steady miss...M20

[chinist]

Developed an engine miss problem on the way home today. It seems to have come on right after I had to get on the brakes pretty hard in traffic. My first thought was that the vacuum boost diaphragm developed a leak, but can see, hear no evidence of that. The miss is steady from idle through rpm range and applying brake doesn't change it at all. So far I've only had time to look and listen for a vacuum leak....nothing.
Anyone have any ideas ? I will check the usual ignition items when I get a chance, but the brake thing is really strange. Also, not showing any check engine light.

[Ferret]

Have a nosey at the ignition system first, it's the only thing that will cause a constant same cylender misfire... unless you've got a mechanical or injector failure.

Whip off the dizzy cap and have a look see if anything's dislodged under there...

[chinist]

a small chunk of carbon grounding #6 plug electrode. Don't know why that would be floating around, but I had changed plugs not too long ago, so maybe dislodged it then. Otherwise runs great. Might get 300k yet.