Hi all this is a similar problem to Fzr400Trev, though my wee 400 doesn't start at all.
It has new plugs,
a new battery,
I took the carbs off and cleaned them,
the tank was cleaned out and re-lined yet it still won't start.

There's petrol in the float bowls so there's petrol coming from the tank through the tap and down the fuel line. I took all the sparks out and checked for a spark which was ok but still no go. I put some petrol in the cylinders under the plugs and it fired but didn't catch and run.

Any thoughts?

Probably the o-ring seals in the carbs need replacing. Have a word with variablevalvesuck for replacements.

Mine had 2 issues preventing it starting, one was the infamous o-rings, and the second was the chuffing starter motor of all things. It would spin the engine, but appeared to be supping all the current from the ignition circuit to do so. Ended up stripping the motor down and rebuilding it.

Before I fixed it, I could get it to go sometimes be squirting a bit of easystart type stuff into the intake.

i had a nightmare getting mine going all started when i took the carbs off
i tried everything.
using the mrs car with jump leads, would get it to fire but not start
cleaning the filter
new petrol pipes
stripping the carbs and cleaning them
bum starting (nearly killed me)
checked the coils
plenty of swaring
kicking the tyres
shouting at the engine
turning my back on it and cusing it
none worked

changed the plugs statred 1st push
my old plugs gave a spark