That sounds weird for sure.  A new cap and rotor might be in order, sometimes they are beyond cleanup so good that new ones are arriving.  New sparkplug wires too, even new sparkplugs.  I once goofed up while fiddling with the metal can that encases distributor (it helps prevent static on radio) but misaligned and cap was not solidly secured and squared up with distributor.  But problems that resulted were immediate.  The guys with wisdom will surely speculate regarding bad timing or O2 sensor issues.  The fact that Vanagon was stored for a long while may be a key element in the mystery.  Could be something as simple as old gasoline.

    On Tuesday, May 2, 2023 at 02:09:47 PM PDT, Spencer Allen <soxndubs1951@gmail.com> wrote:  
 
 I tuned up my 85 westy 1.9 about 2K ago and it ran fine until just before I
stored it last fall but now it's running a bit rough and I'll say rich from
the smell of exhaust. I pulled the cap and saw the cap and rotor had a
heavy layer of carbon build up. I have new ones coming from BD but need to
correct the problem before installing but can't figure out what is causing
the build up but suspect it's been heading to this since the tune. The
wires are not arching but I'm wondering if bad timing or O2 sensor would
cause this. Anyone have a list of probables?
Spencer