Not starting

Hi I have a weird problem with my 86 xj750 x , went on a rally at the weekend and did over 650 k's running ok , just bit of a misfire so yesterday checked plugs swapped one over, fired up went for a blast 20k's ran lovely . This morning trying to get to work she'd start ok but cut out as soon as you open the throttle , took carbs off cleaned jets checked floats all looked ok put everything back together and ... Nothing ?? Where to look next ?

 

Fuel supply? No vacuum to petcock? Faulty petcock? Clogged fuel filter?

 

Also my idea, the vacuum petcock hose tends to crack at the connection on the intake boot when aged. Gets pretty hot there. Happened to me a few times on my X. First time because of age, other times because I didn't use the correct hose. Also first running good, than not so good and than total crap.

 

You could also try an auxillary tank like the ones used for carburettors synchronisation and rev the engine to see if it cuts out. If the vaccum hose is split it will be drawing in air instead of drawing fuel possibly.