tony roberts wrote:

I'm running in my overhauled engine - 4 hrs - all a full rich.

I don't understand why you would want to operate the engine without
leaning. No recommendation I've ever read about post-overhaul break-in
ever included anything like that.

Third time I tried it I had no left mag at all - drop to cut-off - same
4th time.

IMO, an indication of something not right in the mag. It bears watching.

Shut down - check all connections - everything looked good - no loose
wires etc.

So, an intermittent problem.

I added a small amout of carb heat to bring it to 37F. On
reflection I should have leaned slightly to allow for the extra richness
from the cab heat but I didn't.

OAT wouldn't have any bearing on the issue with the mag. Here in south
central USA, we get outside temps over 100 degrees F in summertime, and
mag drops aren't noticeably different than in winter with temps in the
30s-40s.

Question - Could fouling really have totally shut down my left mag?

Answer is no - the mag has no way to know what's going on with the
plugs. **Fouled plugs** from over-rich mixture can, and often do,
result in bad run-ups and mag-checks, but I'm more inclined to suspect
there's something going on in the mag. A look at the plugs for that mag
would be helpful in making that determination.