 4,000 rpm as it normally would. Then after about 10-15 seconds the engine just cut out.

Now after continuing to restart the engine, I cannot get it started. And when it will start in like 1 in every 20 attempts with the choke on, the idle will just hover around 1,000rpm and then the engine again cuts out.

So I went back to everything that I removed to replace the spark plugs and air filter.
I removed the spark plugs, reseated them, and reseated the spark plug leads so they were a snug fit. I rechecked all lines to the air filter housing - all ok. I also checked the vacuum line to the fuel tap - ok. Also the fuel line from the tap to the small 3 piece connector - ok.
I have a feeling that there is no fuel getting to the engine, or that I have no spark. That's the only things I can think of.

Any help will be greatly appreciated.

Err, do you definetely have fuel in the tank? tap in correct position?

Yes, fuel in the tank and tap in on position.
I tried a restart this morning (cold start). Choke on fully, tap in on, kill switch on, tried to start it. It fired up but once again engine rpm just hovered at 1,000 rpm, it would not rev up by itself to 4,000rpm like it normally does. It ran for approx 20 seconds and then just cut out.

When I removed the plugs on the rear cylinders yesterday, I think no. 1 was a little carboned up and no. 3 (I think) looked ok. I will put the old plugs back in it tomorrow and see if it starts and how it runs.

How can I check to see if the fuel tap is actually doing anything?

To me that sounds like a fuel vacuum problem, similar to what i had on my NC a few weeks back. I would make sure the vacuuum is properly on at the tank and the engine, make sure there are no kinks. Then maybe run a piece of spare hose to the vacuum pipe on the tank turn the fuel on and suck and see if fuel comes out of the main line, or run a temporary fuel bottle to the engine and see if it runs alright. process of illimination. is the vacuum pipe on the correct nozzle? the front right side nozzle on the tap??

Take the petrol pipe off the tank and use an upside down small pop bottle with the end cut off / funnel / anything to hold some fuel and try starting it using the 'temporay' tank

it'll soon isolate your fuel tank :0

You can then move onto testing the tank

Once again thanks guys, it seems that I made the dumbest mistake in my life (mechanically) . I fitted the vacuum line to the wrong port on the fuel tap. I think it was on the atmosphere outlet and not the inlet for the fuel tap. Re-fitted the tank, choke on fully, and it fired right up. No problems. Let it warm up by itself and then took it on a good run. No problems.

Problem fixed.