Engine sort of sounds ok apart from the spitting back that’s not quite like an ignition type fault sound causing the spitting, I have had early veteran car engines do something similar as the inlet valves are atmospherically opened and not mechanical, you have to get the valve spring strength fine tuned, to strong and you loose power and to weak they can sort of give a double skip bounce and spit at a particular throttle opening, and a sticking valve or broken valve spring can also causes same symptoms, I did have a valve insert start to drop out on a alloy head, it didn’t spit but just like a intermittent ignition fault causing shorted ignition. I have just had a thought valve timing, if the engine sparks at every top dead centre what I call a lazy spark if the inlet is opening early and exhaust is closing late they can spit back so check valve clearances are correct and are rocking at pistons dead top centre.
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