Engine problem and I'm out of ideas

[351ciofgrunt]

Hey guys,
I know its not a ford but i'm having a problem with my GQ patrol TB42 carbie. and I'm out of ideas.

Symptoms
- car idles okay with a slight miss
- soon as you start driving it there's shuddering, surging and also some backfiring through carb, kind of feels like when a fuel pump is gone and the cars starving for fuel. Problem just started one day.

What I've done so far:

Fuel
First replaced fuel pump, then replaced carbie which made no difference, then eliminated fuel system entirely by ripping off fuel system and replacing with straight LPG gas (impco model E and 425)
- made no difference to problem

Compression
-compression tested all cylinders and they are within 5 psi of 165psi

Spark
Replaced points, ignition coil, checked and gapped spark plugs, checked ignition timing

Engine oil looks good and clean
Coolant looks good and clean

I'm out of ideas

I suspect a cracked cylinder head, perhaps a crack between two combustion chambers however it didnt show up on the compression test, but maybe its small enough to only cause a problem under combustion pressures and not cranking pressure?