i had compressions on numbers 1 and 2 of 100psi, 130/140 on 3 and 4, pull head off grind in valves, all exhausts not sealing, check rings and glaze bust bores. Clean out carbs thoroughly, replace head and shim up valves, not compressions the same as before made no difference. Using an infra red thermometer the offending cylinders are half the temp as the good ones when running..... n e one got any ideas??? is there something i could be missing that is typical of this engine?

I had non stop problems from my engine, number 1 wwould not get hot till the bike was ragged
tried everything, clearances, different carbs, balancing etc it then started to rattle/tap.
i thought sod it i have 2 spare engines, so swapped one.
on taking the old one apart, i found one of the bolts that hold down the cams, (inlet i think) had snapped!!!

You've already done everything I would have and it hasn't fixed it!

Other than what Wig suggested...

What were the plugs on 1 and 2 like compared to 3 and 4 when removed? Did it indicate overly rich?

Are the intake valves sticking? Or the Exhaust? You have ground them in, did you check the valve for any evidence of sticking in the guides? Were the guides okay?

Grasping at straws, keep us posted when you find it!

Wig, that sounds like what my bike is going through!

Wig how can you do the valve clearances and miss a broken cam cap bolt

dunno, did the clearances, no probs, the bolt had snapped just where the thread is cut, so it may have been damaged but not enough to break the bolt. where it has snapped the metyal looked like it had marks like plastic when you bend it.
I only found out as i went to reuse the bolt to fit a radiator, and it just broke when I tightened it.

i pulled the head off again and checked all valves, none sticking and clearances are fine, checked rings in bores, ok. i have been building lorry engines for years and have never haid anything so ridiculous.

could you have intermittant problem with spark plug leads?

May have odd pistons fitted,
cam timing,
crank silped/twisted between 2 and 3,

I know it wont help the compression but have you balanced the carbs and done pilot settings.

you should also consider if you have intermolecular preconstructional defisit crystaline propagation lines across the diffusial cross tangents, that hold the main big end bearings