Don't forget to adjust the valve.clearances, I had similar.problem on a honda GX200 engine 👌

I've had the key on the crank-flywheel fail which allowed the timing to be off a few degrees causing similar a similar problem.

My first port of call would be to get a wideband and EGT sensor installed, as i said on the first vid, running on gas requires quite close air fuel ratio adjustment or you will produce excessive heat and burn the valves, the valves are not being cooled by evaporation of fuel like with petrol, so you cant get away with it as easy. It looks like you have a set amount of fuel flowing from your regulator? as you rev it up and down using the throttle valve that would just cause the mixture to vary, as not much extra fuel will be pulled through the venturi (if you are using one) like it would with a liquid. This looks like knock to me as it decelerates quickly and makes a slapping noise, which would support the AFR being wrong. Check the regulator isn't deteriorating from all the vacuum pulses / backfires, a tiny hole in the diaphragm will affect pressure significantly. The ignition should be easy to test, put a spark plug or otherwise a gap you can watch externally that will still allow the engine to run and watch it though the rev range, take 2 mins to see if this is the issue.

On propane, I take it you have fitted the proper fuel filter? Might need cleaned/renewed, could bypass by temporally removing the fuel filter try it then try the air filter. Carburettor might need serviced, also worth checking the spark plug to see if it is fouled.

How small of a bottle and what size regulator are you using? Tank size and regulator size are important. It’s possible it’s not vaporizing fast enough or the regulator is freezing up.

Worth checking the oil level in case low oil switch is kicking in esp if reving it and the oil is getting pumped out the sump. Close look at wiring to see if anything is fouling the wires etc Sparkplug - is it definitely clean?

hi is the mixture off eg too rich or too lean