Not firing on cylinders 2 or 3

[Disorderlypunk]

cant really blow a gasket between 2and3 as the timing chain is in between so would have to blow twice
can you do a compression check (no dismantling needed)
start her up and let her idle and check downpipe temperatures to make sure its 2+3 (they will be cold)
after a while of just tickover try having a look at the spark plugs to see if they are wet
might just need a tin of fuel system cleaner ran through the tank cause it sounds like blocked idle jets to me
failing that its a carb cleanout time