Misfire cylinders #3 & #4

[Craven]

Lots on here about how, ensure you have the correct fuel level in the carb’s.
Very very important.

[NWElan]

Thank you all for your suggestions.

After muddling around trying various things, the engine finally came right. It now idles evenly at 900 rpm, no spitting back through the carbs, exhaust manifold temperatures uniform around 500 degrees and very little smoke out the exhaust.

I'm not sure exactly what I did right but the major changes were correcting one exhaust valve clearance from 0.014" to 0.009", another exhaust valve clearance from 0.008" to 0.010" and increased advance to 12 deg BTDC.