1850 misfire

[MIG Wielder]

We had a good trip out to SDD yesterday but the 1850 developed a strange intermittent misfire on the way back that sounded - and felt - as if it was running on 2 1/2 or 3 cylinders, mostly the full 4 pots but always worse uphill. Since the rev; counter was steady I deduced it was either H.T. or fuel. Pulling out the choke helped quite a lot and it ran as normal for the rest of the trip back with the choke pushed back in . So its also intermittent. The worst type of problem.
This afternoon I had the spark plugs out. ( Easy things first ). The front 2 were fine but the back 2 plugs looked really odd. Half the electrode on each was bright white ( typically of running very weak ) but the other half had a hard black crystalline deposit on. I've always used the Shell hi-octane fuel with no additive so what is this horrible deposit ? And why over just half the spark plug ( Bosch ).
See photo.
I'll check out the fuel side tomorrow perhaps.
Thanks,
Tony.