Updating on what's happening with mine. So there's a bit of black liquid coming out of the exhaust when the engine is revved.
I think its petrol, but not 100% sure as I wasn't the one revving it and I didn't get a whiff of it because I just saw the black stains on the concrete hours later when I got home. However if it is petrol, it seems consistent with there not being a proper combustion in one of the cylinders. I guess the petrol and air are going in, but there is no spark, so as the air keeps coming in, the petrol is just being flooded out to the exhaust.
However, do any of you think it could be something else? I mean it does make sense the engine would shake a lot for a Boxer engine (due to symmetry) once a cylinder is inactive. If it were an inline engine, there would be a lot less shaking. If it were a V-lined engine, there would be some shaking but not as much as this. Being 4 cylinders, having a 2:1 ratio of power output on the sides would make a big difference so that explains why it shakes so violently?
I guess one advantage of a Boxer this time was that it makes more sense when diagnosing the problem. However, getting to the spark plugs as usual is a difficult task haha.